motion estimation and compensation for very low bitrate video coding

1.4 Some (Very-) Low BitRate Codecs 23
Picture MB DCT Motion Residual Change
type type coef. vector coding quant.
Intra Intra
Intra Intra
Intra Stu ng
Inter Intra
Inter Intra
Inter Inter
Inter Inter
Inter Inter
Inter Stu ng
Table 1.2: Macroblock types and included data elements in H.263
component (one for Cb, one for Cr). The quantization matrix applied
to the frequency-based matrix of coe cients can be regularly updated
during the encoding process. There are thus three ways of transmitting
an intra macroblock, as illustrated in the three rst lines of table 1.2.
\Stu ng" means that the macroblock remains exactly the same as in
the previous frame.
1.4.1.2 Inter Macroblocks and Residues
In inter mode, a motion vector is rst searched for so as to determine the
origin of the macroblock. It is achieved by a Block Matching Algorithm
(BMA, cf. Section 2.5.1). Then, according to the relevance of the result,
the macroblock is either coded in intra mode or via the motion vector
and (possibly) additional residues: one of the inter picture modes of
table 1.2 is selected. In the case of inter coding with residues, the DFD
is coded as an intra macroblock with the DCT.
1.4.1.3 Options
In addition to this basic algorithm, four options can be used to improve
the quality of the decoded images (with the same bitrate). Each option
may be used separately or in combination with some others. The options
are:
Unrestricted motion vectors: the BMA can select vectors that
locate the origin of a MB out of the reference image. The last line