Information Technology Reference
In-Depth Information
Figure 5.48:
(a) The texture decoding system of MPEG-4. (b) A comparison of MPEG-2 and MPEG-4 shows that
the latter is more refined.
When inter-coding, MPEG-4 may use motion-compensated bidirectional coding as for MPEG-2, but in doing so it
also uses a more advanced form of lossless vector compression which may then be used to increase the vector
density to one vector per DCT block. In inter-coded pictures the prediction of the picture is improved so that the
residual to be coded is smaller.
When intra-coding, MPEG-4 looks for further redundancy between coefficients using prediction. When a given DCT
block is to be intracoded, certain of its coefficients will be predicted from adjacent blocks. The choice of the most
appropriate block is made by measuring the picture gradient, defined as the rate of change of the DC coefficient.
Figure 5.49(a) shows that the three adjacent blocks, A, B and C, are analysed to decide whether to predict from the
DCT block above (vertical prediction) or to the left (horizontal prediction). Figure 5.49(b) shows that in vertical
prediction the top row of coefficients is predicted from the block above so that only the differences between them
need to be coded.
Figure 5.49(c) shows that in horizontal prediction the left column of coefficients is predicted from the block on the
left so that again only the differences need be coded.
Choosing the blocks above and to the left is important because these blocks will already be available in both the
encoder and decoder. By making the same picture gradient measurement, the decoder can establish whether
vertical or horizontal prediction has been used and so no flag is needed in the bitstream.
Some extra steps are needed to handle the top row and the left column of a picture or object where true prediction
is impossible. In these cases both encoder and decoder assume standardized constant values for the missing
prediction coefficients.
The picture gradient measurement determines the direction in which there is the least change from block to block.
There will generally be fewer DCT coefficients present in this direction. There will be more coefficients in the other
axis where there is more change. Consequently it is advantageous to alter the scanning sequence so that the
coefficients which are likely to exist are transmitted earlier in the sequence.
