COMPUTER GRAPHICS CS 482 – FALL 2016 CHAPTER 28 COLOR COLOR PERCEPTION CHROMATICITY COLOR MODELS COLOR INTERPOLATION

COLOR PERCEPTION CS 482 – FALL 2016 COLOR SENSITIVITY CHAPTER 28: COLORPAGE 211 THE VISIBLE SPECTRUM OF LIGHT, ILLUSTRATED AT RIGHT, RANGES FROM ABOUT 400 TO 700 NANOMETERS IN THE ELECTROMAGNETIC ENERGY SPECTRUM. EMPIRICAL STUDIES HAVE INDICATED THAT THE CONES IN THE EYE HAVE DIFFERENT LEVELS OF SENSITIVITY TO DIFFERENT COLORS, INDICATING THE EYE’S RESPONSE TO PURE BLUE LIGHT IS MUCH LESS STRONG THAN ITS RESPONSE TO PURE RED OR GREEN LIGHT. THE RGB PHOSPHORS USED IN CATHODE RAY TUBES DO NOT EXACTLY PRODUCE “PURE” SHADES OF RED, GREEN, AND BLUE, AS INDICATED IN THE FIGURE ABOVE, SHOWING THE EYE’S RESPONSE TO THE EXCITED PIXEL COLORS.

COLOR PERCEPTION CS 482 – FALL 2016 VISIBLE SPECTRUM CHAPTER 28: COLORPAGE 212 USING AN XYZ COORDINATE SYSTEM (WHERE THE AXES ARE “SHADES” OF RED, GREEN, AND BLUE, RESPECTIVELY, JUST OUTSIDE THE VISIBLE SPECTRUM), THE COMMISSION INTERNATIONALE DE L’ECLAIRAGE (CIE – INTERNATIONAL COMMISSION ON ILLUMINATION) ESTABLISHED A COLOR STANDARD THAT ENCLOSES THE ENTIRE VISIBLE SPECTRUM.

CHROMATICITY CS 482 – FALL 2016 CIE CROMATICITY DIAGRAM CHAPTER 28: COLORPAGE 213 BECAUSE OF THE DIFFERING SENSITIVITIES OF THE EYE’S CONES, VISIBLE COLOR SPACE IS ACTUALLY QUITE ELABORATE. FULLY SATURATED COLORS LIE AROUND THE DIAGRAM’S BORDER, WHILE UNSATURATED COLORS ARE IN THE CENTER. AN INTERNATIONAL STANDARDS ORGANIZATION, THE CIE, DEVELOPED THREE COLOR-MATCHING FUNCTIONS THAT COULD BE COMBINED TO APPROXIMATE THE ENTIRE RANGE OF VISIBLE COLORS. WHEN THIS SPACE IS SLICED BY THE X+Y+Z=1 PLANE, THE RESULT IS THE CIE CHROMATICITY DIAGRAM, PICTURED AT RIGHT.

CHROMATICITY CS 482 – FALL 2016 COLOR GAMUTS CHAPTER 28: COLORPAGE 214 TRIANGULAR REGIONS OF THE CIE CHROMATICITY DIAGRAM, KNOWN AS GAMUTS, ARE USED TO DEFINE THE RANGE OF PRODUCIBLE COLORS FOR A DEVICE. ALL COLORS WITHIN THE GAMUT MAY BE PRODUCED AS LINEAR COMBINATIONS OF THE VERTEX COLORS. THE US TELEVISION STANDARD, NTSC, USES RED: (0.6700, ), GREEN: (0.2100, ), AND BLUE: (0.1400, ). HDTV USES RED: (0.6400, ), GREEN: (0.3000, ), AND BLUE: ((0.1500, ). COLOR CRTS USE RED: (0.6280, ), GREEN: (0.2680, ), AND BLUE: (0.1500, ), AS ILLUSTRATED ABOVE.

CHROMATICITY CS 482 – FALL 2016 COLOR BLINDNESS CHAPTER 28: COLORPAGE 215 DEUTERANOPES PERCEIVE BLUES & MAGENTAS AS VIOLETS. PERCEIVE GREENS & ORANGES AS YELLOWS. PERCEIVE CYANS & REDS AS WHITE. TRITANOPES PERCEIVE ORANGES & MAGENTAS AS REDS. PERCEIVE GREENS & BLUES AS CYANS. PERCEIVE YELLOWS & VIOLETS AS WHITE.

COLOR MODELS CS 482 – FALL 2016 ADDITIVE SYSTEMS CHAPTER 28: COLORPAGE 216 IN DIGITAL DISPLAY SYSTEMS, EACH PIXEL IN AN IMAGE IS REPRESENTED AS AN ADDITIVE COMBINATION OF THE THREE PRIMARY COLOR COMPONENTS: RED, GREEN, AND BLUE.

