CS 445 / 645 Introduction to Computer GraphicsAssignment 3Last ClassHalftoningSlide 5Halftoning – dot sizeHalftoning – Moire PatternsDitheringSpecifying ColorCombining ColorsHow Do Artists Do It?HSV Color SpaceHSV Color ModelSlide 14Intuitive Color SpacesSlide 16Precise Color SpecificationsElectromagnetic SpectrumVisible LightHow well do we see color?Just noticeable difference (JND)Human Color VisionColor SpacesA Problem ExistsCIE Color SpaceSlide 26CIE Chromaticity Diagram (1931)Device Color GamutsSlide 29A Problem With XYZ ColorsRGB Color Space (Color Cube)RGB Color GamutsConverting Color SpacesYIQ Color SpaceSlide 35Perceptually Uniform Color SpaceThe CMY Color ModelCS 445 / 645Introduction to Computer GraphicsLecture 13Lecture 13ColorColorLecture 13Lecture 13ColorColorAssignment 3Due March 23Due March 23•Fourth years plan for thesis collisionFourth years plan for thesis collision•We’ll provide lots of details to keep the assignment doableWe’ll provide lots of details to keep the assignment doableMorphing algorithmMorphing algorithmDue March 23Due March 23•Fourth years plan for thesis collisionFourth years plan for thesis collision•We’ll provide lots of details to keep the assignment doableWe’ll provide lots of details to keep the assignment doableMorphing algorithmMorphing algorithmLast ClassWe discussed vision physiology andWe discussed vision physiology andperception of gradientsperception of gradientsToday:Today:Color perceptionColor perceptionColor representationsColor representationsWe discussed vision physiology andWe discussed vision physiology andperception of gradientsperception of gradientsToday:Today:Color perceptionColor perceptionColor representationsColor representationsHalftoningA technique used in newspaper printingA technique used in newspaper printingOnly two intensities are possible, blob of ink and no Only two intensities are possible, blob of ink and no blob of inkblob of inkBut, the size of the blob can be variedBut, the size of the blob can be variedAlso, the dither patterns of small dots can be usedAlso, the dither patterns of small dots can be usedA technique used in newspaper printingA technique used in newspaper printingOnly two intensities are possible, blob of ink and no Only two intensities are possible, blob of ink and no blob of inkblob of inkBut, the size of the blob can be variedBut, the size of the blob can be variedAlso, the dither patterns of small dots can be usedAlso, the dither patterns of small dots can be usedHalftoningHalftoning – dot sizeHalftoning – Moire PatternsRepeated use of same dot Repeated use of same dot pattern for particular pattern for particular shade results in shade results in repeated patternrepeated pattern•Perceived as a moire patternPerceived as a moire pattern•Instead, randomize halftone Instead, randomize halftone patternpatternRepeated use of same dot Repeated use of same dot pattern for particular pattern for particular shade results in shade results in repeated patternrepeated pattern•Perceived as a moire patternPerceived as a moire pattern•Instead, randomize halftone Instead, randomize halftone patternpatternDitheringHalftoning for color imagesHalftoning for color imagesHalftoning for color imagesHalftoning for color imagesSpecifying ColorColor perception usually involves three quantities:Color perception usually involves three quantities:•HueHue: Distinguishes between colors like red, green, blue, etc: Distinguishes between colors like red, green, blue, etc•SaturationSaturation: How far the color is from a gray of equal intensity: How far the color is from a gray of equal intensity•LightnessLightness: The perceived intensity of a reflecting object: The perceived intensity of a reflecting objectSometimes lightness is called Sometimes lightness is called brightness brightness if the object is if the object is emitting light instead of reflecting it.emitting light instead of reflecting it.In order to use color precisely in computer graphics, we In order to use color precisely in computer graphics, we need to be able to specify and measure colors.need to be able to specify and measure colors.Color perception usually involves three quantities:Color perception usually involves three quantities:•HueHue: Distinguishes between colors like red, green, blue, etc: Distinguishes between colors like red, green, blue, etc•SaturationSaturation: How far the color is from a gray of equal intensity: How far the color is from a gray of equal intensity•LightnessLightness: The perceived intensity of a reflecting object: The perceived intensity of a reflecting objectSometimes lightness is called Sometimes lightness is called brightness brightness if the object is if the object is emitting light instead of reflecting it.emitting light instead of reflecting it.In order to use color precisely in computer graphics, we In order to use color precisely in computer graphics, we need to be able to specify and measure colors.need to be able to specify and measure colors.Combining ColorsAdditive (RGB)Shining colored lightson a white ballSubtractive (CMYK)Mixing paint colors andilluminating with white lightHow Do Artists Do It?Artists often specify color as tints, shades, and tones Artists often specify color as tints, shades, and tones of saturated (pure) pigmentsof saturated (pure) pigments•TintTint:: Gotten by adding white to a pure pigment, Gotten by adding white to a pure pigment, decreasing saturationdecreasing saturation•ShadeShade:: Gotten by Gotten by adding adding black to a pure pigment, black to a pure pigment, decreasing lightnessdecreasing lightness•ToneTone:: Gotten by adding Gotten by adding white and black to a pure white and black to a pure pigmentpigmentArtists often specify color as tints, shades, and tones Artists often specify color as tints, shades, and tones of saturated (pure) pigmentsof saturated (pure) pigments•TintTint:: Gotten by adding white to a pure pigment, Gotten by adding white to a pure pigment, decreasing saturationdecreasing saturation•ShadeShade:: Gotten by Gotten by adding adding black to a pure pigment, black to a pure pigment, decreasing lightnessdecreasing lightness•ToneTone:: Gotten by adding Gotten by adding white and black to a pure white and black to a pure pigmentpigmentWhitePure ColorBlackGraysTintsShadesTonesHSV Color SpaceComputer scientists frequently use an intuitive Computer scientists frequently use an intuitive color space
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