Color Vision 1 ColorColor Vision 2 Review of last weekColor Vision 3 Review 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 wrtXYZ)Color Vision 4 CIE-Lab (a.k.a. CIE L*a*b*) Image adapted from Wyszecki, G., and W. S. Stiles,Color science: Concepts and methods, quantitative data and formulae. Wiley-Interscience; 2nd Edition. July 2000. ISBN: 0471399183Color Vision 5 Perceptually 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•MacAdamellipses (areas of unperceivable differences) become circles 560 550 540 530 520 510 490 480 470 460 450 .05 x y .10 .15 .20 .45 .50 .55 .60 .65.25 .25 .20.15 .10 .05 .35 .40 .45 Spectrum locus.50 .55 .60 .65 .70 .30 400 nm 700 nm 650 620 610 600 590 580 570 Purple line Wyszecki, G. and W. S. Stiles. Color science: Concepts and Methods, Quantitative Data and Formulae. Wiley-Interscience, 2nd ed. July 2000. ISBN: 0471399183. Image adapted from:Color Vision 6 Hue Saturation Value•Value: from black to white•Hue: dominant color (red, orange, etc)•Saturation: from gray to vivid color•HSV double conevaluesaturationsaturationhueColor Vision 7 Hue Saturation Value • One interpretation in spectrum space • Not the only one because of metamerism • Dominant wavelength (hue) • Intensity • Purity (saturation)Color Vision 8 CIE 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/BC C 0.1 0.1 0.2 400 480 490 500 510 520 540 560 B A D E C F G 580 600 700 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.6 0.7 x y 0.7 0.8 Hunt, R. W. G. The Reproduction of Colour. John Wiley & Sons Incorporated. September 2004. ISBN: 0-470-02425-9. Image adapted from:Color Vision 9 Plan•Color Vision•Color spaces•Color effects–Definitions–Spatial sensitivity–Color illusion and color appearance•Producing colorColor Vision 10 Color terms (Fairchild 1998•Color•Hue•Brightness vs. lightness•Colorfulness and Chroma•Saturation•Unrelated and related colorsColor Vision 11 Color–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 12 Related 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.Color Vision 13 Hue•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 14 Brightness 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 15 Colorfulness & 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 16 Saturation–Colorfulness of an area judged in proportion to its brightness.Color Vision 17 Plan•Color Vision•Color spaces•Color effects–Definitions–Spatial sensitivity–Color illusion and color appearance•Producing colorColor Vision 18 Cornsweetillusion•Two opposite gradients•We judge only the contrast at the edgeColor Vision 19 Contrast processing•Receptors are wired to other neurons•Center-surround organization•Sensitive mostly to local contrast + ++ + + ++++ + + + + +++++ ------------------Color Vision 20 Land Retinexand local contrastColor Vision 21 Mach Bands•Contrast is enhanced atregion boundaries a light Perceived Brightness (e.g. scale) Actual Intensity (e.g. lumina) dark light dark b c d e a b c d e e d c b aColor Vision 22 Hermann Grid -------++ + +++ + +++ -------------------------------------------------------------------------------Color Vision 23 Brightness vs. lightness•Brightness: subjective amount of light•Lightness: how “white”The white cells in shadow are as darkas the black illuminated cells (Courtesy of The Department of Brain and Cognitive Sciences. Used with permission.)Color Vision 24 Brightness vs. lightness•Brightness: subjective amount of light•Lightness: how “white”The white cells in shadow are as darkas the black illuminated cells Color Vision 25 Brightness vs. lightness•Brightness: subjective amount of light•Lightness: how “white”The white cells in shadow are as darkas the black illuminated cells Color Vision 26 Lightness and transparency (Courtesy of The Department of Brain and Cognitive Sciences. Used with permission.)Color Vision 27 Opponents and image compression•JPG, MPG•Color opponents instead of RGB•Compress color more than luminanceColor Vision 28 Contrast Sensitivity•Sine Wave grating•What contrast is necessary to make the grating visible? (Image removed due to copyright considerations.)Color Vision 29 Contrast Sensitivity Function (CSF) (Image removed due to copyright considerations.)Color Vision 30 Contrast Sensitivity Function (CSF) -1 0.01 0.11 0 Luminance Red-Green Blue-Yellow Log Spatial Frequency (cpd) Contrast Sensitivity 1 2 10 100 Spatial contrast sensitivity functions for luminance