CSCE441: Computer Graphics: Color Models Jinxiang ChaiOutlineHuman VisionSlide 4Electromagnetic SpectrumVisible LightSlide 7Slide 8Spectral Energy DistributionSlide 10Slide 11Slide 12Color Representation?Slide 14Light Detection: Rods and ConesTristimulus of Color TheorySlide 17Tristimulus Color TheoryAdditive and Subtractive ColorRGB Color SpaceSlide 21Slide 22Slide 23TristimulusThe CIE XYZ systemColor Matching FunctionsXYZ spaceSlide 28Luminance and ChromaticityChromaticity DiagramSlide 31White PointSpectral ColorsSlide 34Saturation/PuritySlide 36HueSlide 38Slide 39Slide 40Slide 41Slide 42Slide 43Non-Spectral ColorsCombining Two ColorsComplementary ColorsSlide 47Combining Three ColorsGamutDiffering GamutsMonitor/Print/Scanner GamutDevice GamutsGamutsColor ModelsRGBCMYCMYKYIQ / YUVHSV Color ModelRepresenting ColorSlide 61Image RepresentationDisplays – PixelsSlide 64Slide 651CSCE441: Computer Graphics:Color Models Jinxiang Chai2OutlineColor ModelsImage representationRequired readings: HB 12-1 to 12-83Human VisionModel of human vision4Human VisionModel of human visionVision components:• Incoming light• Human eye5Electromagnetic SpectrumVisible light frequencies range between:Red: 3.8x1014 hertz (780nm)Violet: 7.9x1014 hertz (380nm)6Visible LightThe human eye can see “visible” light in the frequency between 380nm-780nmred violet7Visible LightThe human eye can see “visible” light in the frequency between 380nm-780nm380nm 780nm8Visible LightThe human eye can see “visible” light in the frequency between 380nm-780nm380nm 780nm- Each frequency value between 380nm-780nm corresponds to a distinct spectral color - Not strict boundary- Some colors are absent (brown, pink)9Spectral Energy DistributionThree different types of lights10Spectral Energy DistributionThree different types of lightsCan we use spectral energy distribution to represent color?11Spectral Energy DistributionThree different types of lightsCan we use spectral energy distribution to represent color?- Not really, different distribution might result in the same color (metamers)!12Spectral Energy DistributionThe six spectra below look the same purple to normal color-vision people13Color Representation?Why not all ranges of light spectrum are perceived?So how to represent color?380nm 780nm14Human VisionPhotoreceptor cells in the retina: - Rods - Cones15Light Detection: Rods and ConesRods: -120 million rods in retina -1000X more light sensitive than Cones - Discriminate B/W brightness in low illumination - Short wave-length sensitiveCons: - 6-7 million Cones in the retina - Responsible for high-resolution vision - Discriminate Colors - Three types of color sensors (64% red, 32%, 2% blue) - Sensitive to any combination of three colors16Tristimulus of Color TheorySpectral-response functions of each of the three types of cones17Tristimulus of Color TheorySpectral-response functions of each of the three types of conesColor matching function based on RGB - any spectral color can be represented as a linear combination of these primary colors18Tristimulus Color TheorySo, color is psychological- Representing color as a linear combination of red, green, and blue is related to cones, not physics- Most people have the same cones, but there are some people who don’t – the sky might not look blue to them (although they will call it “blue” nonetheless)- But many people (mostly men) are colorblind, missing 1,2 or 3 cones (can buy cheaper TVs)19Additive and Subtractive ColorRGB color modelCMY color modelComplementary color models: R=1-C; G = 1-M; B=1-Y;White: [1 1 1]TGreen: [0 1 0];White: [0 0 0]TGreen: [1 0 1];20RGB Color Spaceredgreenblue21RGB Color SpaceredgreenblueWhite (1,1,1)22RGB Color Spaceredgreenbluemagenta (1,0,1)23RGB Color SpaceRGB cubeEasy for devicesCan represent all the colors?But not perceptualWhere is brightness, hue and saturation?redgreenbluemagenta (1,0,1)24TristimulusSince 3 different cones, the space of colors is 3-dimensional.We need a way to describe color within this 3 dimensional space.We want something that will let us describe any visible color with additive combination…25The CIE XYZ systemCIE – Comission Internationale de l’Eclairage- International Commission on Illumination- Sets international standards related to lightDefined the XYZ color system as an international standard in 1931X, Y, and Z are three Primary colors. - imaginary colors - all visible colors can be defined as an additive combination of these three colors. - defines the 3 dimensional color space26Color Matching FunctionsGiven an input spectrum, , we want to find the X, Y, Z coordinates for that color.Color matching functions, , , and tell how to weight the spectrum when integrating:dzpZdypYdxpX)()()()()()()(x )(y)(z)(pImage taken from http://upload.wikimedia.org/wikipedia/commons/8/87/CIE1931_XYZCMF.png27XYZ spaceAny color can be represented in the XYZ space as an additive combination of three primary colorsZYXZCYCXCC )(28XYZ spaceThe visible colors form a “cone” in XYZ space.For visible colors, X, Y, Z are all positive.But, X, Y, and Z themselves are n ot visible colors!Image taken from http://fourier.eng.hmc.edu/e180/handouts/color1/node27.html29Luminance and ChromaticityThe intensity (luminance) is just X+Y+Z.Scaling X, Y, Z just increases intensity.We can separate this from the remaining part, chromaticity.Color = Luminance + ChromaticityChromaticity is 2D, Luminance is 1DTo help us understand chromaticity, we’ll fix intensity to the X+Y+Z=1 plane.30Chromaticity DiagramProject the X+Y+Z=1 slice along the Z-axisChromaticity is given by the x, y coordinatesImage taken from http://fourier.eng.hmc.edu/e180/handouts/color1/node27.html31Chromaticity DiagramDetermining purity and dominant wave length for a given colorIdentify complementary colorsCompare color gamuts for different primariesImage taken from http://fourier.eng.hmc.edu/e180/handouts/color1/node27.html32White PointWhite: at the center of the diagram.Image taken from http://fourier.eng.hmc.edu/e180/handouts/color1/node27.html33Spectral ColorsVisible Spectrum along outside curveImage taken from http://fourier.eng.hmc.edu/e180/handouts/color1/node27.html34Spectral
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