CS 430 Computer Graphics Color Theory ChiCheng Lin

CS 430 Computer Graphics Color Theory Chi-Cheng Lin, Winona State University

Topics Colors l CIE Color Model l RGB Color Model l CMY Color Model l YIQ Color Model l Intuitive Color Concepts l HSV Color Model l HLS Color Model l 2

l Colors z. A narrow frequency band within the electromagnetic spectrum 3

Colors l Visible band z. Each frequency corresponds to a distinct color z. Low-frequency end (4. 3 x 1014 Hz): Red z. High-frequency end (7. 5 x 1014 Hz): Violet z. Wavelength = v/f, where v=300, 000 km/sec z. Low frequency High frequency red orange yellow green blue violet Long wavelength Short wavelength 700 nm 4

Colors l Colors of an object z. Light source emits “white light” (all frequencies of light) z. Object reflects/absorbs some frequencies z. Color = combination of frequencies reflected l Dominant wavelength (or frequency) z. Hue or color of the light z. E. g. , pink S( ): spectrum (luminance/intensity of light) 400 620 700 5

CIE Color Model l Color models z. Use three primary colors to produce other colors l Primary colors z. Colors used in a color model to produce all the other colors in that model. z. Cannot be made from the other (two) colors defining the model. l CIE color model z. X, Y, and Z: nonexistent, super saturated colors y. Vectors in 3 -D additive color space z. Any color S = AX + BY + CZ 6

CIE Color Model l S = AX + BY + CZ can be normalized to zx = A/(A+B+C) zy = B/(A+B+C) zz = C/(A+B+C) s = x. X + y. Y + z. Z, where x + y + z = 1 s lies in the plane x + y + z = 1 in 3 D y =670 x =400 z 7

CIE Color Model l CIE chromaticity diagram zs'( ) = (x( ), y( )) z. By viewing the 3 D curve in an orthographic projection, looking along the z-axis zhorseshoe shape y =670 x =400 z 8

CIE Chromaticity Diagram 9

CIE Chromaticity Diagram 10

Uses of CIE Chromaticity Diagram 11

Uses of CIE Chromaticity Diagram l Any colors on the line l between two colors a and b z. Is a convex combination of a and b z. Is a legitimate color zcan be generated by shining various amounts of a and b onto a screen (like “tweening”) l Complementary colors z. Any two colors on a line passing through white and added up to be white are complementary e. g. , e and f zred cyan green magenta blue yellow 12

Uses of CIE Chromaticity Diagram l Measure dominant wavelength and saturation z. Color g: Some combination of h and white z. Dominant wavelength of g = wavelength at h z. Saturation (purity) of g = (g - w) / (h - w) l Color j has no dominant wavelength because k is not a pure color (k lies on the purple line) z. Represented by dominant wavelength of k’s complement m, with by a c suffix, e. g. , 498 c 13

Uses of CIE Chromaticity Diagram l Any color within a triangle can be generated by the three vertices of the triangle z. Any point inside IJK is a convex combination of points I, J, and K 14

Uses of CIE Chromaticity Diagram l Define color gamuts z. Range of colors that can be produced on a device CRT monitor’s gamut is different from printer’s (See Plate 33 in the textbook) l Any choice of three primaries can never encompass all visible colors l RGB are natural choices for primaries as they can cover the largest part of the “horseshoe” l 15

Gamut Example 16

RGB Color Model l Used in light emitting devices z. Color CRT monitors l Additive z. Result = individual contributions of each primary color added together z. C = r. R + g. G + b. B, where r, g, b [0, 1] z. R = (1, 0, 0) z. G = (0, 1, 0) z. B = (0, 0, 1) 17

RGB Color Model 18

RGB Color Model l Color Cube z. R + G = (1, 0, 0) + (0, 1, 0) = (1, 1, 0) = Y z. R + B = (1, 0, 0) + (0, 0, 1) = (1, 0, 1) = M z. B + G = (0, 0, 1) + (0, 1, 0) = (0, 1, 1) = C z. R + G + B = (1, 1, 1) = W z 1 – W = (0, 0, 0) = BLK z. Grays = (x, x, x), where x (0, 1) 19