COLOR MODELS CS 482 – FALL 2016 SUBTRACTIVE SYSTEMS CHAPTER 28: COLORPAGE 217 PRINTERS USE A SUBTRACTIVE COLOR SYSTEM, IN WHICH THE COMPLEMENTARY COLORS OF RED, GREEN, AND BLUE (CYAN, MAGENTA, AND YELLOW) ARE APPLIED IN INKS AND TONERS IN ORDER TO SUBTRACT COLORS FROM A VIEWER’S PERCEPTION. CYAN (GREEN+BLUE, THE RED SUBTRACTOR) + MAGENTA (RED+BLUE, THE GREEN SUBTRACTOR) BLUE MAGENTA (RED+BLUE, THE GREEN SUBTRACTOR) + YELLOW (RED+GREEN, THE BLUE SUBTRACTOR) RED YELLOW (RED+GREEN, THE BLUE SUBTRACTOR) + CYAN (GREEN+BLUE, THE RED SUBTRACTOR) GREEN

COLOR MODELS CS 482 – FALL 2016 RGB CUBE CHAPTER 28: COLORPAGE 218 A MUCH SIMPLER COLOR MODEL IS THE RGB CUBE, WHICH MERELY SETS A PARTICULAR SHADE OF RED, GREEN, AND BLUE ALONG THE THREE COORDINATE AXES, AND LINEARLY COMBINES COLORS TO OBTAIN THE COLORS INSIDE THE CUBE. THE HARDWARE STANDARD FOR COLOR PRODUCTION IS STILL A VARIATION OF CIE CHROMATICITY; SOFTWARE AND COLOR PRINTER STANDARDS TEND TO USE A VARIATION OF THE RGB CUBE. WHILE THIS APPROACH IS CONCEPTUALLY SIMPLE, IT IS QUITE INACCURATE, SINCE COLOR PERCEPTION IS NOT, IN FACT, DEFINABLE AS A LINEAR PROCESS.

COLOR MODELS CS 482 – FALL 2016 HSV HEXCONE CHAPTER 28: COLORPAGE 219 THIS COLOR MODEL TAKES A DIFFERENT APPROACH, USING HUE (I.E., PURE COLOR, REPRESENTED BY THE ANGLE AROUND THE CENTRAL AXIS TO THE CONE), SATURATION (I.E., LEVEL OF PURITY, REPRESENTED BY THE RADIAL DISTANCE FROM THE AXIS), AND VALUE (I.E., BRIGHTNESS, REPRESENTED BY THE DISTANCE UP THE CENTRAL AXIS). THIS MODEL HAS THE INTUITIVE APPEAL OF THE ARTIST’S TINT, SHADE, AND TONE MODEL: PURE RED = H =0, S =1, V = 1; PURE PIGMENTS ARE (I,1,1). TINTS: ADDING WHITE PIGMENT IS EQUIVALENT TO DECREASING S AT CONSTANT V SHADES: ADDING BLACK PIGMENT IS EQUIVALENT TO DECREASING V AT CONSTANT S TONES: “GRAYING” BY DECREASING S AND V A VARIATION ON THIS APPROACH IS THE HSL (HUE-SATURATION- LIGHTNESS) TECHNIQUE, WHICH USES A DOUBLE-HEXCONE APPROACH INSTEAD OF A SINGLE HEXCONE.

COLOR MODELS CS 482 – FALL 2016 POLAR COORDINATES CHAPTER 28: COLORPAGE 220 RGB CUBE HSV HEXCONE HSL DOUBLE HEXCONE Tilt cube and add seams Force RGBCMY into a plane Expand horizontal slices Round off hexagonal exterior Example vertical cross- sections

COLOR INTERPOLATION CS 482 – FALL 2016 RGB VS. HSV (CLOCKWISE) VS. HSV (COUNTER-CLOCKWISE) CHAPTER 28: COLORPAGE 221 LINEAR INTERPOLATION BETWEEN RGB VALUES IS LAZY, SOMETIMES PRODUCING UNSATISFACTORY RESULTS. WHY PASS THROUGH GRAY WHEN MOVING FROM CYAN TO RED? THE MORE “ARTIST-FRIENDLY” APPROACH OF USING HUE INTERPOLATION CAN ALSO PRODUCE DISAPPOINTING RESULTS. WHY PASS THROUGH GREEN AND YELLOW WHEN MOVING FROM CYAN TO RED? HUE INTERPOLATION IN THE OPPOSITE DIRECTION WILL SOMETIMES BE MORE PLEASING TO VIEWERS, EVEN THOUGH THE ACTUAL DISTANCE TRAVELED IS THE SAME. PASSING THROUGH BLUE AND MAGENTA WHEN MOVING FROM CYAN TO RED SEEMS MORE “NATURAL”.