CIE tri-stimulus experiment diffuse reflecting screen diffuse reflecting screen 770 769 768 test light 382 381 380 observer test light 445 535 630 445 535 630 observer light intensity for visual color match Value Functions 535nm control light 445nm control light 630nm control light Problem: negative values 0 due to poor choice of 3 lights 445 535 630 400 500 600 700 Test light wavelength in nm CIE 1931 Standard Diagram Normalization: x = X/(X+Y+Z) y = Y/(X+Y+Z) z = Z/(X+Y+Z) x+y+z = 1 1
Standardized Table Properties Standardized internationally accepted color system! Device independent! Applicable in many different areas! specification, complementary colors, mixtures of 2 & 3 colors, equal color tone wave lengths (see book)! Y encodes luminance! CIE is a 2-D color system Complete color specification is (x,y) + Y!! Not perceptionally uniform Ellipses define regions of same perceived color Tests by MacAdam CIE Luv (UCS) Model - 1976 More uniform color steps, based on MacAdam Defined as rational transformation of CIE 1931 color space 2
CIE Lab Model Mapping into Luv No fundamental difference to Luv Models for Devices! RGB system! HLS system! CMYK system cyan The RGB Model blue black (in back) magenta green white red Grayscale on diagonal axis (view along green axis) yellow 3
HLS (HSV) Model white Hue (color) green L H red S blue Lightness black Saturation Simple algebraic transformation RGB HLS HSV system similar, only bottom part of cone CMYK Model on paper behaves as subtractive mixture basic colors: Cyan, Magenta, Yellow RGB: CMY: Black often used as 4th ink to get perfect black: CMYK Gamuts RGB cube in CIE space Problem: how to map between different gamuts? 4
Gamut in Luv gamut of TV (SMPTE) and NCS (color system) shown Gamma Correction Relation between electron beam intensity and screen brightness is nonlinear! a γ 1 γ 2.5 for almost all devices a a γ Correction with hardware - LUT (even for 24 bit display) brightness beam intensity use beam intensity a instead of a Note: requires correct blackness/contrast setting! 1 γ Alternative Systems! Munsell color system! NCS color system! CNS color system 5
Munsell System Defined by A. Munsell in 1905 as color atlas! specification by hue, value and chroma! Perceptual color system (perceptionally uniform distances won by experiments)! Important in science for comparison purposes! Minor importance in computer graphics (no simple conversion to XYZ, no analytic definition) Munsell System Natural System (NCS) Swedish color system, color atlas! Based on complementary color theory by Hering! specification by blackness, chromaticeness and hue! blackness defines the proportion of black,! chromaticeness defines the color portion in %,! hue defines the similarity degree to the 4 basic colors red, green, blue and yellow 6
Natural System (NCS) Frequent usage in Scandinavian countries and in architecture example: 4020 - Y60R 40% black portion 20% color portion 40% yellow 60% red Naming System (CNS) Related to HLS, verbal system Basic colors: red green blue yellow purple orange brown Intermediate colors between neighbors are possible in the proportions 25:75, 50:50, 75:25 e.g. "red-purple" (= 50:50) "reddish-purple" (= 25:75) "purplish-red" (= 25:75) Naming System (CNS) Lightness: very dark, dark, medium, light, very light additionally: black, white Saturation: grayish, moderate, strong, vivid additionally: grey examples: light moderate bluish-green very dark vivid red CNS enables the formulation of 627 colors 7
Effects of s passive cold heavylight warm active Statements about s (1) changes with! background! surround! environment Influence of the Background s look different on different backgrounds A A A A A A 8
Influence of Surround (1) Many thanks to S. Shevell, Chicago for this example Identical color Influence of Surround (2) Large color shift due to surround! This does not work well with constant color surround. Illusion based on textured surround. Influence of Surround (3) Large and unexpected color shift due to surround! The only difference is the phase of the concentric rings! 9
Influence of Surround (4) Another example: Effect of Environment Daylight illumination Tungsten filament illumination Statements about s (2) Not all colors are equally well readable Narrow (thin) blue parts on black background are difficult to detect Brilliant colors look very unnatural Some colors are associated with meanings Similar colors should have similar meanings s have different depth effects 10
Unequal Readability Thin Blue Parts on Black Background small text small text small text Relative luminance of RGB colors Primaries Relative luminance (percent) White RGB 100 Yellow RG 90 Cyan GB 70 Green G 60 Magenta R B 40 Red R 30 Blue B 10 Black 0 11
Meaning of s We associate instinctively different meanings with different colors possible combination: black, white, blue green, cyan yellow, magenta red blinking red background neutral information attention! alarm!! ALARM!!! Note: cultural dependencies s have different Depth Effects good bad Works for about 60% of population, 30% see opposite effect, 10% no effect. The Stroop Effect Task: Say the colour of each word as quickly as possible (Stroop, 1935) 12