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Color Appearance Models

Color Appearance Models. Should account for chromatic adaptation and illumination. CIELab does account for illumination and can be modified to account for chromatic adaptation by scaling cone responses independently after converting XYZ to cone coordinates. But:.

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Color Appearance Models

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  1. Color Appearance Models • Should account for chromatic adaptation and illumination. • CIELab does account for illumination and can be modified to account for chromatic adaptation by scaling cone responses independently after converting XYZ to cone coordinates. But:

  2. Color Appearance ModelsProblems with CIELab • CIELab is not a simple adaptation model:[Xa,Ya,Za]t = M-1diag(kL kM kS) M [X Y Z] t • is not itself a linear adaptation model, i.e.is not of the form[Xa,Ya,Za]t = diag(kL0 kM0 kS0) [X Y Z] t • (“A wrong Von-Kries transformation”

  3. Color Appearance ModelsProblems with CIELab • Trouble for Munsell colors: • Constant Munsell chroma and hue are not constant in a* b* space (Fairchild Fig 10-4) • Munsell R, G, Y, B do not lie on a*,b* axes • No prediction of luminence dependencies

  4. Nayatani Color Appearance Model • Foundation in illumination engineering • need to model illumination color rendering • how general? • Models appearance of uniform patches on gray background • Not good for complex stimuli or backgrounds

  5. Nayatani Color Appearance Model • Data for the model • Background reflectance Y0 > 18% • CIE chromaticity x0 y0 of illuminant • CIE chromaticity x y and reflectance Y of stimulus • Absolute illuminance E0 of viewing fieldin lux • http://www.intl-light.com/handbook/irrad.html

  6. Nayatani Color Appearance Model • Model components • Nonlinear chromatic adaptation • One achromatic, two chromatic color opponent channels weighted by cone population ratios

  7. Nayatani Color Appearance Model • Model outputs • Brightness as linear function of adapted cone responses (which are non-linear!) • Lighness: achromatic channel origin translated to black=0, white = 100 • Brightness of “ideal white” (=perfect reflector) • Hue angle (from the chromatic channels)

  8. Nayatani Color Appearance Model • Model outputs • Hue quadrature: interpolation between 4 hues defined by chromatic channels red (20.14), yellow (90 .00), green (164.25), blue (231.00) • Saturation: depends on hue and luminance (predicts changes of chromaticity with luminance) • Chroma = saturation*lightness • Colorfullness: Chroma*brightness of ideal white.

  9. Nayatani Color Appearance Model Advantages • Invertible for many outputs, i.e. measure output quantities, predict inputs • Accounts for changes in color appearance with chromatic adaptation and luminance

  10. Nayatani Color Appearance Model Weaknesses • Doesn’t predict: • Effects of changes in background color or relative luminance • incomplete chromatic adaptation • cognitive discounting the illuminant • appearance of complex patches or background • mesopic color vision

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