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Exploring Color: Perception, Technology, and Representation

Discover the fascinating world of color through its scientific principles, visual perception, and technological applications in cameras and image formats. Learn about the complexities of hues, saturation, and intensity, and how they are represented in digital images. Explore the standards defining color, from CIE XYZ to RGB and YUV, as well as various color spaces like HSI. Delve into the storage and compression of color images, from lossless schemes like TIFF to lossy formats like JPG, and the unique characteristics of GIF. Unravel the intricacies of color encoding and decoding, and gain insights into the diverse aspects of color in both natural perception and artificial display.

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Exploring Color: Perception, Technology, and Representation

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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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