CS-184: Computer GraphicsLecture #2: Color Prof. James O’BrienUniversity of California, BerkeleyV2006-F-03-1.02TodayColor and Light3What is Light?Radiation in a particular frequency range4Spectral ColorsLight at a single frequencyBright and distinct in appearanceReproduction only, not a real spectral color!4Spectral ColorsLight at a single frequencyBright and distinct in appearanceR o y G. B i vReproduction only, not a real spectral color!5Other ColorsMost colors seen are a mix light of several frequenciesImage from David Forsyth6Most colors seen are a mix light of several frequenciesOther ColorsImage from David Forsyth7Most colors seen are a mix light of several frequenciesOther ColorsImage from David Forsyth8Perception -vs- MeasurementYou do not “see” the spectrum of lightEyes make limited measurementsEyes physically adapt to circumstanceYou brain adapts in various ways alsoWeird psychological stuff happens9Everything is Relative10Everything is Relative11Adapt12Adapt13It’s all in your mind...14Mach Bands14Mach Bands15Everything’s Still Relative15Everything’s Still Relative16Eyes as SensorsThe human eye contains cells that sense lightRodsNo color (sort of)Spread over the retinaMore sensitiveConesThree types of conesEach sensitive to different frequency distributionConcentrated in fovea (center of the retina)Less sensitiveImage from Stephen Chenney17ConesEach type of cone responds to different range of frequencies/wavelengthsLong, medium, shortRatio: L10/M40/S1Also called by colorRed, green, blueMisleading:“Red” does not mean your red cones are firing...Image from David ForsythNote: Rod response peaks between S&M18ConesResponse of a cone is given by a convolution integral :Images from David Forsythr(L, S) =ZL(λ) · S(λ)dλ19ConesYou can see that “red” and “green” respond to more more than just red and green...Images from David Forsyth2021Cones (repeat)Response of a cone is given by a convolution integral :Images from David Forsythr(L, S) =ZL(λ) · S(λ)dλ22Cones (repeat)Response of a cone is given by a convolution integral :Different light inputs (L) may produce the same response (r) in all three conesMetamers: different “colors” that look the sameCan be quite useful... Odd interactions between illumination and surfaces can be odd...r(L, S) =ZL(λ) · S(λ)dλ23TrichromaticityEye records color by 3 measurementsWe can “fool” it with combination of 3 signalsConsequence: monitors, printers, etc...PS: The cone responses are linear24Additive ColorShow color on leftMix “primaries” on right until they matchThe primaries need not be RGB25Color Matching FunctionsFor primaries at 645.2, 526.3, and 444.4 nmNote negative region...26Additive MixingGiven three colors we agree onMake generic color withNegative not realizableColor now described by If we match onExample: computer monitor [RGB], paintα, β, γABCM αA βB γC27M = W − (αA + βB + γC)Subtractive MixingGiven three colors we agree onMake generic color withMax limited by Color now described by If we match onExample: ink [CMYK]α, β, γABCWWhy 4th ink for black?28CIE XYZImaginary set of color basesMatch across spectrum with positive valuesX, Y, ZNormalized:x = X / ( X+Y+Z )y = Y / ( X+Y+Z )29CIE Color Horseshoe Thinggy30GamutsConstraints on additive/ subtractive mixing limit the range of color a given device can realize.Devices may differ.Matching between devices can be difficult.31Dynamic RangeMax/min values also limited on devices“blackest black” “brightest white”Jack Tumblin32Tone Mapping“Day for night”(not the best example, done in Photoshop)33Color SpacesRGB color cube34Color SpacesRGB color cubeHSV color cone35Color SpacesRGB color cubeHSV color coneCIEMacAdam Ellipses (10x)Colors in ellipses indistinguishable from center.36Color SpacesRGB color cubeHSV color coneCIE (x,y)CIE (u,v)Scaled to be closer to circles.++=′′YXZYXvu943151x,yu,v37Color SpacesRGB color cubeHSV color coneCIE (x,y)CIE (u,v)CMYKMany others...38Color PhenomenaLight sources seldom shine directly in eyeLight follows some transport path, i.e.:SourceAirObject surfaceAirEyeColor effected by interactions39ReflectionLight strikes objectSome frequencies reflectSome adsorbedReflected spectrum is light times surfaceRecall metamers...Unknown?40TransmissionLight strikes objectSome frequencies passSome adsorbed (or reflected)Unknown?41ScatteringInteractions with small particles in mediumLong wavelengths ignoreShort ones scatterUnknown?42InterferenceWave behavior of lightCancelationReinforcementWavelength dependentUnknown?43IridescenceInteraction of light withSmall structuresThin transparent surfacesUnknown?44Iridescence45Iridescence46Fluorescence / PhosphorescencePhoton come in, knocks up electronElectron drops and emits photon at other frequencyMay be some latencyRadio active decay can also emit visible photons47Fluorescence / Phosphorescence48Black Body RadiationHot objects radiate energyFrequency is temperature dependentModerately hot objects get into visible rangeSpectral distribution is given byLeads to notion of “color temperature”€ Eλ( )∝1λ5 1exp hc kλT( )−1 49Black Body
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