Each 8 8 block of pixels is
Each 8 8 block of pixels is transformed to another 8 8 block of transform coefficients. Vertical edges in the picture transform as a horizontal frequency, and horizontal edges as vertical frequency coefficients. At this stage there has been no compression of the data. The goal is to gain a representation of the video signal in a form where perceptual and statistical redundancies can be used to reduce the data rate. The coefficients are normalized and quantized. The quantization process allows the high-energy, low-frequency coefficients to be coded with a greater number of bits, while using fewer for the high-frequency coefficients. So as we go from top left to bottom right, the coefficients tend toward zero. Video compression 85 low frequencies mid frequencies DC high frequencies vertical edges diagonal edges horizontal edges Figure 5.3 Spatial frequencies. Variable-length coding (VLC) By scanning the coefficients in a zigzag the result is long runs of zeros. The coefficients then are converted to a series of run-amplitude pairs. Each pair indicates the number of zero-value coefficients and the amplitude of the nonzero coefficient. These run-amplitude coefficients are then coded with a variablelength code. This uses shorter codes for commonly occurring pairs and longer codes for less common pairs a form of entropy coding. These can be runlength encoded to reduce to the overall data rate. Temporal or interframe compression Video is a sequence of similar images, with step changes at scene boundaries. In many sequences there is virtually no change from one frame to the next. In scenes with subject motion, or where the camera is moving, there will be
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