CS-184: Computer GraphicsLecture #2: Color Prof. James O’BrienUniversity of California, BerkeleyV2011-F-02-1.0Slides revised using additional materials from Maneesh Agrawala2Announcments•Account sheets available after class•Sign up for Google Group•Assignment 1: due Friday, Sept 2•Assignment 2: due Tuesday, Sept 6•New section: Wed 4:00-5:00 pm in 405 Soda•Waitlist...3Today•Color, Light, and Perceptions•The basics4What is Light?•Radiation in a particular frequency range5Spectral Colors•Light at a single frequency•Also called monochromatic (an overloaded term)•Bright and distinct in appearanceR o y G. B i vReproduction only, not a real spectral color!6Other Colors•Most colors seen are a mix light of several frequenciesImage from David ForsythCurves describe spectral composition of stimulus()7•Most colors seen are a mix light of several frequenciesOther ColorsImage from David Forsyth8•Most colors seen are a mix light of several frequenciesOther ColorsImage from David Forsyth9Perception -vs- Measurement•You do not “see” the spectrum of light•Eyes make limited measurements•Eyes physically adapt to circumstance•You brain adapts in various ways also•Weird psychological/psychophysical stuff also happens10Everything is Relative11Everything is Relative12Adapt13Adapt14It’s all in your mind...15Mach Bands16Everything’s Still RelativeBezold Effect1718PerceptionThe eye does not see intensity values...The eye does not see intensity values...19PerceptionThe eye does not see intensity values...20Perception21Eyes as Sensors•The human eye contains cells that sense light•Rods•No color (sort of)•Spread over the retina•More sensitive•Cones•Three types of cones•Each sensitive to different frequency distribution•Concentrated in fovea (center of the retina)•Less sensitiveImage from Stephen Chenney22Cones•Each type of cone responds to different range of frequencies/wavelengths•Long, medium, short•Also called by color•Red, green, blue•Misleading:“Red” does not mean your red cones are firing...Normalized sensitivity curves23Cones•You can see that “red” and “green” respond to more more than just red and green...Images from David ForsythRods vs Cones24350 400 450 500 550 600 650 700 750 800 0 200 400 600 800 1000 1200 1400 1600 1800 Wavelength (nm) Luminous efficacy (lumens/watt) Scotopic (rod - dark adjusted) Photopic (cones - bright light)Eyes as Sensors25Monochromatic scotopic vision (low light levels) Chromatic photopic vision (high light levels) 26Cones•Response of a cone is given by a convolution integral :L =Z()L()dM =Z()M ()dS =Z()S()dcontinuous version of a dot product2728TrichromaticityEye records color by 3 measurementsWe can “fool” it with combination of 3 signalsSo display devices (monitors, printers, etc.) can generate perceivable colors as mix of 3 primariesCone Responses are LinearResponse to stimulus isResponse to stimulus is Then response to is Response to is 291(L1,M1,S1)(L2,M2,S2)21+ 2(L1+ L2,M1+ M2,S1+ S2)n1(nL1,nM2,nS1)30Cones and MetamersCone response is an integralMetamers: Different light input produce same cone response•Different spectra look the same•Useful for measuring colorL =Z()L()dM =Z()M ()dS =Z()S()d1(), 2()L, M, S31Additive MixingGiven three primaries we agree onMatch generic input light withNegative not realizable, but can add primary to test lightColor now described by Example: computer monitor [RGB]α, β, γp1,p2,p3 = p1+ ⇥p2+ ⇤p332Additive Color MatchingShow test light spectrum on leftMix “primaries” on right until they matchThe primaries need not be RGBExperiment 133Slide from Durand and Freeman 06Experiment 134p1 p2 p3 Slide from Durand and Freeman 06Experiment 135p1 p2 p3 Slide from Durand and Freeman 06Experiment 136p1 p2 p3 The primary color amounts needed for a match p1 p2 p3 The primary color amounts needed for a match Slide from Durand and Freeman 06Experiment 237Slide from Durand and Freeman 06Experiment 238p1 p2 p3 Slide from Durand and Freeman 06Experiment 239p1 p2 p3 Slide from Durand and Freeman 06Experiment 240p1 p2 p3 p1 p2 p3 We say a “negative” amount of p2 was needed to make the match, because we added it to the test color’s side. The primary color amounts needed for a match: p1 p2 p3 Slide from Durand and Freeman 0641Color Matching Functions¯r()¯g()¯b()Input wavelengths are CIE 1931 monochromatic primaries =0B@(1)...(N)1CAUsing Color Matching FunctionsFor a monochromatic light of wavelength we know the amount of each primarynecessary to match it:Given a new light input signal Compute the primaries necessary to match it42i¯r(i), ¯g( i),¯b(i)Using Color Matching FunctionsGiven color matching functions in matrix form and new lightamount of each primary necessary to match is given by 43C =0@¯r(1) ... ¯r(N)¯g(1) ... ¯g(N)¯b(1) ...¯b(N)1AC =0B@(1)...(N)1CA¯r()¯g()¯b()44CIE XYZImaginary set of color primaries with positive values, X, Y, Z45Rescaled XYZ to xyzRescale X, Y, and Z to remove luminance, leaving chromaticity:Because the sum of the chromaticity values x, y, and z is always 1.0, a plot of any two of them loses no informationSuch a plot is a chromaticity diagramx = X / ( X+Y+Z )y = Y / ( X+Y+Z )z = Z / ( X+Y+Z )x+y+z = 1CIE Chromaticity Diagram46Intended property: white point at (1/3, 1/3, 1/3) Intended property: non-negative Intended property: fitted to edge of right triangleCIE Chromaticity Diagram47Pure (saturated) spectral colors around the edge of the plot Less pure (desaturated) colors in the interior of the plot White at the centroid of the plot (1/3, 1/3) Are the colors correct ? GamutGamut is the chromaticities generated by a set of primariesBecause everything we’ve done is linear, interpolation between chromaticities on a chromaticity plot is also linearThus the gamut is the convex hull of the primary chromaticitiesWhat is the gamut of the CIE 1931 primaries?48CIE 1931 RGB Gamut49R = 700 nm G = 546 nm B = 438 nm Other Gamuts (LCDs and NTSC)50Given three primaries we agree onMake generic color withMax limited by Color now described by
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