Information Technology Reference
In-Depth Information
The remote vectors which are used and the weighting factors are both a function of the pixel position in the block.
Figure 5.51 shows that the block to be coded is divided into quadrants. The remote vectors are selected from the
blocks closest to the quadrant in which the pixel resides. For example, a pixel in the bottom-right quadrant would
be predicted using remote vectors from the DCT block immediately below and the block immediately to the right.
Figure 5.51:
(a) MPEG-4 inter-coded pixel values may be predicted from three remote vectors as well as the true
vector. (b) The three remote vectors are selected according to the macroblock quadrant. Macroblocks may have
one or four vectors and the prediction mechanism allows prediction between blocks of either type.
Not all blocks can be coded in this way. In P-VOPs it is permissible to have blocks which are not coded or intra-
blocks which contain no vector. Remote vectors will not all be available at the boundaries of a VOP. In the normal
sequence of macroblock transmission, vectors from macroblocks below the current block are not yet available.
Some additional steps are needed to handle these conditions. Adjacent to boundaries where a remote vector is not
available it is replaced by a copy of the actual vector. This is also done where an adjacent block is intra-coded and
for blocks at the bottom of a macroblock where the vectors for the macroblocks below will not be available yet. In
the case of non-coded blocks the remote vector is set to zero.
Figure 5.52(a) shows that the weighting factors for pixels near the centre of a block favour that block. In the case of
pixels at the corner of the block, the weighting is even between the value obtained from the true vector and the sum
of the two pixel values obtained from remote vectors.
Figure 5.52:
Vector prediction.
The weighted sum produces a predicted pixel which is subtracted from the actual pixel in the current VOP to be
coded to produce a residual pixel. Blocks of residual pixels are DCT coded as usual. OBMC reduces the magnitude
of residual pixels and gives a corrsponding reduction in the number or magnitude of DCT coefficients to be coded.
OBMC is lossless because the decoder already has access to all the vectors and knows the weighting tables.
Consequently the only over- head is the transmission of a flag which enables or disables the mechanism.