Color Cube 20

CMY Color Model CMY: Complements of RGB l Used in light absorbing devices l z. Hardcopy output devices l Subtractive z. Color specified by what is subtracted from white light z. Cyan absorbs red, magenta absorbs green, and yellow absorbs blue 21

CMY Color Model 22

CMY Color Model l W = (0, 0, 0) B = (1, 1, 1) l Conversion from RGB to CMY l Conversion from CMY to RGB 23

CMYK Color Model l Motivations z. Do we get black if paint cyan, magenta and yellow on a white paper? z. Which cartridge is more expensive? l CMYK model z. K = greatest gray that can be extracted l Given C, M, and Y z. K = min(C, M, Y) z. C = C – K z. M = M – K z. Y = Y – K Try some examples… 24

YIQ Color Model l Used in U. S. commercial color-TV broadcasting z. Recoding of RGB for transmission efficiency z. Backward compatible with black-and-white TV z. Transmitted using NTSC (National Television System Committee) standard 25

YIQ Color Model l YIQ z. Y: luminance z. I, Q: chromaticity z. Only Y shown in black-and-white TV l RGB YIQ 26

YIQ Color Model l Human’s visual properties z. More sensitive to changes in luminance than in hue or saturation more bits should be used to represent Y than I and Q z. Limited color sensation to objects covering extremely small part of our field of view One, rather than two color dimensions would be adequate I or Q can have a lower bandwidth than the others 27

YIQ Color Model l NTSC encoding of YIQ into broadcast signal z. Uses human’s visual system properties to maximize information transmitted in a fixed bandwidth z. Y: 4 MHz z. I: 1. 5 MHz z. Q: 0. 6 MHz 28

Intuitive Color Concepts l Terminology Perceptual Term hue Colorimetry Comments dominated wavelength saturation excitation purity Lightness (reflecting luminance objects) Brightness (selfluminance luminous objects) to distinguish colors e. g. , red and pink e. g. , Sun, CRT 29

Intuitive Color Concepts white grays tints pure color tones shades black z. Tint: white pigment added to pure pigment saturation reduced z. Shade: black pigment added to pure pigment lightness reduced z. Tone: consequence of adding both white and black pigments to pure pigments 30

Intuitive Color Concepts Tints, shades, and tones different colors of same hue are produced l Grays = black pigments + white pigments l Graphics packages that provide color palettes to users often employ two or more color models l 31

HSV Color Model l HSV = Hue, Saturation, and Value z. A. k. a. HSB, where B is Brightness RGB, CMY, and YIQ: hardware-oriented l HSV and HLS: user-oriented l Cylinder coordinate system l z. Space: hexcone zhexagon is obtained from the color cube in isometric projection z(h, s, v), where h [0, 360) and s, v [0, 1] yhue: angle round the hexagon ysaturation: distance from the center yvalue: axis through the center 32

HSV Color Model Color Cube Hexcone 33

HSV Color Model W = (-, 0, 1) l B = (-, 0, 0) l R = (0, 1, 1) Y = (60, 1, 1) l : M = (300, 1, 1) l Adding white pigments S l Adding black pigments V l Creating tones S and V 34

HSV Color Model True color system: 16 million colors l Q: Do we need that many? l Human eyes can distinguish l z 128 hues z 130 tints (saturation levels) z 23 shades of yellow colors, 16 of blue colors 128 x 130 x 23 = 82720 colors 35

HLS Color Model HLS: Hue, Lightness, and Saturation l Cylinder coordinate system l z. Space: double cone zbase is from the hexagon as in HSV z(h, l, s), where h [0, 360) and s, v [0, 1] yhue: angle round the base ylightness: axis through the center ysaturation: distance from the center W = (-, 0, 1) l B = (-, 0, 0) l R = (0, 0. 5, 1), Y = (60, 0. 5, 1), … l 36

HLS Color Model l Double cones white pure color h black 37