1Color Vision 1ColorColor Vision 2Review of last weekColor Vision 3Review of color• Spectrum• Cone sensitivity function• Metamers– same color, different spectrum• Opponent – black-white, blue-yellow, red-green• Color spaces– Linear algebra– Problem with negative values– Standard: CIE XYZ• Perceptually uniform color space– CIE Lab (non-linear wrt XYZ)SpectralPowerDistributionUnder F1Color Vision 4CIE-Lab (a.k.a. CIE L*a*b*)Source: [Wyszecki and Stiles ’82]Color Vision 5Perceptually Uniform Space: MacAdam• In color space CIE-XYZ, the perceived distance between colors is not equal everywhere• In perceptually uniform color space, Euclidean distances reflect perceived differences between colors• MacAdam ellipses (areas of unperceivable differences) become circles Source: [Wyszecki and Stiles ’82]Color Vision 6Hue Saturation Value• Value: from black to white• Hue: dominant color (red, orange, etc)• Saturation: from gray to vivid color• HSV double conevaluesaturationsaturationhue2Color Vision 7Hue Saturation Value• One interpretation in spectrum space• Not the only onebecause of metamerism• Dominant wavelength (hue)• Intensity• Purity (saturation)Color Vision 8CIE color space• Match color at some point A• A is mix of white C, spectral B!• What is dominant wavelength of A?• What is excitation purity (%) of A?– Move along AC/BCCColor Vision 9Plan• Color Vision• Color spaces• Color effects– Definitions– Spatial sensitivity– Color illusion and color appearance• Producing colorColor Vision 10Color terms (Fairchild 1998• Color• Hue• Brightness vs. lightness• Colorfulness and Chroma• Saturation• Unrelated and related colorsColor Vision 11Color– chromatic and achromatic content. This attribute can be described by chromatic color names such as yellow, orange, brown, red, pink, green, blue, purple, etc., or by achromatic color names such as white, gray, black, etc., and qualified by bright, dim, light, dark, etc., or by combinations of such names.– Note: Perceived color depends on the spectral distribution of the color stimulus, on the size, shape, structure, and surround of the stimulus area, on the state of adaptation of the observer's visual system, and on the observer's experience of the prevailing and similar situations of observations.Color Vision 12Related and Unrelated Colors• Unrelated Color– Color perceived to belong to an area or object seen in isolation from other colors.• Related Color – Color perceived to belong to an area or object seen in relation to other colors.3Color Vision 13Hue• Hue– Attribute of a visual sensation according to which an area appears be similar to one of the perceived colors: red, yellow, green, and blue, or to a combination of two of them.• Achromatic Color– Perceived color devoid of hue. • Chromatic Color– Perceived color possessing a hue.Color Vision 14Brightness vs. Lightness• Brightness– Attribute of a visual sensation according to which an area appears to emit more or less light.• Lightness: – The brightness of an area judged relative to the brightness of a similarly illuminated area that appears to be white or highly transmitting.Color Vision 15Colorfulness & Chroma• Colorfulness– Attribute of a visual sensation according to which the perceived color of an area appears to be more or less chromatic.• Chroma: – Colorfulness of an area judged as a proportion of the brightness of a similarly illuminated area that appears white or highly transmitting.Color Vision 16Saturation– Colorfulness of an area judged in proportion to its brightness.Color Vision 17Plan• Color Vision• Color spaces• Color effects– Definitions– Spatial sensitivity– Color illusion and color appearance• Producing colorColor Vision 18Cornsweet illusion• Two opposite gradients• We judge only the contrast at the edge4Color Vision 19Contrast processing• Receptors are wired to other neurons• Center-surround organization• Sensitive mostly to local contrastColor Vision 20Land Retinex and local contrastColor Vision 21Mach Bands• Contrast is enhanced atregion boundariesabcdeColor Vision 22Hermann Grid----------------+++++-----------------------------------------+++++-------------------------------Color Vision 23Hermann GridColor Vision 24Brightness vs. lightness• Brightness: subjective amount of light• Lightness: how “white”The white cells in shadow are as darkas the black illuminated cells5Color Vision 25Brightness vs. lightness• Brightness: subjective amount of light• Lightness: how “white”The white cells in shadow are as darkas the black illuminated cells Color Vision 26Brightness vs. lightness• Brightness: subjective amount of light• Lightness: how “white”The white cells in shadow are as darkas the black illuminated cells Color Vision 27 Color Vision 28Lightness and transparencyColor Vision 29Opponents and image compression• JPG, MPG• Color opponents instead of RGB• Compress color more than luminanceColor Vision 30Contrast Sensitivity• Sine Wave grating• What contrast is necessary to make the grating visible?6Color Vision 31Contrast Sensitivity Function (CSF)Color Vision 32Contrast Sensitivity Function (CSF)Color Vision 33JPEG Compression• Perform DCT to work in frequency space– Local DCT, 8x8 blocks• Use CSF for quantization (more bits for sensitivity with more contrast)• Other usual coding tricksColor Vision 34Question?Color Vision 35Plan• Color Vision• Color spaces• Color effects– Definitions– Spatial sensitivity– Color illusion and color appearance• Producing colorColor Vision 36Color constancy• Chromaticity of light sources vary• Chromatic adaptation– Similar to white balance on camera– Different films, filtersObjective colorsunder neon lightingWith chromaticadaptation7Color Vision 37Chromatic adaptation• Von Kries adaptation• Different gain control on L, M, S• Similar to white balance on camera0.75, 1, 1Gain control:*1.33, *1, *10.75, 1, 10.15, 1, 0.20.2, 1, 0.20.2, 1, 0.2Color Vision 38Crispening• Increased sensitivityColor Vision 39Stevens effect• Stevens effect– Perceived contrastincreases with luminanceColor Vision 40Hunt and Stevens effect• Stevens effect– Perceived contrast increases with luminance• Bartleson-Breneman effect– Image contrast changes with surround– A dark surround decreases contrast(make the black of the image look less deep)–
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