color.slides.printing.6

Why not examples red vs blue red vs pink dark red

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Unformatted text preview: h How pure that color is (ratio of color to white) – Some models use different chromaticity parameters • Tints and shades are not inverses! – Why not? • Examples: – Red vs. Blue? – Red vs. Pink? – Dark red vs. Bright red? The CIE RGB Model The CIE XYZ Model (1931) • Replaces R, G, B with “imaginary” primaries X, Y, Z • Standardized “red”, “green”, and “blue” to try to match perception: 438.1, 546.1, and 700 mn R = Km G = Km B = Km – Don’t correspond to single wavelengths – Weights are always positive – More saturated than monochromatic light – Can model all physically realizable colors Ú L(l) r(l) dl Ú L(l) g(l) dl Ú L(l) b(l) dl X = Km Y = Km Notice negative weight of red component for † wavelengths near 500 nm—can’t physically mix Z = Km † Ú L(l) x (l) dl Ú L(l) y (l) dl Ú L(l) z (l) dl Normalizing CIE XYZ Model CIE Chromaticity Diagram • Normalize X, Y, Z so that we can talk about chromaticity: X X +Y + Z Y y= X +Y + Z Z z= X +Y + Z x= † Gamut Notice that x + y + z = 1 forms a plane in color space — can just use x and y (because z = 1 - x - y) Color Gamuts • A color gamut is the space of colors spanned by a set of primaries • No three physical primaries can span the entire space of physically realizable colors – Use “negative” weights for some colors, or – Use “imaginary” primaries that lie outside the color space – Both are physically impossible • Better with more primarie...
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