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Chapter 13: Color Processing

Chapter 13: Color Processing. 。 Color: An important descriptor of the world 。 The world is itself colorless 。 Color is caused by the vision system responding differently to different wavelengths of light. 。 Image color depends on:

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Chapter 13: Color Processing

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  1. Chapter 13: Color Processing • 。 Color: An important descriptor of the • world • 。 The world is itself colorless • 。 Color is caused by the vision system • responding differently to different • wavelengths of light.

  2. 。 Image color depends on: • (1) The color of the incidence light • (2) The color of the scene surface • (3) The nature of the visual sensor

  3. ○ The Human Eye

  4. Two kinds of photoreceptors: rods, cones

  5. Rods -- sensitive to light Cones -- sensitive to color Three types of cones:

  6. ○ RGB Color Space -- Many colors are made up • of varying amounts of red, green and blue • R, G, B: primary colors, real : color matching functions may be negative

  7. ○ CIE XYZ Color Space • CIE (Commission Internationale d’Eclairage): • an organization responsible for color standard X,Y,Z: not real primaries, Y: luminance Their color matching functions are positive everywhere 。 The volume of visible colors in CIE XYZ space is a cone

  8. 。The relationship between RGB and XYZ

  9. ○ CIE xy Color Space -- A constant brightness section intersects the XYZ space with the plane Since x + y + z = 1, a color can be specified by x and y alone.

  10. 。 Chromaticity Diagram • Spectral locus: the • curved boundary • along which colors • of single wavelengths • are viewed • (ii) Neutral point: the • color whose weights • are equal for all • three primaries • (iii) Colors that lie farther away from the • neutral point are more saturated

  11. 。RGB Gamut – The colors correspond to • positive matching values

  12. 。Secondary colors (primaries of pigments): • Magenta (purple) = R + B = W - G • Cyan = G + B = W - R • Yellow = R + G = W - B • 。Pigments remove color from incident light, • which is reflected from paper • e.g., Red ink absorbs green and blue light; • incident red light passes through the • ink and is reflected from the paper

  13. Hue: varies from red  green Saturation: varies from red  pink Brightness: varies from black  white • ○HSV (Hue, Saturation, Value) Color Space

  14. ○ (i) RGBHSV • If R = V, then • If G = V, then • If B = V, then • If H ends up being negative, add 1 • If (R,G,B) = (0,0,0), then (H,S,V) = (0,0,0)

  15. 。 Example: (R, G, B) = (0.2, 0.4, 0.6)

  16. (ii) HSV RGB

  17. 。 Example: (H, S, V) = (0.5833, 0.6667, 0.6)

  18. ○YIQ Color Space – Used for TV and video • Y : luminance information • I, Q : color information

  19. ○ Uniform Color Space • -- The distance in the space is a guide to • color difference • 。 Noticeable difference – the difference when • modifying a color until one can tell it has • changed • 。 Macadam ellipse -- the noticeable difference • of a color forms the boundary of the color in • a color space and can be fitted with an ellipse

  20. The color difference in CIE xy space is poor: (a) the ellipses at the top are larger than those at the bottom (b) the ellipses rotate as they move up

  21. 。 CIE u’v’ Color Space – a more uniform • space than the CIE xy color space

  22. ○ CIE Lab Color Space • – another substantial uniform space where : the XYZ coordinates of a reference white patch

  23. ◎ Color Images

  24. ◎ Pseucoloring • 。 Intensity Slicing

  25. 。Transformation • Define colormap functions:

  26. ◎ Processing of Color Images • Two methods: • (a) (b)

  27. ○ Noise Reduction R G B Apply median filter to Y Apply median filter to R,G,B

  28. ○ Contrast Enhancement • Perform on the intensity component • (1) RGB  YIQ • (2) Apply histogram equalization to • Y  Y’ • (3) Y’IQ  R’G’B’

  29. ○ Spatial Filtering • Both low- and high- pass filters are better • off applying to the intensity component

  30. ○ Edge Detection • Two ways: • (1) Apply edge detection to the intensity • component • (2) Apply edge detection to each RGB • component

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