Information Technology Reference
In-Depth Information
Figure 5.49:
(a) DC coefficients are used to measure the picture gradient. (b) In vertical prediction the top row of
coefficients are predicted using those above as a basis. (c) In horizontal prediction the left column of coefficients
are predicted from those to the left.
Figure 5.50 shows the two alternate scans for MPEG-4. The alternate horizontal scan concentrates on horizontal
coefficients early in the scan and will be used in conjunction with vertical prediction. Conversely the alternate
vertical scan concentrates on vertical coefficients early in the scan and will be used in conjunction with horizontal
prediction. The decoder can establish which scan has been used in the encoder from the picture gradient.
Figure 5.50:
The alternate zig-zag scans employed with vertical or horizontal prediction.
Coefficient prediction is not employed when inter-coding because the statistics of residual images are different.
Instead of attempting to predict residual coefficients, in inter-coded texture, pixel-based prediction may be used to
reduce the magnitude of texture residuals. This technique is known as
overlapped block motion compensation
(OBMC) which is only used in P-VOPs. With only one vector per DCT block, clearly in many cases the vector
cannot apply to every pixel in the block. If the vector is considered to describe the motion of the centre of the block,
the vector accuracy falls towards the edge of the block. A pixel in the corner of a block is almost equidistant from a
vector in the centre of an adjacent block.
OBMC uses vectors from adjacent blocks, known as
remote vectors
in addition to the vector of the current block for
prediction. Figure 5.51 shows that the motion-compensation process of MPEG-1 and MPEG-2 which uses a single
vector is modified by the addition of the pixel prediction system which considers three vectors. A given pixel in the
block to be coded is predicted from the weighted sum of three motion- compensated pixels taken from the previous
I- or P-VOP. One of these pixels is obtained in the normal way by accessing the previous VOP with a shift given by
the vector of this block. The other two are obtained by accessing the same VOP pixels using the remote vectors of
two adjacent blocks.
