A macroblock consists of a 4x4 array of blocks, and there
are three possible ways of splitting a MB:
Splitting level 0: no split, a single MV per reference frame for
the MB;
Splitting level 1: split into four sub-macroblocks (sub-MBs), each
a 2x2 array of blocks, one MV per reference frame per
sub-MB;
Splitting level 2: split into the 16 constituent blocks.
Figure: Macroblock splitting modes
The splitting mode is chosen by redoing motion estimation
for the sub-MBs and the MB as a whole, again using the
RDO metric , suitably scaled to
take into account the different sizes of the blocks. At the
same time, the best prediction mode for each prediction unit (block,
sub-MB or MB) is chosen. Four
prediction modes are available:
INTRA: intra coded, predicted by DC value;
REF1_ONLY: only predict from the first reference;
REF2_ONLY: only predict from the second reference (if
one exists);
REF1AND2: bi-directional prediction.
The result is a hierarchy of parameters: the splitting level
determines what modes, motion
vectors and block DC values (in the case of INTRA) need to be present.
In motion estimation, an overall cost for each MB is
computed, and compared for each legal combination of these
parameters. This is a tricky operation, and has a very significant
effect on performance. The decisions interact very heavily with
those made in coding the wavelet coefficients of the resulting
residuals, and the best results probably depend on picture material,
bit rate, the block size and its relationship to the size of the video frames, and the
degree of perceptual weighting used in selecting quantisers for wavelet
coefficients.
Dirac can use any block sizes, although blocks parameters do have to
meet some constraints, so
that the overlapping process works properly, especially in conjunction with
subsampled chroma components (for which the blocks will be correspondingly smaller).
For example, the block separations and corresponding lengths must differ by a multiple
of four, so that overlap is symmetric for luma and sub-sampled chroma.