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Color Representation Color Representation Foley & Van Dam, Chapter 13 Color Representation • Visible Light Spectrum • Color Matching • Trichromatic Color Theory • Psychophysics • CIE standard • RGB and CMYK Color Spaces • HLS Color Model • YIQ Color Model Visible Light Spectrum and Colors Light is an electro-magnetic radiation Ultra- Short- AC Gamma X raysviolet Infrared Radar FM TVwave AM electricity -12 -8 -4 4 8 10 10 10 11010 Wavelength in meters (m) Visible light 400 nm 500 nm 600 nm 700 nm Wavelength in nanometers (1nm=10-9 m) • Hue: distinguished among colors • Saturation: how far is color from a gray of equal intensity • Lightness: perceived intensity of a reflective surface • Brightness: perceived intensity of emitting surface Spectral Power Distribution • The Spectral Power Distribution of a light is a function f(λ) defining the energy at each wavelength 1 1 0.5 0.5 0 0 400 500 600 700 400 500 600 700 Blue Skylight Tungsten bulb 1 1 0.5 0.5 0 0 400 500 600 700 400 500 600 700 Red monitor phosphor Monochromatic light Color Matching Experiment • Three primary lights are set to match a test light • Metamer: two lights visually undistinguishable (they might have different spectral power distributions) test match + - + - + - Test light Match light (tungsten light) (monitor emission) Power =~ 0 400 500 600 700 0 400 500 600 700 Trichromatic Color Theory • Trichromatic: “tri”=three “chroma”=color also tristimulus color vision is based on three primaries (three dimensional) •Thomas Young • A few different retinal receptors operating with different wavelength sensitivities allow humans to perceive colors • Suggested 3 receptors • Helmholtz & Maxwell • Color matching with 3 primaries The Human Eye Lens Cornea Fovea Optic Nerve cones Pupil Vitreous rods Humor Iris Optic Disc Retina Ocular Muscle bipolar horizontal amacrine ganglion light Retinal Photoreceptors • Cones: Sensitive to high illumination levels (Photopic vision) Less sensitive than rods 5 million cones in each eye Only cones in fovea (approx. 50,000) Density decreases with distance from fovea 3 types differing in their spectral sensitivity: L , M, and S 1 L M 0.75 S 0.5 0.25 Relative sensitivity 0 400 500 600 700 Wavelength (nm) Retinal Photoreceptors Linear Color Spaces • Colors in 3D color space can be described as linear combinations of 3 basis colors called primaries = A • + B • + C • The representation of the color having spectrum: Is given by (A, B, C) Choosing The Primaries • Stiles & Burch (1959) used 3 monochromatic primaries of wavelengths 444.4, 525.3 and 645.2 3 r(λ) 2 b(λ) g(λ) 1 Primary Intensity 0 400 500 600 700 Wavelength (nm) Color Matching Diagram Problem: Subtractive components CIE Color Standard • CIE: Commision Internationale d’Eclairage (1931) defined a standard system (CIE- XYZ) for color representation 1.8 z(λ) 1.4 y(λ) 1 x(λ) 0.6 0.2 Tristimulus values 400 500 600 700 Wavelength (nm) • Weights are non negative over the visible wavelengths • The 3 primaries associated with x y z color matching functions cannot be easily realized in hardware • y was chosen to equal luminance of monochromatic lights CIE Color Standard If X, Y and Z are the weights used to define a color C, then the chromaticity values x, y, z (independent from the luminosity) are given by: x= X/(X+Y+Z) y=Y/(X+Y+Z) z=Z/(X+Y+Z) (x,y,z) is a point on the plane X+Y+Z=1 Y 0.9 520 530 510 540 550 y 505 560 X 500 570 0.5 580 495 590 Z 600 610 490 650 485 CIE Chromaticity Diagram 480 470 x 0.0 450 0.0 0.5 1.0 CIE Color Standard • Color Gamut: A convex sum of several colors RGB Color Representation Cathode shadow mask electron guns (electron gun) and phosphor coated screen shadow mask focusing phosphors on anode deflection glass screen yoke • In a CRT each color can be defined by the required power of each electron gun: CrRgGbB= ++ • The intensity is defined as: I = r + g + b •The chroma(ticy) is defined as: rR + g GbB+ C = r ++g b RGB Color Images 111 14 126 36 12 36 10 128 126 20012 111 200 36 12 3614 36 36 111 36 12 17 111 111 14 126 36 12 36 200 36 12 3614 36 17 36 36 14 36 72 17 111 20036 12 36 200 111 14 126 17 111 200 1111414 3612612 17 3611114 36 36 111 36 12 17 111 10 128 126 20012 111 12 17126 17 111 200 36 36 1711111136 1414 12636 17 111 36 36 111 36 14 36 17 36 1736 126141272361261267220017 3611112 36 17 111 20036 12 36 12 17126 17 111 200 14 200 36 12 126 17 200 36 12 36 12 126 17 126 72 126 17 111 14 2007236 1212 1712611117 14 36 14 36 12 36 14 36 126 200 111 14 36 72 126 200 111 14 36 72 200 36 12 36 12 126 36 12 17 72 106 155 17 111 14 126 17 111 36 12 17 72 106 155 72 12 17 111 14 36 white 12 126 20036 12 36 blue green red black RGB to CIE-XYZ Conversion • RGB to CIE-XYZ is a linear transformation: 2.365− 0.515 0.005 R X −−=0.897 1.426 0.014 GY − 0.468 0.089 1.009 B Z • R = monochromatic primary 700nm • G = monochromatic primary 546.1nm • B = monochromatic primary 435.8nm RGB vs. CMY(K) Color Scheme • RGB and CMYK (Cyan, Magenta, Yellow and blacK) are hardware-oriented representations • CMY is used in color photography and (with K) in most color printers CR1 M =−1 G YB1 RGB is Additive CMY is Subtractive The HLS Color Model • HLS: Hue Lightness, Saturation similar to HSV: Hue Saturation Value Munsell Book of Colors Hue (red, green, yellow, blue ...) Saturation (pink, bright red, ....) Lightness (black, grey, white ....) The YIQ Color Model • Based on the concept of opponent colors • Used in NTSC Television (National Television Systems Committee) • Similar method (YCbCr) used in JPEG and MPEG 0.299 0.587 0.114 RY 0.596−− 0.275 0.321 GI = 0.212− 0.523 0.311 BQ • Y = Luminance + • I = Red-Green - + • Q = Blue-Yellow - The YIQ Color Model • The human eye is more sensitive to luminosity than to colors, so it is possible to save space by encoding colors more coarsely • Preferred by the NTSC because of backward compatibility with B/W TV Original Y - Blur I - Blur Q - Blur Summary • CIE-XYZ • Tristimulus Coordinates • Device Independent • Universal standard • CIE-Lab • Perceptual Space, used to assess image quality • RGB and CMY • Hardware oriented • Additive spaces used for CRT, printers, photography • YIQ and YCbCr • Opponent Space • Used for color television broadcast and image compression • HLS • Perceptual Digitized Space • Used for Human Interactive Painting.
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