ENEE631 Digital Image Processing (Spring'04) Visual Perception Spring ’04 Instructor: Min Wu ECE Department, Univ. of Maryland, College Park .

ENEE631 Digital Image Processing (Spring'04)

Visual PerceptionVisual Perception

Spring ’04 Instructor: Min Wu

ECE Department, Univ. of Maryland, College Park

www.ajconline.umd.edu (select ENEE631 S’04) [email protected]

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Based on ENEE631 Based on ENEE631 Spring’04Spring’04Section 2 Section 2

ENEE631 Digital Image Processing (Spring'04) Lec2 – HVS [3]

Information Processing by Human ObserverInformation Processing by Human Observer

Visual perception– Concerns how an image is perceived by a human observer

preliminary processing by eye this lecture further processing by brains

– Important for developing image fidelity measures needed for design and evaluate DIP/DVP algorithms &

systems

imageimage eyeeye perceived perceived imageimage

understanding of content

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The EyeThe Eye

– Cross section illustrationFigure is from slides at Gonzalez/ Woods DIP book website (Chapter 2)

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Two Types of Photoreceptors at RetinaTwo Types of Photoreceptors at Retina

Rods– Long and thin– Large quantity (~ 100 million)– Provide scotopic vision (i.e., dim light vision or at low illumination)– Only extract luminance information and provide a general overall picture

Cones– Short and thick, densely packed in fovea (center of retina)– Much fewer (~ 6.5 million) and less sensitive to light than rods– Provide photopic vision (i.e., bright light vision or at high illumination)– Help resolve fine details as each cone is connected to its own nerve end– Responsible for color vision

– Mesopic vision provided at intermediate illumination by both rod and cones

our interest (well-lighted display)

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LightLight

Light is an electromagnetic wave– with wavelength of 350nm to 780nm stimulating human visual response

Expressed as spectral energy distribution I()

– The range of light intensity levels that human visual system can adapt is huge: ~ on 10 orders of magnitude (1010) but not simultaneously

– Brightness adaptation: small intensity range to discriminate simultaneously

Figure is from slides at Gonzalez/ Woods DIP book website (Chapter 2)

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Luminance vs. BrightnessLuminance vs. Brightness

Luminance (or intensity)– Independent of the luminance of surroundings

I(x,y,) -- spatial light distributionV() -- relative luminous efficiency func. of visual system ~ bell shape

(different for scotopic vs. photopic vision; highest for green wavelength, second for red, and least for blue )

Brightness– Perceived luminance– Depends on surrounding luminance

Same lum. Different brightness

Different lum.

Similar brightness

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Luminance vs. Brightness (cont’d)Luminance vs. Brightness (cont’d)

Example: visible digital watermark– How to make the watermark

appears the same graylevelall over the image?

from IBM Watson web page“Vatican Digital Library”

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Look into Simultaneous Contrast PhenomenonLook into Simultaneous Contrast Phenomenon

Human perception more sensitive to luminance contrast than absolute luminance

Weber’s Law: | Ls – L0 | / L0 = const

– Luminance of an object (L0) is set to be just noticeable from luminance of surround (Ls)

– For just-noticeable luminance difference L: L / L d( log L ) 0.02 (const)

equal increments in log luminance are perceived as equally different

Empirical luminance-to-contrast models

– Assume L [1, 100], and c [0, 100]– c = 50 log10 L (logarithmic law, widely used)

– c = 21.9 L1/3 (cubic root law)

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Mach BandsMach Bands

Visual system tends to undershoot or overshoot around the boundary of regions of different intensities

Demonstrates the perceived brightness is not a simple function of light intensity


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Visual Angle and Spatial FrequencyVisual Angle and Spatial Frequency

Visual angle matters more than absolute distance– Smaller but closer object vs. larger but farther object– Eyes can distinguish about 25-30 lines per degree in bright illumination

25 lines per degree translate to 500 lines if distance=4*screenheight

Spatial Frequency– Measures the extent of spatial transition

in unit of “cycles per visual degree”

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Visibility Threshold at Various Spatial FrequencyVisibility Threshold at Various Spatial Frequency

Preliminaries on 2-D linear (spatial) invariant system

[ Extending from 1-D LTI systems ]

– Can be characterized by “Point Spread Function (PSF)” (i.e. impulse response) and the 2-D transfer function

– The magnitude of the (normalized) transfer function is called the “Modulation Transfer Function (MTF)”

