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?