Chapter 4 Color in Image and Video.ppt

* YIQ Color Model YIQ is used in NTSC color TV broadcasting. Again, gray pixels generate zero (I, Q) chrominance signal. I and Q are a rotated version of U and V . Y’ in YIQ is the same as in YUV; U and V are rotated by 330 I= 0.492111(R’? Y’) cos 330 ? 0.877283(B’? Y’) sin 330 Q= 0.492111(R’? Y’) sin 330 + 0.877283(B’? Y’) cos 330 This leads to the following matrix transform: Fig. 4.19 shows the decomposition of the same color image as above, into YIQ components. * YIQ Color Model Fig.4.19: I and Q components of color image. * YCbCr Color Model The Rec. 601 standard for digital video uses another color space,Y CbCr, often simply written YCbCr — closely related to the YUV transform. YUV is changed by scaling such thatCb is U , but with a coefficient of 0.5 multiplying B’. In some software systems, Cb and Cr are also shifted such that values are between 0 and 1. This makes the equations as follows: Cb= ((B’?Y’)/1.772) + 0.5 Cr= ((R’?Y’)/1.402) + 0.5 (4.33) Written out: * YCbCr Color Model In practice, however, Recommendation 601 speci?es 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 ?oats in [0.. + 1], then we obtain Y’, C’, Cr in [0..255] via the transform: The YCbCr transform is used in JPEG image compression and MPEG video compression. * 4.4 Further Exploration Links in the Chapter 4 section of the “Further Exploration” directory on the text website include: More details on gamma correction for publication on the WWW. The full speci?cation of the new sRGB standard color space for WWW applications. A link to an excellent review of color transforms. A Matlab script to exercise (and expand upon) the color transform functions that are part of the Image Toolbox in Matlab: the standard ‘Lena’ image is transformed to YIQ and to YCbCr. The new MPEG standard, MPEG-7