Image Perception and Color Space

Human Eye Photoreceptors: 1. Rods more sensitive, for low light 2. Cones less sensitive, for day light (So, human can’t perceive color in low light) 100M rods vs. 6.5M cones. -> 16:1 -> chrominance down-sampling can be 4:1 in each dimension The Fovea (center of retina) All the cone sensors of the retina are concentrated in this area. It is a small pale spot, about the size of a pinhead and, on our big picture, is in the darkened area just to the right of the optic disc - it has been electronically enhanced in this image. It is totally responsible for our colour vision and our critical vision

Simultaneous Contrast luminance vs. brightness Contrast Weber’s Law: If background is f, than foreground of (f+delta f) gives you the same contract

Mach Band Lateral inhibition Eye sees contrast, not absolute luminance So, when the neighbor is bright, the area looks darker Can be modeled as a sinc low pass filter for the impulse response of visual system

Herman Grid Can be explain by later inhibition (when there are white above/below/left/right, it looks dark) When you focus on one intersection, the other intersection turn dark, which proves that lateral inhibition is more significant in the peripheral

Blind Spot Test Where the optical nerve is Ask the viewer to use right eye, focus on the cross

Blind Spot Test Where the optical nerve is Ask the viewer to use right eye, focus on the cross

Blind Spot Test Where the optical nerve is Ask the viewer to use right eye, focus on the cross

Blind Spot Test Where the optical nerve is Ask the viewer to use right eye, focus on the cross

Color Representation

Visual Spectrum Blue 435.8 nm Green 546.1 nm Red 700 nm

Perceptual Representation (HSV)

CIE Color Chart CIE XYZ Not uniform chromaticity scale (UCS) CIE: Intl Committee on Color Standards CIE XYZ

RGB primary colors CMY secondary colors

Lower right: saturated colors, farthest from the line connecting black and white Upper right: pure red of different intensity/luminance The upper left is similar to CIE XYZ

YUV Color Space Y is luminance of a color Y = 0.299*R’ + 0.587*G’ + 0.114*B’ U and V are color differences U = 0.492*(B’-Y) V = 0.877*(R’-Y) This simplifies recovery of R’,G’,B’ R’ = Y - 1.140V G’ = Y - 0.394U - 0.581V B’ = Y + 2.032U R’,G’,B’ here are same as R_N,G_N,B_N in Jain’s

YIQ Color Space Rotate UV vectors by 30o U V I Q

YIQ Color Space I = 0.736*(R’-Y) - 0.268(B’-Y) Q = 0.478*(R’-Y) + 0.413*(B’-Y) Recovery of R,G,B R’ = Y - 0.956*I + 0.620*Q G’ = Y - 0.272*I - 0.647*Q B’ = Y - 1.108*I - 1.705*Q

YCbCr Scaled and offset version of YUV Range of signals Suitable for digital images/video

YCbCr Color Space Begin by calculating R-Y and B-Y vectors B’-Y = -0.299*R’ - 0.587*G’ + 0.886*B’ R’-Y = 0.701*R’ - 0.587*G - 0.114*B’ The difference signals have ranges B’-Y (-.866 to .866) R’-Y (-.701 to .701) Scale to range (-.5 to .5) to give same range as Y (0 to 1) Cb = -.169*R’ - .331*G’ + .500*B’ = .564(B’-Y) Cr = .500*R’ - .419*G’ - .081*B’ = .713(R’-Y)

YCbCr Color Space For computer representation, Scale and offset these values to keep in range 16 to 240 (Cb and Cr) or 16 to 235 (Y) Cb = 224*Cb + 128 = 126.336(B’-Y) + 128 Cr = 224*Cr + 128 = 159.712(B’-Y) + 128 Y = 219*Y + 16 Finally,

CMYK Color Space Subtractive color space Most common use is for printers K (black) is added for efficiency and consistency White cannot be generated w/o white paper Example: Cyan represents green and blue, by adding cyan we subtract the color red from sum.

Perception? Illusion?

Masking Effect Quantization step size = 32

Masking Effect Quantization step size = 32 But less visible artifacts

Necker Cube Tiring effect: the more you look at it, the more unstable it appears to jump between different interpretation

How many? 6 or 7?

3 different interpretations

Which is longer? Explanation: eye trends to compare closer parts of the two objects

Poggendorff Illusion

Ponzo Illusion The gray lines produce the distance effect, human eyes magnify object that is farther The one that “fills” the space looks longer

Adaptation Eyes are sensitive to changes, but get used to stead states

Adaptation

Adaptation

Adaptation

Are they straight lines?

Which is longer? The horizontal is “broken” by the vertical, so looks shorter

Stereo Vision

Autostereogram

What do you see?

What do you see?