We’ll study 2-D systems and transforms in detail in 2 lectures

Visibility threshold at different spatial frequency– Eyes are most sensitive to mid frequencies,

and least sensitive to high frequencies– Most sensitive to horizontal and vertical ones than other orientations

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Image Fidelity CriteriaImage Fidelity Criteria

Subjective measures– Examination by human viewers– Goodness scale: excellent, good, fair, poor, unsatisfactory– Impairment scale: unnoticeable, just noticeable, … – Comparative measures

with another image or among a group of images

Objective (Quantitative) measures– Mean square error and variations– Pro:

Simple, less dependent on human subjects, & easy to handle mathematically

– Con: Not always reflect human perception

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Mean-square CriterionMean-square Criterion

Average (or sum) of squared difference of pixel luminance between two images

Signal-to-noise ratio (SNR)– SNR = 10 log10 ( s

2 / e2 ) in unit of decibel (dB)

s2 – image variance, e

2 – variance of noise or error

– PSNR = 10 log10 ( A2 / e2 ) with A being peak-to-peak value

PSNR is about 12-15 dB higher than SNR

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Color of LightColor of Light

Perceived color depends on spectral content (wavelength composition)

– e.g., 700nm ~ red.– “spectral color”

A light with very narrow bandwidth

A light with equal energy in all visible bands appears white

“Spectrum” from http://www.physics.sfasu.edu/astro/color.html

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Perceptual Attributes of Color Perceptual Attributes of Color

Value of Brightness (perceived luminance)

Chrominance– Hue

specify color tone (redness, greenness, etc.)

depend on peak wavelength

– Saturation describe how pure the color is depend on the spread

(bandwidth) of light spectrum reflect how much white light is

added

RGB HSV Conversion ~ nonlinear

HSV circular cone is from online documentation of Matlab image processing toolbox

http://www.mathworks.com/access/helpdesk/help/toolbox/images/color10.shtml

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Absorption of Light by R/G/B ConesAbsorption of Light by R/G/B Cones


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Representation by Three Primary ColorsRepresentation by Three Primary Colors

Any color can be reproduced by mixing an appropriate set of three primary colors (Thomas Young, 1802)

Three types of cones in human retina– Absorption response Si() has peaks around 450nm (blue), 550nm

(green), 620nm (yellow-green) – Color sensation depends on the spectral response {1(C), 2(C),

3(C) } rather than the complete light spectrum C()

S1() C() d

S2() C() d

S3() C() d

C()

color light

1(C)

2(C)

3(C)

Identically perceived colors if i (C1) = i (C2)

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Example: Seeing Yellow Without YellowExample: Seeing Yellow Without Yellow

mix green and red light to obtain perception of yellow, without shining a single yellow photon

520nm 630nm570nm

=

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“Seeing Yellow” figure is from B.Liu ELE330 S’01 lecture notes @ Princeton; R/G/B cone response is from slides at Gonzalez/ Woods DIP book website


RGB Primaries and Color RepresentationRGB Primaries and Color Representation

– Use red, green, blue light to represent a large number of visible colors– The contribution from each primary is normalized to [0, 1]

Color-cube figures: left figure is from B.Liu ELE330 S’01 lecture notes @ Princeton, right figure is from slides at Gonzalez/ Woods DIP book website

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Color Coordinate for PrintingColor Coordinate for Printing

Primary colors for pigment– Defined as one that subtracts/absorbs a

primary color of light & reflects the other two

CMY – Cyan, Magenta, Yellow – Complementary to RGB– Proper mix of them produces black

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ExamplesExamples

HSV

YUV

RGB

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Color Coordinates Used in TV TransmissionColor Coordinates Used in TV Transmission

Facilitate sending color video via 6MHz mono TV channel

YIQ for NTSC (National Television Systems Committee) transmission system

– Use receiver primary system (RN, GN, BN) as TV receivers standard

– Transmission system use (Y, I, Q) color coordinate Y ~ luminance, I & Q ~ chrominance I & Q are transmitted in through orthogonal carriers at the

same freq.

YUV (YCbCr) for PAL and digital video– Y ~ luminance, Cb and Cr ~ chrominance

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ENEE631 Digital Image Processing (Spring'04) Visual Perception Spring ’04 Instructor: Min Wu ECE Department, Univ. of Maryland, College Park .

Documents

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examples image

information processing

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