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UVA CS 445 - Raster Graphics and Color

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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 20062OverviewDisplay hardwareHow are images displayed?Raster graphics systemsHow are imaging systems organized?Color modelsHow can we describe and represent colors?All non-credited images in this slide set are from Wikipedia3OverviewDisplay hardwareHow are images displayed?Raster graphics systemsHow are imaging systems organized?Color modelsHow can we describe and represent colors?4Display HardwareVideo display devicesCathode Ray Tube (CRT)Liquid Crystal Display (LCD)Plasma panelsThin-film electroluminescent displaysLight-emitting diodes (LED)Hard-copy devicesInk-jet printerLaser printerFilm recorderElectrostatic printerPen 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 coilsImage via Wikipedia: http://en.wikipedia.org/wiki/Cathode_ray_tube6Liquid Crystal Display (LCD)Figure 2.16 from H&B7Display HardwareVideo display devices»Cathode Ray Tube (CRT)»Liquid Crystal Display (LCD)Plasma panelsThin-film electroluminescent displaysLight-emitting diodes (LED)Hard-copy devicesInk-jet printerLaser printerFilm recorderElectrostatic printerPen plotter8OverviewDisplay hardwareHow are images displayed?Raster graphics systemsHow are imaging systems organized?Color modelsHow 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 FramebufferStore the actual intensities of R, G, and B individually in the framebuffer24 bits per pixel = 8 bits red, 8 bits green, 8 bits blue13Red component vs. monochromaticThe 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 FramebufferStore indices (usually 8 bits) in framebufferDisplay controller looks up the R,G,B values before triggering the electron gunsColor indicesDAC15Color CRTFigure 2.8 from H&B16OverviewDisplay hardwareHow are images displayed?Raster graphics systemsHow are imaging systems organized?»Color modelsHow can we describe and represent colors?17Specifying ColorColor perception usually involves three quantities:Hue: Distinguishes between colors like red, green, blue, etcSaturation: How far the color is from a gray of equal intensityLightness: The perceived intensity of a reflecting objectSometimes 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) pigmentsTint: Adding white to a pure pigment, decreasing saturationShade: Adding black to a pure pigment, decreasing lightnessTone: Adding white and black to a pure pigmentWhitePure ColorBlackGraysTintsShadesTones19Additive color vs. Subtractive colorAdditive colors models are used in lightStart with black, and add colored light to make your desired shadeSubtractive color models are used with paintStart with white, and add colorsA given color – red – subtracts away (from the reflected light) any wavelength that is not redAdditive 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 SpacesHSV is an intuitive color spaceCorresponds to our perceptual notions of tint, shade,and toneHue (H) is the angle around the vertical axisSaturation (S) is a value from 0 to 1 indicating how far fromthe vertical axis the color liesValue (V) is the height of the “hexcone”22Precise Color SpecificationsPigment-mixing is subjective --- depends on human observer, surrounding colors, lighting of the environment, etcWe need an objective color specificationLight is electromagnetic energy in the 400 to 700 nm wavelength rangeDominant wavelength is the wavelength of the color we “see”Excitation purity is the proportion of pure colored light to white lightLuminance is the amount (or intensity) of the light23Electromagnetic SpectrumVisible light frequencies range between ...Red = 4.3 x 1014 hertz (700nm)Violet = 7.5 x 1014 hertz (400nm)Figures 15.1 from H&B24Visible LightHue = 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 MatchingIn 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 matchedRGB = laser sources (R=700nm, G=546nm, B=435nm)Humans


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UVA CS 445 - Raster Graphics and Color

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