Raster Graphics and ColorOverviewSlide 3Display HardwareCathode Ray Tube (CRT)Liquid Crystal Display (LCD)Slide 7Slide 8Raster Graphics SystemsFrame BufferFrame Buffer RefreshDirect Color FramebufferRed component vs. monochromaticColor Lookup FramebufferColor CRTSlide 16Specifying ColorHow Do Artists Do It?Additive color vs. Subtractive colorHSV Color ModelIntuitive Color SpacesPrecise Color SpecificationsElectromagnetic SpectrumVisible LightColor MatchingLinear Color MatchingRGB Spectral ColorsPerception of color intensitiesHuman Color VisionJust Noticeable DifferencesLuminanceChromaticity and the CIEXYZ Matching FunctionsCIE Color SpaceSpectral LocusChromaticity DiagramMeasuring ColorGamutsGamut problemsA Problem With XYZ ColorsThe RGB Color ModelMore on RGBThe CMY Color ModelCMYKCMY vs CMYKThe YIQ Color ModelHSV color space asideThe YCbCr Color ModelJPEG Image CompressionFuture of color displaysPhoto printersRaster Graphics and ColorAaron BloomfieldCS 445: Introduction to GraphicsFall 20062OverviewDisplay hardwareHow are images displayed?Raster graphics systemsHow are imaging systems organized?Color modelsHow can we describe and represent colors?All non-credited images in this slide set are from Wikipedia3OverviewDisplay hardwareHow are images displayed?Raster graphics systemsHow are imaging systems organized?Color modelsHow can we describe and represent colors?4Display HardwareVideo display devicesCathode Ray Tube (CRT)Liquid Crystal Display (LCD)Plasma panelsThin-film electroluminescent displaysLight-emitting diodes (LED)Hard-copy devicesInk-jet printerLaser printerFilm recorderElectrostatic printerPen plotter5Cathode Ray Tube (CRT)5. Anode connection6. Mask for separating beams for RGB part of displayed image 7. Phosphor layer with RGB zones8. Close-up of the phos-phor-coated inner side of the screen 1. Electron guns2. Electron beams3. Focusing coils4. Deflection coilsImage via Wikipedia: http://en.wikipedia.org/wiki/Cathode_ray_tube6Liquid Crystal Display (LCD)Figure 2.16 from H&B7Display HardwareVideo display devices»Cathode Ray Tube (CRT)»Liquid Crystal Display (LCD)Plasma panelsThin-film electroluminescent displaysLight-emitting diodes (LED)Hard-copy devicesInk-jet printerLaser printerFilm recorderElectrostatic printerPen plotter8OverviewDisplay hardwareHow are images displayed?Raster graphics systemsHow are imaging systems organized?Color modelsHow can we describe and represent colors?9Raster Graphics SystemsDisplayProcessorDisplayProcessorSystemMemorySystemMemoryCPUCPUFrameBufferFrameBufferMonitorVideoControllerVideoControllerSystem BusI/O DevicesFigure 2.29 from H&B10Frame BufferFrame BufferFigure 1.2 from FvDFH11Frame Buffer RefreshFigure 1.3 from FvDFH Refresh rate is usually 60-120 Hz for CRTs12DACDirect Color FramebufferStore the actual intensities of R, G, and B individually in the framebuffer24 bits per pixel = 8 bits red, 8 bits green, 8 bits blue13Red component vs. monochromaticThe red component only has the red components of each pixel (duh!)Monochromatic is a gray-scale image that uses another color instead of white14Color Lookup FramebufferStore indices (usually 8 bits) in framebufferDisplay controller looks up the R,G,B values before triggering the electron gunsColor indicesDAC15Color CRTFigure 2.8 from H&B16OverviewDisplay hardwareHow are images displayed?Raster graphics systemsHow are imaging systems organized?»Color modelsHow can we describe and represent colors?17Specifying ColorColor perception usually involves three quantities:Hue: Distinguishes between colors like red, green, blue, etcSaturation: How far the color is from a gray of equal intensityLightness: The perceived intensity of a reflecting objectSometimes lightness is called brightness if the object is emitting light instead of reflecting it.In order to use color precisely in computer graphics, we need to be able to specify and measure colors.18How Do Artists Do It?Artists often specify color as tints, shades, and tones of saturated (pure) pigmentsTint: Adding white to a pure pigment, decreasing saturationShade: Adding black to a pure pigment, decreasing lightnessTone: Adding white and black to a pure pigmentWhitePure ColorBlackGraysTintsShadesTones19Additive color vs. Subtractive colorAdditive colors models are used in lightStart with black, and add colored light to make your desired shadeSubtractive color models are used with paintStart with white, and add colorsA given color – red – subtracts away (from the reflected light) any wavelength that is not redAdditive color mixing:Subtractive color mixing:20HSV Color ModelFigure 15.16&15.17 from H&B H S V Color 0 1.0 1.0 Red120 1.0 1.0 Green240 1.0 1.0 Blue * 0.0 1.0 White * 0.0 0.5 Gray * * 0.0 Black 60 1.0 1.0 ?270 0.5 1.0 ?270 0.0 0.7 ?21Intuitive Color SpacesHSV is an intuitive color spaceCorresponds to our perceptual notions of tint, shade,and toneHue (H) is the angle around the vertical axisSaturation (S) is a value from 0 to 1 indicating how far fromthe vertical axis the color liesValue (V) is the height of the “hexcone”22Precise Color SpecificationsPigment-mixing is subjective --- depends on human observer, surrounding colors, lighting of the environment, etcWe need an objective color specificationLight is electromagnetic energy in the 400 to 700 nm wavelength rangeDominant wavelength is the wavelength of the color we “see”Excitation purity is the proportion of pure colored light to white lightLuminance is the amount (or intensity) of the light23Electromagnetic SpectrumVisible light frequencies range between ...Red = 4.3 x 1014 hertz (700nm)Violet = 7.5 x 1014 hertz (400nm)Figures 15.1 from H&B24Visible LightHue = dominant frequency (highest peak)Saturation = excitation purity (ratio of highest to rest)Lightness = luminance (area under curve)White Light Orange LightFigures 15.3-4 from H&B25Color MatchingIn order to match a color, we can adjust the brightness of 3 overlapping primaries until the two colors look the same.C = color to be matchedRGB = laser sources (R=700nm, G=546nm, B=435nm)Humans
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