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ECE 638: Principles of Digital Color Imaging Systems

ECE 638: Principles of Digital Color Imaging Systems. Lecture 11: Color Opponency. From “ Xkcd A Webcomic of Romance, Sarcasm, Math, and Language ” – courtesy Steven C. Rausch, ECE 638 student, Fall 2017. Basic spatiochromatic model structure. Opponent stage.

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ECE 638: Principles of Digital Color Imaging Systems

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  1. ECE 638: Principles ofDigital 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 even more red to green, the color appears red, but never reddish green

  5. Red-green color opponency

  6. Blue-yellow color opponency

  7. Red-blue and green-blue combinations

  8. 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

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

  10. 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

  11. 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.

  12. See Figs. 4 and 5 for plots shown on Slide 12

  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 Underlined 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. L* -a* -b* + b* + a* CIE L*a*b* and its linearizedversion YCxCz in terms of CIE XYZ CIE L*a*b* • { x L* = 116 f(Y/Y ) - 16 7.787x +16/116 0 0.008856 n f(x) = x 1/3 0.008856 x 1 [ ] a* = 500 f(X/X ) - f(Y/Y ) n n [ ] b* = 200 f(Y/Y ) - f(Z/Z ) white point :(X Y Z ) n n , , n n n • Linearized opponent color space Y C C y x z Y 116 (Y/Y ) correlate of luminance y = n [ ] C = 500 (X/X ) - (Y/Y ) R - G opponent color chrominance channel x n n [ ] C = 200 (Y/Y ) - (Z/Z ) Y - B opponent color chrominance channel z n n

  15. Formulas for conversion for CIE XYZ to 1976 CIE L*a*b* From Wyszecki and Stiles, Color Science Concepts and Methods, Quantitative Data and Formulae, 2nd Ed., p. 167.

  16. Basic spatiochromatic model structure

  17. text

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

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

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

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

  22. 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

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

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