Image Perception and Color Space
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Transcript of Image Perception and Color Space
Image Perception and Color Space
Human Eye
Simultaneous Contrast
• luminance vs. brightness
• Contrast– Weber’s Law: constant
f
f
Mach Band
• Lateral inhibition
0 50 100 150 200 250-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Herman Grid
Blind Spot Test
Blind Spot Test
Blind Spot Test
Blind Spot Test
Color Representation
Visual Spectrum
Red 700 nm
Green 546.1 nm
Blue 435.8 nm
Perceptual Representation (HSV)
CIE Color Chart
CIE XYZ
YUV Color Space
• Y is luminance of a colorY = 0.299*R’ + 0.587*G’ + 0.114*B’
• U and V are color differencesU = 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
YIQ Color Space
• Rotate UV vectors by 30o
V
U
Q
I
)33cos(
)33sin(
)33sin(
)33cos(
0
1
1
0
U
V
Q
I
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 signalsY: 16~235
Cb,Cr: 16~240
• Suitable for digital images/video
YCbCr Color SpaceBegin 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 SpaceFor 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,
128
128
16
071.0368.0439.0
439.0291.0148.0
098.0504.0257.0
B
G
R
C
C
Y
r
b
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
Masking Effect
Necker Cube
Which is longer?
Poggendorff Illusion
Ponzo Illusion
Adaptation
Adaptation
Adaptation
Adaptation
Are they straight lines?
Which is longer?
Stereo Vision
Autostereogram
What do you see?
What do you see?