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ELE 488 Fall 2006Image Processing and Transmission
Syllabus
1. Human Visual System 2. Image Representations (gray level, color)3. Simple Processing: point operations and filtering 4. Still Image Coding5. Resampling, Resizing, Interpolation, and Registration6. Probability Models, Quantization, Estimating Densities7. Synthesizing Pixels, Segmentation8. Radon Transform, Other imaging modes9. Video, Video Compression 10. Selected Topics: watermarking, feature description, face recognition, . . .
9/19/06
What is an Image?
• What we perceive as an image is a pattern of light intensity over an image plane. It can be described by a real valued function J(x,y) of two spatial coordinates on an image plane. J(x,y) is the intensity of the image at the point (x,y).• For image processing purposes, the image is usually defined on a bounded rectangle only.
x
yAnalog image – (x,y) can take on any value in the image plane.Digital Image – (x,y) takes on discrete values.
Grayscale image – the image values J(x,y) are scalars, e.g. real numbers. Color image – J(x,y) is a vector
From 9/14
From Min Wu @ U Md
Light – electromagnetic wave
Visible Light - wavelength between 350nm and 780nm
wavelength
Few objects are of a single colorA color is not necessarily of a single wavelength
Energy distribution of light
From Kodek
Human Vision System
• Visible spectrum• brightness• color• limit of vision• seeing imperfections• sensitivity: ~ 3photons• range : 10 order of
magnitude • (range of hearing: 7
order)
dot
dot
What We Can See
• Visual angle matters more than absolute distance– Smaller but closer object vs larger but farther object– Can distinguish ~ 25-30 lines per degree in bright illumination
• 25 lines per degree translate to 500 lines if distance=4*screenheight
• Spatial Frequency– Periodic light intensity variation in space variable– Unit: cycles per degreeU
Two types of receptors~200 million Rods – brightness~ 10 million Cones – color B <10%, G ~30%, R ~60%
only 1 million ganglion cells to carry information to the brain
Neural Structure of Retina
COMPRESSIONCOMPRESSION
Two Types of Photoreceptors at Retina
~ 200 million rods - sensitive to brightness
~ 7 million cones - sensitive to color (red, green, blue)
Rela
tive S
ensi
tivit
y
Wavelength [nm]
400 500 600 7000
0.5
1.5
1.0
Spectral Response of R, G, B Cones
Two Types of Photoreceptors at Retina
• Rods– 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
– 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
• Perceived color depends on spectral content – e.g., 700nm ~ red.
• A color is not necessarily of a single wavelength • A color is defined by its spectral distribution C()• “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