and chromatic contrast. Image adapted from: Mark D. Fairchild. Color Appearance Models. Prentice Hall PTR, November 1997. ASIN: 0201634643Color Vision 31 JPEG 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 32 Plan•Color Vision•Color spaces•Color effects–Definitions–Spatial sensitivity–Color illusion and color appearance•Producing colorColor Vision 33 Color constancy•Chromaticity of light sources vary•Chromatic adaptation–Similar to white balance on camera–Different films, filtersColor Vision 34 Chromatic 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 35 Crispening•Increased sensitivityColor Vision 36 Stevens effect•Stevens effect–Perceived contrastincreases with luminance 0.1 1.0 1.0 L (97 dB) L (79 dB) L (63 dB) L (Dark) Image adapted from: Fairchild, Mark D. Color Appearance Models. Prentice Hall PTR. November 1997. ASIN: 0201634643. Relative Brightness Relative LuminanceColor Vision 37 Hunt 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)–Important for movies•Hunt effect–Colorfulness increases with luminance•Hence the need for gamma correction (see later)Color Vision 38 Color appearance models•Predict the appearance of a color depending on–Objective stimulus–Surrounding, contextColor Vision 39 Plan•Color Vision•Color spaces•Color effects•Producing colorColor Vision 40 Color synthesisAdditiveSubtractivered, green, bluecyan, magenta, yellowAdd light (CRT, video projector)Remove light (e.g. filter) printer, photosColor Vision 41 Color synthesis: a wrongexampleAdditiveSubtractivered, green, blue cyan, magenta, yellowRIGHTWRONGColor Vision 42 Device Color Models(Subtractive)Start with white, remove energy (e.g.,w/ink, filters)Example: CMY color printerCyan ink absorbs red lightMagenta ink absorbs green lightYellow ink absorbs blue lightC+M+Y absorbs all light: Black!RGB => CMY conversionSome digital cameras use CMYColor Vision 43 CMYKCMY model plus black ink (K)Saves inkHigher-quality blackIncreases gamutConversion not completely trivialColor Vision 44 The infamous gamma curveA gamma curve x->xγis used for many reasons:•CRT response–The relation between voltage and intensity is non-linear•Color quantization–We do not want a linear color resolution–More resolution for darker tones–Because we are sensitive to intensity ratios•Perceptual effect–We perceive colors in darker environment less vivid–Hunt and Stevens effect•Contrast reduction–Keep some contrast in the highlightsColor Vision 45 Gamut•Every device with three primaries can produce only colors inside some (approx.) triangle–Convexity!•This set is called a color gamut–(Why can’t RGB can’t give all visible colors?)•Usually, nonlinearities warp the triangle–Also, gamut varies with luminance 0.1 0.1 0.2 400 480 490 500 510 520 540 o560 580600 700 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.6 0.7 x x y y 0.7 0.8 R. W. G. Hunt. The Reproduction of Colour. John Wiley & Sons, Incorporated, September 2004. ISBN: 0-470-02425-9. Image adapted from:Color Vision 46 Gamut MappingGamut Mapping•Color gamut of different processes may be different (e.g. CRT display and 4-color printing process)•Need to map one 3D color space into another•Color gamut of different processes may be different (e.g. CRT display and 4-color printing process)•Need to map one 3D color space into anotherCIE-LabPerceptually-uniform Color spaceTypical CRT gamut4-color printing gamutColor Vision 47 Gamut MappingGamut MappingTypical CRTgamut4-color CMYK printinggamuta*b*Gamut mapping is a morphing of 3D color space according to adopted schemeGamut mapping is a morphing of 3D color space according to adopted schemeColor Vision 48 ICC standard•Every device has a different color response (gamut, spectrum)•Picture file should store this color profile•ICC standard•Unfortunately not spread enoughhttp://www.color.org/Color Vision 49 Selected BibliographyVision Scienceby Stephen E. PalmerMIT Press; ISBN: 0262161834760 pages (May 7, 1999)Billmeyerand Saltzman'sPrinciples of Color Technology, 3rd Editionby Roy S. Berns, Fred W. Billmeyer, Max SaltzmanWiley-Interscience; ISBN: 047119459X304 pages 3 edition (March 31, 2000) Vision and Art : The Biology of Seeingby Margaret Livingstone, David H. HubelHarry N Abrams; ISBN: 0810904063 208 pages (May 2002) Color Vision 50 Selected BibliographyThe Reproduction of Colorby R. W. G. HuntFountain Press, 1995Color Appearance Modelsby Mark FairchildAddison Wesley, 1998