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Color as Attention

Color as Attention. Color. What is Color?. Colors are different wavelengths (frequencies) of light Only some colors occur as pure wavelengths; many are mixtures of pure colors (e.g. white). What is Color?. What is Color?.

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Color as Attention

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  1. Color as Attention

  2. Color

  3. What is Color? • Colors are different wavelengths (frequencies) of light • Only some colors occur as pure wavelengths; many are mixtures of pure colors (e.g. white)

  4. What is Color?

  5. What is Color? • We almost never see a “pure” wavelength of light; rather a mixture of wavelengths, each with a different “power” • Only some colors occur as pure wavelengths; many are mixtures of pure colors (e.g. white)

  6. How Do We See Color? There are three kinds of cones in our retina: S, M, and L. They see Blue, Green, and Reddish light. “Red” cone “Blue” cone “Green” cone

  7. EXAMPLE: THE HUMAN EYE

  8. THE HUMAN EYE: RESPONSE R(l) b(l) g(l) r(l)

  9. Color The human eye can see 7,000,000 colors Yellow is the most fatiguing color (reflectance) (Babies cry more in yellow rooms)

  10. 1 CCD cameras A Bayer pattern is placed in front of the CCD A Demosaicingprocess reads the pixels in a region and computes color and intensity How Color Cameras Work * CCD - charge-coupled device

  11. 3 CCD camera use a beam splitter and 3 separate CCDs higher color fidelity needs lots of light requires careful alignment of ccds How Color Cameras Work

  12. Standard Linear Color Systems • Several standards are used to define “color” based on spectral response functions • CIE (Commission International d’Eclairage) establishes standards • CIE XYZ is a popular standard with everywhere positive response • RGB requires a negative (subtractive) component in R response • YIQ, YUV

  13. RGB Color System RED magenta yellow BLUE GREEN cyan RGB

  14. ANOTHER LINEAR SCHEME FOR REPRESENTING COLOR • Invented for color television (NTSC) • Backward compatible with B/W TV (Y val) • Y given higher bandwidth than I/Q YIQ (also YUV)

  15. cos-1(x) if G > B H = p - cos-1(x) if G < B Dividing Up Color Space Hue HSI is a nonlinear representation of color space. Note the non-uniform treatment of color Saturation I = (R+G+B)/3 or L = .3 R + .6G + .1B Intensity S = 1- 3 min(R,G,B)/I x= (R-G) + (R-B)/((R-G)2 + (R-B)(G-B))1/2

  16. THE ORGANIZATION OF A 2D IMAGE Pixel Binary 1 bit Grey 1 byte Color 3 bytes

  17. Storing Images • Non-lossy schemes • pbm/pgm/ppm/pnm • code for file type, size, number of bands, and maximum brightness • tif (lossless and lossy versions) • bmp • gif (grayscale) • Lossy schemes • gif (color) • jpg • uses Y Cb Cr color representation; subsamples the color • Uses DCT on result • Uses the fact the human system is less sensitive to color than spatial detail

  18. RED GREEN BLUE 8-bit index 0 R0 G0 B0 1 R1 G1 B1 2 R2 G2 B2 254 R254 G254 B254 255 R255 G255 B255 GIF IMAGE FORMAT • GIF (Graphics Interchange Format) • Limited to 8 bits/pixel for both color and gray-scale.

  19. TIFF IMAGE FORMAT • TIFF (Tagged Image File Format) • More general than GIF • Allows 24 bits/pixel • Supports 5 types of image compression including: • RLE (Run length encoding) • LZW (Lempel-Ziv-Welch) • JPEG (Joint Photographic Experts Group)

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