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Light and Color

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Light and Color CS148: Introduction to Computer Graphics and Imaging Light and Color Painting by Cheryl Yaney Topics Physics of light Electromagnetic spectrum Fundamental operations: add, filter, measure Perception of color Trichromatic theory Luminance Color spaces Art Color terms Color wheels and intuitive color spaces CS148 Lecture 10 Pat Hanrahan, Winter 2009 Page 1 Physics of Light Light Image from Clive Maxfield CS148 Lecture 10 Pat Hanrahan, Winter 2009 Page 2 Electromagnetic Spectrum Image from Clive Maxfield CS148 Lecture 10 Pat Hanrahan, Winter 2009 Sources Visible (300-800 nm) spectra power distribution CS148 Lecture 10 Pat Hanrahan, Winter 2009 Page 3 Sources Visible (300 – 800 nm) spectra power distribution CS148 Lecture 10 Pat Hanrahan, Winter 2009 Adding Light Energy CS148 Lecture 10 Pat Hanrahan, Winter 2009 Page 4 Reflecting Light CS148 Lecture 10 Pat Hanrahan, Winter 2009 Light Operations Add spectra Multiply spectra CS148 Lecture 10 Pat Hanrahan, Winter 2009 Page 5 Measuring Light Photon detector CS148 Lecture 10 Pat Hanrahan, Winter 2009 Perception of Color Page 6 Color Matching Adjust brightness of three primaries to “match” color C – color to be matched Lasers: R = 700 nm, G = 546 nm, B = 435 nm C R G B C = R + G + B Result: all colors can be matched with three colors Therefore: humans have trichromatic color vision CS148 Lecture 10 Pat Hanrahan, Winter 2009 RGB Color CS148 Lecture 10 Pat Hanrahan, Winter 2009 Page 7 Human Retina: Three Types of Cones From http://webvision.med.utah.edu/imageswv/fovmoswv.jpeg CS148 Lecture 10 Pat Hanrahan, Winter 2009 Response of Three Cones CS148 Lecture 10 Pat Hanrahan, Winter 2009 Page 8 Cone Response Three cones L (long) M (medium) S (short) Metamerism: Different spectra, same color response CS148 Lecture 10 Pat Hanrahan, Winter 2009 Color Blindness: Ishihara Test Normal http://www.toledo-bend.com/colorblind/Ishihara.html CS148 Lecture 10 Pat Hanrahan, Winter 2009 Page 9 Color Blindness: Ishihara Test Normal Deuteranopia http://www.toledo-bend.com/colorblind/Ishihara.html CS148 Lecture 10 Pat Hanrahan, Winter 2009 Types of Color Blindness Dichromacy: missing pigment (genetic, 10% M, 1% F) Protanopia – missing L Deuteranopia – missing M (red-green) Tritanopia – missing S Normal Deuteranopia Tritanopia www.vischeck.com CS148 Lecture 10 Pat Hanrahan, Winter 2009 Page 10 Rod CS148 Lecture 10 Pat Hanrahan, Winter 2009 Luminance Compare color to a gray source Luminance (B&W TV) CS148 Lecture 10 Pat Hanrahan, Winter 2009 Page 11 Spectral Matching Functions Match each pure (monochromatic) color in the visible spectrum (rainbow) and record the color coordinates as a function of wavelength CS148 Lecture 10 Pat Hanrahan, Winter 2009 Absolute/Calibrated Color Space Spectral matching functions define a horseshoe curve CIE XYZ Y is luminance CS148 Lecture 10 Pat Hanrahan, Winter 2009 Page 12 CIE XYZ Chromaticity Coordinates CS148 Lecture 10 Pat Hanrahan, Winter 2009 Grassman’s Laws Color matching is linear! 1. Scaling the color and the primaries by the same factor preserves the match 2. To match a color formed by adding two colors, add the primaries that match each color CS148 Lecture 10 Pat Hanrahan, Winter 2009 Page 13 Gamut All perceivable colors are inside the horseshoe curve (gamut) - A spectra is a sum of monochromatic colors with positive weights equal to the amount of energy of that color - Sum of vectors with positive weights must lie inside the convex hull of the vectors CS148 Lecture 10 Pat Hanrahan, Winter 2009 Early Visual Processing + - + + + + + + - A R-G Y-B CS148 Lecture 10 Pat Hanrahan, Winter 2009 Page 14 Perceptual Organization Not “RGB” as in Colorimetry Lightness Colorfulness Hue CS148 Lecture 10 Pat Hanrahan, Winter 2009 Munsell Color Space Book of painted chips Perceptually uniform CS148 Lecture 10 Pat Hanrahan, Winter 2009 Page 15 CIE Lab CS148 Lecture 10 Pat Hanrahan, Winter 2009 CIELAB Color distance: CS148 Lecture 10 Pat Hanrahan, Winter 2009 Page 16 Intuitive Color Space Tints White Color Tones Gray Shades Black MetaDesign Color Picker CS148 Lecture 10 Pat Hanrahan, Winter 2009 Page 17 HSV M=max(R,G,B) K m=min(R,G,B) M = r=(M-R)/(M-m) m g=(M-G)/(M-m) W b=(M-B)/(M-m) R G B R G B if(M==R) V=M H=(6+b-g)/6 S=(M-m)/M .... CS148 Lecture 10 Pat Hanrahan, Winter 2009 Things to Remember Physics of color Addition, transmission/reflection, measurement Perception of color Three colors will match another color Trichromatic because we have 3 cones Explains metamerism, color blindness Color matching is linear Perceptual organization in terms of BW, YB, RG Hue-Saturation-Value CS148 Lecture 10 Pat Hanrahan, Winter 2009 Page 18 .
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