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Color ImagingDescribing Chromatic lightsPrimary ColorsPrimary and Secondary Colors of Lights and PigmentsCharacterization of ColorChromaticity DiagramColor GamutColor ModelsRGB Color ModelHSI Color ModelColor Coordinate Transform© 2002-2003 by Yu Hen Hu1ECE533 Digital Image ProcessingColor Imaging© 2002-2003 by Yu Hen Hu2ECE533 Digital Image ProcessingDescribing Chromatic lightsRadiance (watt): »Total amount of energy flow from the light source.Luminance (lumens, lm): »measure of amount of energy an observer perceives from a light source. It varies based on distance from the source, wavelength, etc.Brightness: »a subjective descriptor, describing color sensation.© 2002-2003 by Yu Hen Hu3ECE533 Digital Image ProcessingPrimary ColorsPrimary colors of light (additive):»Red (700 nm), 65% cones sensitive to red light.»Green (546.1nm), 33%»Blue(435.8nm). 2% cones sensitive to blue light.Mixing of R,G,B may NOT generate ALL visible colors.© 2002-2003 by Yu Hen Hu4ECE533 Digital Image ProcessingPrimary and Secondary Colors of Lights and PigmentsPrimary colors of pigment (subtractive): »magenta,»cyan, and »yellow.© 2002-2003 by Yu Hen Hu5ECE533 Digital Image ProcessingCharacterization of ColorColors are distinguished from one another based on brightness, hue, and saturation.Hue: »an attribute associated with the dominant wavelength in a mixture of light waves. It represents the dominant color as perceived by an observer. Saturation: »specifies relative purity or the amount of white lights mixed with a hue. Hue and saturation together are called chromaticity.Example: color palette© 2002-2003 by Yu Hen Hu6ECE533 Digital Image ProcessingChromaticity DiagramTri-chromatic coefficients:»Let X, Y, Z: tri-stimulus values representing the amounts of red, green, and blue needed to form any particular color.»Since x + y + z = 1, x and y along will make a chromaticity diagramCIE Chromaticity diagram»x-axis: red, y-axis: green»Color on boundary are completely saturated.»Saturation @ pts of equal energy is zero, ,X Yx yX Y Z X Y ZZzX Y Z= =+ + + +=+ +© 2002-2003 by Yu Hen Hu7ECE533 Digital Image ProcessingColor GamutAny 3 points in the chromaticity diagram can produce all colors within that triangle. Due to the tongue-shape indicates that no mixing of three primary color can produce ALL possible colors© 2002-2003 by Yu Hen Hu8ECE533 Digital Image ProcessingColor ModelsRGB color model: monitor, videoCMY (CMYK) color model: printingHIS: close to HVS© 2002-2003 by Yu Hen Hu9ECE533 Digital Image ProcessingRGB Color ModelR, G, B at 3 axis ranging in [0 1] eachGray scale along the diagonalIf each component is quantized into 256 levels [0:255], the total number of different colors that can be produced is (28)3 = 224 = 16,777,216 colors.RGB safe color:»Quantize each components into 6 levels from 0 to 255.24-bit RGB color cubeRGB safe color cube© 2002-2003 by Yu Hen Hu10ECE533 Digital Image ProcessingHSI Color ModelHue: »an attribute describing pure colorSaturation:»The degree of which a pure color is diluted by white light.HSI model»Hue and saturation lie in a plane perpendicular to an intensity axis.© 2002-2003 by Yu Hen Hu11ECE533 Digital Image ProcessingColor Coordinate TransformRGB  CYMHSI  RGB»Others see text bookRGB  HSI111C RY GM B� � �� � �� � �� � �= -� � �� � �� � �� � �� � �� � �0 120(1 )cos1cos(60 )1 ( )ooHB I SS HR IHG R B� �= -� ��= � +� �-� �= - +[ ]( )12( ) ( ) / 2cos( ) ( )( )3603 min( , , )1/ 3R G R BR G R B G BB GHB GR G BSR G BI R G Bqqq-� �- + -� �=� �- + - -� ����=�- >��= -+ += +


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UW-Madison ECE 533 - Color Imaging

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