1. ECE 638: Principles of Digital Color Imaging Systems
Lecture 11: Color Opponency

2. From “Xkcd A Webcomic of Romance, Sarcasm, Math, and Language” – courtesy Steven C. Rausch, ECE 638 student, Fall 2017

3. Basic spatiochromatic model structure

4. Opponent stage
Trichromatic theory provides the basis for understanding whether or not two spectral power distributions will appear the same to an observer when viewed under the same conditions.
However, the trichromatic theory will tell us nothing about the appearance of a stimulus.
In the early 1900’s, Ewald Hering observed some properties of color appearance
Red and green never occur together – there is no such thing as a reddish green, or a greenish red
If I add a small amount of blue to green, it looks bluish-green. If I add more blue to green, it becomes cyan.
In contrast, if I add red to green, the green becomes less saturated. If I add enough red to green, the color appears gray, blue, or yellow
If I add enough red to green, the color appears red, but never reddish green

6. Red-green color opponency

7. Blue-yellow color opponency

8. Red-blue and green-blue combinations

9. Opponent stage (cont.)
Hering postulated that there existed two kinds of neural pathways in the visual system
Red-Green pathway fires fast if there is a lot of red, fires slowly if there is a lot of green
Blue-Yellow pathway fires fast if there is a lot of blue, fires slowly if there is a lot of yellow
Hering provided no experimental evidence for his theory; and it was ignored for over 50 years

10. Hue Cancellation Observer looks at patch & makes two observations
(no yet)
1) Reddish or greenish (or neither)
2) Bluish or yellowish (or neither)
Stimulus
Patch
Cancelling
Monochromatic Source Test Stimulus

11. Hue Cancellation (cont.)
Do two experiments separately
1) a. If subject said reddish, add enough green
to cancel reddish appearance
b. If subject said greenish, add enough red
to cancel greenish appearance
2) Perform similar experiment for blue-yellow

12. Experimental evidence for opponency
Hurvitch and Jameson hue cancellation experiment (1955)
Savaetichin electrophysiological evidence from the retinal neurons of a fish (1956)
Boynton’s color naming experiment (1965)
Wandell’s color decorrelation experiment
Left and right plots show data for two different observers. Open triangles show cancellation of red-green appearance. Closed circles show cancellation of blue-yellow appearance.

13. Color spaces that incorporate opponency
YUV (NTSC video standard space)
YCrCb (Kodak PhotoCD space)
L*a*b* (CIE uniform color space)
YCxCz (Linearized CIE L*a*b* space)
O1O2O3 (Wandell’s optimally decorrelated space)
O1O2O3 forms the basis for the Zhang-Wandell S-CIELAB color space
Underline colors indicate approximate opponent components
Wandell used cone response curves to compute LMS tristimulus values for the colors in the Macbeth Color Checker.
He then found a linear transformation to new color coordinates O1O2O3 that are maximally decorrelated.

14. CIE L*a*b* and its linearized version YCxCz in terms of CIE XYZ•
CIE L*a*b* { L*
= 116 f(Y/Y
) - 16
7.787x +16/116 x n
f(x) = x
1/3 x 1 a*
= 200 [
f(X/X
) - f(Y/Y ) ] n n b*
= 500 [
f(Y/Y
) - f(Z/Z ) ] n n
white point :(X , Y n n , Z ) n L*
-a* + a*
-b* b* •
Linearized opponent color space Y y C x C z Y
116 (Y/Y )
correlate of luminance y = n C
= 200 [
(X/X
) - (Y/Y ) ] x n n R - G
opponent color chrominance channel C
= 500 [
(Y/Y
) - (Z/Z ) ] z n n Y - B
opponent color chrominance channel

15. Basic spatiochromatic model structure

16. Wandell’s Experiment Background
Wandelll’s PCA of Macbeth Color Checker (LMS) Tristimulus data set:
i-th patch from Macbeth color checker T
yellow

18. Wandell’s Experiment (cont.)
- Achromatic channel measuring lightness
- Green-Red
- Blue-Yellow

19. Spectral sensitivities of the Wandell channels
Wandell’s sensitivities
Hurvetch-Jameson cancelation curves (similar to negative of Wandell sensitivities

20. Applications of color opponency
Example 1: ( ) Kodak PhotoCD
: R-G
: B-Y
Example 2: YUV (D.E. Pearson)
Example 3: CIE L*a*b*
Gamma-corrected
Primary

21. Wandell’s space in terms of CIE XYZ*
*Wen Wu, “Two Problems in Digital Color Imaging: Colorimetry and Image Fidelity Assessor,” Ph.D. Dissertation, Purdue University, Dec. 2000

22. Visualization of opponent color representation
(Y,o2,o3)
(Y,0.24,0.17)
(13.3,o2,0.17)
(13.3,0.24,o3)