1Color Vision 1ColorFrédo Durand and Seth TellerMIT- EECSColorSome slides courtesy of Victor Ostromoukhov and Leonard McMillanColor Vision 3Puzzles• How comes a continuous spectrum ends into a 3D color space• Why is violet “close” to red• Primaries: 3 or 4? Which ones– Red, blue, yellow, green– Cyan and magenta are not “spontaneous” primaries• Color mixing• What is the color of Henry IV’s white horse?Color Vision 4What is Color?ReflectanceSpectrumSpectralPowerDistributionSpectralPowerDistributionIlluminant D65Electromagnetic Wave(nm)Color Vision 5What is Color?ReflectanceSpectrumSpectralPowerDistributionUnder F1SpectralPowerDistributionIlluminant F1SpectralPowerDistributionUnder D65Neon LampColor Vision 6What is Color?StimulusObserverObserver2Color Vision 7What is Color?SpectralSensibilityof the L, M and SConesSM LRodsRodsConesConesDistribution of Distribution of Cones and RodsCones and RodsLightLightRetinaOptic NerveAmacrineCellsGanglionCellsHorizontalCellsBipolarCellsRodConeColor Vision 8What is Color?VisualVisualCortexCortexRight LGNRight LGNLeft LGNLeft LGNLGN = Lateral Geniculate NucleusColor Vision 9Plan• Color Vision• Color spaces• Producing color• Color effectsColor Vision 10Cone spectral sensitivity• Short, Medium and Long wavelengthwavelength0.751.000.500.250.00400 500 600 700S M LColor Vision 11Cones do not “see” colorswavelength0.751.000.500.250.00400 500 600 700MColor Vision 12Cones do not “see” colors• Different wavelength, different intensity• Same responsewavelength0.751.000.500.250.00400 500 600 700M3Color Vision 13Response comparison• Different wavelength, different intensity • But different response for different coneswavelength0.751.000.500.250.00400 500 600 700S M LColor Vision 14von Helmholtz 1859: Trichromatic theoryVioletBlueGreenYellowOrangeRedShort wavelength receptorsMedium wavelength receptorsLong wavelength receptorsReceptor ResponsesWavelengths (nm)400 500 600 700VioletBlueGreenYellowOrangeRedColor Vision 15Cones distribution• LMS 40:20:1• No S (blue) in retina centerColor Vision 16Metamers• Different spectrum• Same responseColor Vision 17Color matching• Reproduce the color of a test lampwith the addition of 3 primary lightsColor Vision 18Metamerism & light source• Metamersunder a given light source• May not be metamersunder a different lamp4Color Vision 19Color blindness• Dalton • 8% male, 0.6% female• Genetic• Dichromate (2% male)– One type of cone missing– L (protanope), M (deuteranope), S (tritanope)• Anomalous trichromat– Shifted sensitivityColor Vision 20We are all color blind• Center of retina• No S (blue) • We compensatevia gaze movement• Not well understoodColor Vision 21Questions?Meryon (a colorblind painter), Le Vaisseau FantômeColor Vision 22Hering 1874: Opponent Colors+0-+0-+0-Red/GreenReceptorsBlue/YellowReceptorsBlack/WhiteReceptors• Hypothesis of 3 types of receptors: Red/Green, Blue/Yellow, Black/White• Explains well several visual phenomena• Hypothesis of 3 types of receptors: Red/Green, Blue/Yellow, Black/White• Explains well several visual phenomenaColor Vision 23Dual Process Theory• The input is LMS• The output has a different parameterization:– Light-dark– Blue-yellow– Red-greenTrichromaticStageOpponent-ProcessStageSLMGYWhBkBRColor Vision 24Color opponents wiring• Sums for brightness• Differences for color opponentsB+Y-W+B-R +G-MLS M LS-M-L S+M+L L-M++++++--Y+B-B+W-G+R-MLS M L-S+M+L -S-M-L M-L++++----5Color Vision 25Simultaneous contrast• In color opponent direction• Center-surroundColor Vision 26Land RetinexColor Vision 27Simultaneous Color ContrastColor Vision 28After-ImageColor Vision 29After-Image-whiteColor Vision 30Opponent ColorsImageImageAfterimageAfterimage6Color Vision 31Opponents and image compression• JPG, MPG• Color opponents instead of RGB• Compress color more than luminanceColor Vision 32Color reparameterization• The input is LMS• The output has a different parameterization:– Light-dark– Blue-yellow– Red-green• A later stage may reparameterize:– Brightness or Luminance or Value– Hue– SaturationSLMGYWhBkBRSL (or B)HColor Vision 33Hue Saturation ValueColor Vision 34Hue Saturation Value• One interpretation in spectrum space• Not the only onebecause of metamerism• Dominant wavelength (hue)• Intensity• Purity (saturation)Color Vision 35Color categories• Prototypes• Harder to classify colors at boundariesColor Vision 36Questions?Van Gogh Jawlensky7Color Vision 37Plan• Color Vision• Color spaces• Producing color• Color effectsColor Vision 38Color response linear subspace• Project the infinite-D spectrum onto a subspace defined by 3 basis functions• We can use 3x3 matrices to change the colorspace– E.g. LMS to RGB– E.g. RGB to CIE XYZColor Vision 39Color response and RGB or LMS• Project the infinite-D spectrum onto a subspace defined by 3 basis functions• Small problem: this basis is NOT orthogonal• What does orthogonal mean in our case?• Second problem: the orthogonal basis is NOT physically realizableColor Vision 40Color response and RGB or LMS• Project the infinite-D spectrum onto a subspace defined by 3 basis functions• Small problem: this basis is NOT orthogonal• What does orthogonal mean in our case?• Second problem: the orthogonal basis is NOT physically realizableColor Vision 41Color Matching Problem• Some colors can not be produced using only positively weighted primaries– Negative values are a problem– (e.g. for measurement devices, requires 6 channels instead of 3)• Solution 1: add light on the other side!Color Vision 42Color Matching Problem• Some colors can not be produced using only positively weighted primaries– Negative values are a problem– (e.g. for measurement device, requires 6 channels instead of 3)• Solution 2: standardize!• In 1931, the CIE (Commission Internationale de L’Eclairage) defined three new primaries• Called X, Y , Z,– with positive color matching functions8Color Vision 43CIE color space• Can think of X, Y , Z as coordinates• Odd-shaped cone contains visible colors– Note that many points in XYZ do not correspond to visible colors!Color Vision 44CIE color space• Objective, quantitative color descriptions– Dominant wavelength:• Wavelength “seen” (corresponds to Hue)– Excitation purity:• Saturation, expressed objectively– Luminance:• Intensity• Chromaticity
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