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Chapter 6 Color

Chapter 6 Color. Multimedia Systems. Key Points. Colour is a subjective sensation produced in the brain. The measurable physical quantity that best models colour is a spectral power distribution (SPD) .

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Chapter 6 Color

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  1. Chapter 6Color Multimedia Systems

  2. Key Points • Colour is a subjective sensation produced in the brain. • The measurable physical quantity that best models colour is a spectral power distribution (SPD). • In the RGB colour model, a colour is represented by three values, giving the proportions of red (R), green (G) and blue (B) light which make up light of the desired colour.

  3. Key Points • The RGB colour gamut is a proper subset of the visible colours. • The colour depth of an image is number of bits used to hold a colour value. • Indexed colour: The stored 8-bit value is used as an offset into a colour lookup table (CLUT) or palette containing up to 256 24-bit colour values.

  4. Key Points • Dithering helps avoid the posterization that can occur when indexed colour is used, by using patterns of dots to simulate a wider range of colours using optical mixing. • The CMYK model is important for printing, as it models the way colour is produced by mixing ink or paint. • Other colour models include HSV, YUV and its variants, and the CIE device-independent spaces Lab and Luv.

  5. Key Points • A 24-bit image can be thought of as being made up of three 8-bit channels, one each for red, green and blue in RGB colour.Colour correction etc. can be performed by using techniques from Chapter 5 on individual channels. • Colour management software uses device profiles to help maintain consistent colour.

  6. Color & Science • Color is a subjective sensation produced in the brain.

  7. Color & Science • Light is a form of electromagnetic radiation, we can measure its wavelength. • http://micro.magnet.fsu.edu/primer/lightandcolor/index.html

  8. Microwave Ovens • The wavelength is tuned to frequencies that are readily absorbed by water molecules in food causing them to absorb energy and release heat as they vibrate. • Containers holding the food in microwave ovens usually contain very little water and tend to remain cool.

  9. Color & Science • Visible light (可見光): 400nm~700nm

  10. Color & Science • Spectral power distribution (SPD)

  11. Color & Science • Receptor Cells • Rods: night-vision and cannot distinguish color • Cones: in three sorts which respond to different wavelengths

  12. Cone Response

  13. Tristimulus Theory • Any colors can be specified by just three values, giving weights of three components. • Each type of cone responds to R, G, B

  14. Primary Colors • Additive mixing • TV, monitors • Subtractive mixing • Paint absorbs light

  15. Color Temperature • The color temperature model is based on the relationship between the temperature of a theoretical standardized material, called a black body radiator, and the energy distribution of its emitted light as the radiator is brought to increasingly higher temperatures, measured in Kelvin (K). • Although this radiator does not exist, many metals behave very similar to a black body so we can take a metal pot as an example for this discussion.

  16. Color Temperature

  17. RGB Colors • RGB color model • Not possible to represent all visible color as a combination of fixed R, G, B.

  18. Other Color Spaces

  19. Color Depth • Color depth • 24, 30, 36, 48 bits • Grey scale image: R=G=B • PNG format: up to 48 bits • Scanner: 48 bits • Accurate approximations when reduced to a lower resolution for display • Millions of colors, true color: 24 bitsThousands of colors, hi-color: 16 bits256 colors: 8 bits • Grey-level: 256 = 8 bits

  20. Indexed Color • Direct color • Indexed color • Palette of 256 colors • Color lookup table (CLUT) • Logical colors => physical colors • 24 bits color => indexed color • 1/3 of data • PNG, BMP, TGA, TIFF: paletteGIF: only 256 colors JPEG: 24 bit color • Missing color: the nearest color • Visible artifacts • Posterization: gradations replace by sharp boundaries • Web-safe Palette • 216 colors, Displayed on any platform

  21. Posterization • Area of a single color are replaced by a pattern of dots of several colors • Pink = red + white • Dithering: high resolution • Halftoning: greyscale images • N by N pixels = N2+1 grey levels

  22. Other Color Models • CMYK • C = G+B = W-RM = R+B = W-GY = R+G = W-B • Complementary color • Art and design • The dyes do not supply color to light reflected off surface, but to light that penetrates through them and gets reflected or scattered back from beneath it.

  23. Mixing Colored Inks White light White light Cyan light Cyan light Absorption of green light Absorption of red light Magenta Cyan ink Absorption of red light Cyan ink

  24. Not possible to manufacture inks which absorb only light of precisely the complementary color.Some unwanted colors are absorbed. • RGB gamut > CMY gamut • C+M+M != perfect black • With black: CMYK • Hue, saturation, brightness • As white is mixed in, saturation decreases.

  25. Color wheel

  26. HSV

  27. Color Pickers

  28. Color Differences • Brightness=(R+G+B)/3 • LuminanceY=0.2125R + 0.7154G + 0.0721BGreen contributes far more • Color differences • U=B-Y and V=R-Y • YUV • YCBCR

  29. Device-independent Color Spaces • RGB, CMYK: device-dependent • CIE XYZ: device-independent • Not perceptually uniformthe same change in one of vales produced the same change in appearanceR: (1 to 11) != (101 to 111) • Perceptually uniform • CIE L*a*b*: as in CMYK for prepress industry • CIE L*u*v*: as in RGB for monitors and scanners

  30. Channels and Color Correction • Color images = R + B + C channels • Each channel can be manipulated separately. • Color correction • Compensate for the deficiencies of scanners and other input devices • Time-consuming • Wizards, assistants

  31. Color Correction • Color balance + hue & saturation adjustment • Color balance • 3 sliders: C+R, M+G, Y+B

  32. Replace a specified color with a different color • CMYK: printing YCBCR: video • Alpha channeldifferent from color channels • Blue screen: alpha = blue channel • Mask out background • JPEG: process each channel separately • YCBCR: downsample color difference

  33. Consistent Color • Color profiles • White=(255,255,255)specified in a device-independent color space • Gamma

  34. Gamma Correction Sample Input Graph of Input Gamma Corrected Input Graph of Correction L' = L ^ (1/2.5) Monitor Output Graph of Output

  35. Color Profile • EPS, TIFF JFIF, PNG: color profile • At least R, G, B, white point, gamma • Fig. 6.13

  36. ICC • International Color Consortium • Apple’s ColorSync • Kodak Precision Color Management System • TIFF, EPS: complete ICC profiles • sRGB: standard RGB • ITU-R BT.709 standard • Gamma=2.2 and white point= 6500 K

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