1. Chapter 6 Color 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/inde x.html http://micro.magnet.fsu.edu/primer/lightandcolor/inde x.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 Rods: night-vision and cannot distinguish color Cones: in three sorts which respond to different wavelengths 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 TV, monitors Subtractive mixing Paint absorbs light 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 24, 30, 36, 48 bits Grey scale image: R=G=B Grey scale image: R=G=B PNG format: up to 48 bits PNG format: up to 48 bits Scanner: 48 bits Scanner: 48 bits Accurate approximations when reduced to a lower resolution for display Millions of colors, true color: 24 bits Thousands of colors, hi-color: 16 bits 256 colors: 8 bits Millions of colors, true color: 24 bits Thousands of colors, hi-color: 16 bits 256 colors: 8 bits Grey-level: 256 = 8 bits Grey-level: 256 = 8 bits

20. Indexed Color Direct color Indexed color Palette of 256 colors Palette of 256 colors Color lookup table (CLUT) Color lookup table (CLUT) Logical colors => physical colors 24 bits color => indexed color 24 bits color => indexed color 1/3 of data PNG, BMP, TGA, TIFF: palette GIF: only 256 colors JPEG: 24 bit color PNG, BMP, TGA, TIFF: palette GIF: only 256 colors JPEG: 24 bit color Missing color: the nearest color Visible artifacts Visible artifacts Posterization: gradations replace by sharp boundaries Posterization: gradations replace by sharp boundaries Web-safe Palette 216 colors, Displayed on any platform 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 Pink = red + white Dithering: high resolution Halftoning: greyscale images N by N pixels = N 2 +1 grey levels N by N pixels = N 2 +1 grey levels

22. Other Color Models CMYK C = G+B = W-R M = R+B = W-G Y = R+G = W-B C = G+B = W-R M = R+B = W-G Y = 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 Cyan ink Absorption of red light Cyan light White light Cyan ink Absorption of green light Blue light White light Magenta Absorption of red light

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+Y != perfect black C+M+Y != perfect black With black: CMYK Hue, saturation, brightness As white is mixed in, saturation decreases. As white is mixed in, saturation decreases.

25. Color wheel

26. HSV

27. Color Pickers

28. Color Differences Brightness=(R+G+B)/3 Luminance Y=0.2125R + 0.7154G + 0.0721B Green contributes far more Color differences U=B-Y and V=R-Y U=B-Y and V=R-Y YUV YUV YC B C R YC B C R

29. Device-independent Color Spaces RGB, CMYK: device-dependent CIE XYZ: device-independent Not perceptually uniform the same change in one of vales produced the same change in appearance R: (1 to 11) != (101 to 111) Perceptually uniform CIE L*a*b*: as in CMYK for prepress industry CIE L*a*b*: as in CMYK for prepress industry CIE L*u*v*: as in RGB for monitors and scanners 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 Compensate for the deficiencies of scanners and other input devices Time-consuming Time-consuming Wizards, assistants

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

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

33. Chromakey

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

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

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

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