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ColorSusan Campbell and Sandro FouchéMotivating ExampleMotivating ExampleColor Vision•trichromatic receptors•blue•green•red•combine into opponent channels•blue-yellow•red-greenresponses of the standard observer. Hence no two peo-ple see a given color stimulus in exactly the same way.V(!), the spectral luminous efficiency function of CIE(the International Commission on Lighting), describes therelative sensitivity of the eye to monochromatic colorsthroughout the visible spectrum (see Figure 2). V(!) peaksat a wavelength of 555 nm, corresponding to a greenish-yellow hue. For computer graphics, Table 1 lists the approx-imate ordering of the relative luminance of the primaryand secondary colors (rounded to the nearest 10 percent).This is not the same order as the spectral hue sequence.Light is refracted as it passes though the cornea andlens of the eye, which disperse the spectral componentsin the same way that a prism does. Normally the eyefocuses to bring into sharpest focus yellow wavelengthsof 578 nm.3The longer red wavelengths converge beyond the reti-nal surface, whereas the shorter green and blue wave-lengths converge in front of the retina. To focus on redcolors, the lens must become more convex, as if for anobject nearer to the observer. Conversely, to focus onblues, the lens must become less convex (more relaxed),as if for an object farther away. These factors contributeto the apparent color-depth effect called chromostere-opsis: for the majority of observers, red appears toadvance while blue appears to recede.4To preventunwanted depth effects on a display, avoid juxtaposingstrong red and strong blue.Suppression of the eye’s scanning movements can fix-ate a strongly colored image on the retina. This tem-porarily reduces the sensitivity of the photoreceptors,leading to afterimages. Try this by fixing your gaze onthe black dot at the center of Figure 3 for 10 seconds ormore and then looking away at a sheet of white paper.You should see an afterimage with the same layout asFigure 3 but in complementary colors. In the regionswhere the image was red, you now see cyan because youhave temporarily suppressed the response from yourlong-wavelength (red) cones, and only the medium(green) and short (blue) cones are responding normal-ly to the white light. For this reason, it’s unwise to uselarge areas of bright color in a display.5The signals transmitted via the optic nerve from theeye to the brain are not the basic long-, medium- andshort-wavelength responses of the three cone types.Instead they consist of an achromatic (luminance) sig-nal, representing the sum of the cone responses, andtwo opponent chromatic signals defining the red-greenand yellow-blue color differences, as shown schemati-cally in Figure 4. These signals are formed by what is lit-erally a neural network in the retina, consisting of threeIEEE Computer Graphics and Applications 211.00.50Relative sensitivity400 500 600 700Wavelength (nm)2 Relativesensitivity of theeye to huesthroughout thespectrum.3 Strong colors can give rise to complementary after-images.Phot orecept ors+!!Neural channelsAchromatic(luminance)Red-greenYellow-blueShort Medium Long4 Interconnections of the retinal cone signals produceachromatic and opponent chromatic signals.Table 1. Relative luminance ordering ofcomputer graphics primary and secondarycolors.Relat iveluminanceColor Primaries (pe rcent )White R G B 100Yellow R G 90Cyan G B 70Green G 60Magenta R B 40Red R 30Blue B 10Black — 0Color Vision (2)•afterimagesColor Vision (2)•afterimagesColor Vision (3)•achromatic perception of contrast •opponent colors more difficult to detectColor Vision (4)•color deficiency and color blindness•sex-linked: red/green•recessive: blue/yellowColor Perception•colors depend on context•chromatic induction•inferential proceses•memory affects perception•lightness, hue, colorfulnessPerception IllusionCIE ModelDisplay Technology•Subtractive Color•RGB - Three light-emitting elements•Gamma - intensity of emissions per channel (γ = 1.6 - 2.5; 2.2 is somewhat standard)•Gamut - range of color possible on a displayDisplay GamutPrimary ColorsGreenRedBlueSecondary ColorsYellowCyanMagentaTertiary ColorsColor GroupingsComplementary DyadPrimary TriadSplit TriadComplentary PairsColor Models•RGB•most staightforward,•perceptually non-uniform•device dependent luminance / hue color biasHSV model•Hue : chroma values•Saturation : scale to white•Value / Brightness : scale to blackLab* color•L : lightness•a : chroma green to red•b : chroma (blue to yellow)Design Principles•Association : visual similarity•Differentiation : visual distinction•Harmony / DiscordColor is one of the most effective visual attributes for coding information in displays and is capable, when used correctly, of achieving powerful and memorable effects.Design Principles•Association : visual similarity•Differentiation : visual distinction•Harmony / DiscordDesign Principles•Association : visual similarity•Differentiation : visual distinction•Harmony / DiscordDesign Principles•Association : visual similarity•Differentiation : visual distinction•Harmony / DiscordText•luminance contrast•minimum of 3:1•ideal of 10:1 (blue on white)•color text or background, but not bothTextTextTextTextTextTextInformation•nominal variables: discriminability and memorability imply 5-7 colors maximum•ordinal variables: graded sequence of colors•include color key alongside figure•see ColorBrewer exampleVisualization•Don’t use color that doesn’t support or add to the meaning of the information displayed. •Use color to indicate changes of state. •Use color saturation to depict depth layering and priority of object categories. •In scientific visualization, use only enough color to create a realistic effect.Imaging•The most important criterion is to preserve the intended appearance of the image.•Always use a neutral gray background when displaying color images. •Put a narrow white border around an image to stabilize its color appearance. •Use color management software when accurate color reproduction is required.Design Guidelines•function•Treat color design as part of a user-centered design process. •Use color for association and differentiation of a design’s elements. •aesthetics•Choose a harmonious palette of colors for use throughout an application. •Unify each design by using common


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