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Computer Graphics. Color. Basics Of Color. elements of color:. Basics of Color. Physics: Illumination Electromagnetic spectra Reflection Material properties Surface geometry and microgeometry Perception Physiology and neurophysiology Perceptual psychology. Electromagnetic Spectrum.
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Computer Graphics Color
Basics Of Color • elements of color:
Basics of Color • Physics: • Illumination • Electromagnetic spectra • Reflection • Material properties • Surface geometry and microgeometry • Perception • Physiology and neurophysiology • Perceptual psychology
How well do we see color? • What color do we see the best? • Yellow-green at 550 nm • What color do we see the worst? • Blue at 440 nm • Flashback: Color tables (color maps) for color storage
Humans and Light • when we view a source of light, our eyes respond to • hue: the color we see (red, green, purple) • dominant frequency • saturation: how far is color from grey • how far is the color from gray (pink is less saturated than red, sky blue is less saturated than royal blue) • brightness: how bright is the color • how bright are the lights illuminating the object?
Hue • hue (or simply, "color") is dominant wavelength • integration of energy for all visible wavelengths is proportional to intensity of color
Saturation or Purity of Light • how washed out or how pure the color of the light appears • contribution of dominant light vs. other frequencies producing white light
Intensity, Brightness • intensity : radiant energy emitted per unit of time, per unit solid angle, and per unit projected area of the source (related to the luminance of the source) • brightness : perceived intensity of light
Combining Colors Additive (RGB) Shining colored lightson a white ball Subtractive (CMYK) Mixing paint colors andilluminating with white light
Colour Matching Experiment Mixing of 3 primaries Target colour overlap Adjust intensities to match the colour
RGB Color Space (Color Cube) • Define colors with (r, g, b) amounts of red, green, and blue
CMY Color Model CMY(short for Cyan, Magenta, Yellow, and key) is a subtractive color model.
The CMY Color Model • Cyan, magenta, and yellow are the complements of red, green, and blue • We can use them as filters to subtract from white • The space is the same as RGB except the origin is white instead of black • This is useful for hardcopy devices like laser printers • If you put cyan ink on the page, no red light is reflected
YIQ Color Space • YIQis the color model used for color TV in America. Y is brightness,I & Qare color • Note: Yis the same as CIE’s Y • Result: Use the Y alone and backwards compatibility with B/W TV! • These days when you convert RGB image to B/W image, the green and blue components are thrown away and red is used to control shades of grey (usually)
Converting Color Spaces • Converting between color models can also be expressed as such a matrix transform: • Note the relative unimportance of blue in computing the Y
HSV Color Space • A more intuitive color space • H = Hue • S = Saturation • V = Value (or brightness) Saturation Value Hue
HSV Color Model H S V Color 0 1.0 1.0 Red 120 1.0 1.0 Green 240 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 ? Figure 15.16&15.17 from H&B
Halftoning • A technique used in newspaper printing • Only two intensities are possible, blob of ink and no blob of ink • But, the size of the blob can be varied • Also, the dither patterns of small dots can be used
Spatial versus Intensity Resolution • Halftone Approximation: Dither • n ´ n pixels encode n2 + 1 intensity levels • The distribution of intensities is randomized: dither noise, to avoid repeating visual artifacts
Dithering • Halftoning for color images
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