Perceptual Color Spaces John C. Hart CS 418 Interactive Computer Graphics
Selecting Colors H S V HSV = Hue, Saturation, Value • 1978, Alvy Ray Smith • Hue [0,360] is angle about color wheel 0 ° = red, 60 ° = yellow, 120 ° = green, ∆ = maxRGB – minRGB 180 ° = cyan, 240 ° = blue, 300 ° = magenta maxRGB = R H = (G – B)/ ∆ • Saturation [0,1] is distance from gray maxRGB = G H = 2+(B – R)/ ∆ maxRGB = B H =4+ (R – G)/ ∆ S = (maxRGB – minRGB)/maxRGB H = (60*H) mod 360 • Value [0,1] is distance from black V = maxRGB HLS = Hue, Saturation, Lightness • Double cone, saturation in middle
CIE XYZ • CIE: International Commission on 0.9 520 Illumination 0.8 540 • 3-D space defined by three color- 0.7 matching functions 560 – X ≅ R, Y ≅ G, Z ≅ B 0.6 500 580 0.5 – Y indicates brightness y 0.4 600 • Projected to 2-D using 620 x = X/(X + Y + Z) 0.3 490 700 y = Y/(X + Y + Z) 0.2 480 • Perceptually designed 0.1 470 460 • L,a*,b* space measures perceptual 380 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 x distance between colors
Gamut • Portion of the spectrum 0.9 520 reproduced by a given 0.8 540 color space 0.7 • TV’s (even HDTV’s) 560 can only display a small 0.6 500 portion of perceivable 580 0.5 y colors 0.4 600 • Printers can display a 620 0.3 490 700 slightly different portion 0.2 of colors 480 0.1 470 460 0.0 380 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 x
Gamma
Gamma • We perceive differences in intensity more carefully for darker shades • Monitors accommodate this feature I = cV γ • Gamma usually between 2 and 2.5 • Need to correct pixel values so they display correct intensity log I = log c + γ log V
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