7.5 Quantisation weighting matrix

7.5 Quantisation weighting matrix

The insensitivity of the human visual system to high frequency distortions can be exploited for further bandwidth compression. In this case, the higher orders of DCT coefficients are quantised with coarser quantisation step sizes than the lower frequency ones. Experience has shown that for SIF pictures a suitable distortion weighting matrix for the intra DCT coefficients is the one shown in Figure 7.8. This intra matrix is used as the default quantisation matrix for intraframe coded macroblocks.

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Figure 7.8: Default intra and inter quantisation weighting matrices

If the picture resolution departs significantly from the SIF size, then some other matrix may give perceptively better results. The reason is that this matrix is derived from the vision contrast sensitivity curve, for a nominal viewing distance (e.g. viewing distances of 4–6 times the picture height) [7]. For higher or lower picture resolutions, or changing the viewing distance, the spatial frequency will then change, and hence different weighting will be derived.

It should be noted that different weightings may not be used for interframe coded macroblocks. This is because high frequency interframe error does not necessarily mean high spatial frequency. It might be due to poor motion compensation or block boundary artefacts. Hence interframe coded macroblocks use a flat quantisation matrix. This matrix is called the inter or nonintra quantisation weighting matrix.

Note that, since in H.261 all the pictures are interframe coded and a very few macroblocks might be intra coded, then only the nonintra weighting matrix is defined. Little work has been performed to determine the optimum nonintra matrix for MPEG-1, but evidence suggests that the coding performance is more related to the motion and the texture of the scene than the nonintra quantisation matrix. If there is any optimum matrix, it should then be somewhere between the flat default inter matrix and the strongly frequency-dependent values of the default intra matrix.

The DCT coefficients, prior to quantisation, are divided by the weighting matrix. Note that the DCT coefficients prior to weighting have a dynamic range from -2047 to +2047. Weighted coefficients are then quantised by the quantisation step size and at the decoder, reconstructed quantised coefficients are multiplied to the weighting matrix to reconstruct the coefficients.



Standard Codecs(c) Image Compression to Advanced Video Coding
Standard Codecs: Image Compression to Advanced Video Coding (IET Telecommunications Series)
ISBN: 0852967101
EAN: 2147483647
Year: 2005
Pages: 148
Authors: M. Ghanbari

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