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FRS 123: Technology in Art and Cultural Heritage Color
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Color Two types of receptors: rods and conesTwo types of receptors: rods and cones Rods and cones Cones in fovea
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Rods and Cones RodsRods – More sensitive in low light: “scotopic” vision – More dense near periphery ConesCones – Only function with higher light levels: “photopic” vision – Densely packed at center of eye: fovea – Different types of cones color vision
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Color Perception 3 types of cones: L, M, S 3 types of cones: L, M, S Tristimulus theory of color S L M
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Tristimulus Color Any distribution of light can be summarized by its effect on 3 types of conesAny distribution of light can be summarized by its effect on 3 types of cones Therefore, human perception of color is a 3-dimensional spaceTherefore, human perception of color is a 3-dimensional space Metamerism: different spectra, same responseMetamerism: different spectra, same response Color blindness: fewer than 3 types of conesColor blindness: fewer than 3 types of cones – Most commonly L cone = M cone
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Color Models RGBRGB CMYCMY HSVHSV XYZXYZ …etc…etc
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Color Models Different ways of parameterizing 3D spaceDifferent ways of parameterizing 3D space RGBRGB – Official standard: R = 645.16 nm, G = 526.32 nm, B = 444.44 nm – Most monitors are some approximation to this
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RGB Color Model R G B Color R G B Color 0.00.00.0Black 1.00.00.0Red 0.01.00.0Green 0.00.01.0Blue 1.01.00.0Yellow 1.00.01.0Magenta 0.01.01.0Cyan 1.01.01.0White 0.50.00.0? 1.00.50.5? 1.00.50.0? 0.50.30.1? Colors are additive
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CMY Color Model C M Y Color C M Y Color 0.00.00.0White 1.00.00.0Cyan 0.01.00.0Magenta 0.00.01.0Yellow 1.01.00.0Blue 1.00.01.0Green 0.01.01.0Red 1.01.01.0Black 0.50.00.0? 1.00.50.5? 1.00.50.0? Colors are subtractive
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HSV Color Model H S V Color 01.01.0Red 1201.01.0Green 2401.01.0Blue *0.01.0White *0.00.5Gray * *0.0Black 601.01.0? 2700.51.0? 2700.00.7?
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XYZ Colorspace RGB can’t represent all pure wavelengths with positive valuesRGB can’t represent all pure wavelengths with positive values – Saturated greens would require negative red XYZ colorspace is a linear transform of RGB so that all pure wavelengths have positive valuesXYZ colorspace is a linear transform of RGB so that all pure wavelengths have positive values
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CIE Chromaticity Diagram
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(White)
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CIE Chromaticity Diagram CompareColorGamutsIdentifyComplementaryColorsDetermine Dominant Wavelength and Purity
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RGB Color Gamut for Typical Monitor
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Colorspaces for Television Differences in brightness more important than differences in colorDifferences in brightness more important than differences in color YC r C b, YUV, YIQ colorspaces = linear transforms of RGBYC r C b, YUV, YIQ colorspaces = linear transforms of RGB – Lightness: Y=0.299R+0.587G+0.114B – Other color components typically allocated less bandwidth than Y
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Perceptually-Uniform Colorspaces Most colorspaces not perceptually uniformMost colorspaces not perceptually uniform MacAdam ellipses: color within each ellipse appears constant (shown here 10X size)MacAdam ellipses: color within each ellipse appears constant (shown here 10X size)
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Perceptually-Uniform Colorspaces u’v’ spaceu’v’ space Not perfect, but better than XYZNot perfect, but better than XYZ
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L*a*b* Color Space Another choice: L*a*b*Another choice: L*a*b*
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L*a*b* Color Space Often used for color comparison when “perceptual” differences matterOften used for color comparison when “perceptual” differences matter
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Summary Perception and representation ofPerception and representation of – Intensity, frequency, color ColorColor – Tristimulus theory of color – CIE Chromaticity Diagram – Different color models
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