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Fundamentals of Multimedia Chapter 4 Color in Image and Video Ze-Nian Li and Mark S. Drew

Fundamentals of Multimedia Chapter 4 Color in Image and Video Ze-Nian Li and Mark S. Drew. 건국대학교 인터넷미디어공학부 임 창 훈. Outline. 4.1 Color Science (skip) 4.2 Color Models in Images 4.3 Color Models in Video. 4.1 Color Models in Images. Fig. 4.15: RGB and CMY color cubes.

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Fundamentals of Multimedia Chapter 4 Color in Image and Video Ze-Nian Li and Mark S. Drew

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  1. Fundamentals of MultimediaChapter 4Color in Image and VideoZe-Nian Li and Mark S. Drew 건국대학교 인터넷미디어공학부 임 창 훈

  2. Outline 4.1 Color Science (skip) 4.2 Color Models in Images 4.3 Color Models in Video Li & Drew; 인터넷미디어공학부 임창훈

  3. 4.1 Color Models in Images Fig. 4.15: RGB and CMY color cubes Li & Drew; 인터넷미디어공학부 임창훈

  4. Additive and Subtractive Color • Additive color: When two light beams impinge on a target, their colors add. When two phosphors on a CRT screen are turned on, their colors add. (red + green = yellow) • Subtractive color: For ink on paper, the opposite situation holds: yellow ink subtracts blue from white illumination, but reflects red and green; it appears yellow. (white – blue = yellow) Li & Drew; 인터넷미디어공학부 임창훈

  5. Subtractive Color: CMY Color Model • Instead of red, green, and blue primaries, we need primaries that amount to -red, -green, and -blue. I.e., we need to subtractR, or G, or Bfrom White (W). • These subtractive color primaries are • Cyan (C), Magenta (M) and Yellow (Y) inks. • C = W – R, (0, 1, 1) = (1, 1, 1) – (1, 0, 0) • M = W – G, (1, 0, 1) = (1, 1, 1) – (0, 1, 0) • Y = W – B, (1, 1, 0) = (1, 1, 1) – (0, 0, 1) Li & Drew; 인터넷미디어공학부 임창훈

  6. Transformation from RGB to CMY • Simplest model we can invent to specify what ink density to lay down on paper, to make a certain desired RGB color: • Then the inverse transform is: Li & Drew; 인터넷미디어공학부 임창훈

  7. Fig. 4.16: Additive and subtractive color. (a): RGB is used to specify additive color. (b): CMY is used to specify subtractive color Li & Drew; 인터넷미디어공학부 임창훈

  8. Undercolor Removal: CMYK System • Undercolor removal • Sharper and cheaper printer colors: • Calculate that part of the CMY mix that would be black, remove it from the color proportions, and add it back as real black (K). • The new specification of inks is thus: Li & Drew; 인터넷미디어공학부 임창훈

  9. 4.2 Color Models in Video • Largely derive from older analog methods of coding color for TV. Luminance is separated from color information. • YIQ is used to transmit TV signals in North America and Japan (NTSC). • In Europe, video tape uses the PAL or SECAM, which are based on TV that uses a matrix transform called YUV. • Digital video mostly uses a matrix transform called YCbCr that is closely related to YUV Li & Drew; 인터넷미디어공학부 임창훈

  10. YUV Color Model • YUV codes a luminance (luma) signal equal to Y’ • (for gamma-corrected signals) • Chrominance (chroma) refers to the difference • between color and luminance. • color differences U, V • U = B’ – Y’, V = R’ – Y’ Li & Drew; 인터넷미디어공학부 임창훈

  11. YUV Color Model • For composite video, • Chrominance signal C in composite video Li & Drew; 인터넷미디어공학부 임창훈

  12. YUV Color Model original color image V Y’ U Li & Drew; 인터넷미디어공학부 임창훈

  13. YIQ Color Model • YIQ is used in analog NTSC color TV broadcasting • I and Q are rotated version of U and V Li & Drew; 인터넷미디어공학부 임창훈

  14. YCbCr Color Model • Digital video uses YCbCr model – closely related to YUV • YUV is changed to YCbCr by scaling. • Cb andCrare shifted between 0 and 1. Li & Drew; 인터넷미디어공학부 임창훈

  15. YCbCr Color Model • In practice, in 8-bit coding, with a maximum Y’ value of • only 219, and a minimum of +16. • Cb and Cr have a range of 112 and offset of +128. If R’, G’, B’ are floats in [0, 1], then we obtain Y’, Cb, Crin [0, 255] via the transform: • The YCbCr transform is used in JPEGimage compression and MPEGvideo compression. Li & Drew; 인터넷미디어공학부 임창훈

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