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In-Depth Information
macroblock covers only a very small area of a picture, much smaller than the regions
that can typically be described by the same motion parameters. Taking into account
that some of the CTUs will be subdivided for assigning different prediction modes
and parameters, as will be described in more detail in the following, the partitioning
into 64
64 CTUs provides a more suitable description.
3.2.2
Coding Trees, Coding Blocks, and Coding Units
In the prior ITU-T and ISO/IEC video coding standards, a macroblock is not only
used for partitioning a video picture; it also represents the processing unit for
which a coding mode is chosen by the encoder. For each macroblock, it is decided
whether all samples of the corresponding luma and chroma blocks are transmitted
using inter-picture coding (i.e., motion-compensated prediction) or intra-picture
coding. Furthermore, in both intra-picture and inter-picture coding, a macroblock
can typically be subdivided into smaller blocks for the purpose of prediction and
signaling of prediction parameters.
In the widely used High profile of H.264
j
MPEG-4 AVC, three intra macroblock
coding modes
4
are supported, which are referred to as Intra-4
4,Intra-8
8,and
Intra-16
16.IntheIntra-4
4 coding mode, the luma component of the macroblock
is subdivided into 4
4 blocks. The samples of each 4
4 block are predicted based
on the samples of already coded neighboring blocks and the prediction residual is
coded using a 4
4 transform. When using the Intra-8
8 mode, the intra prediction
and transform coding of the luma component is done for 8
8 blocks. For the Intra-
16
16 mode, the entire 16
16 luma block is predicted using the samples of
already coded neighboring macroblocks and the prediction residual is coded using
a two-stage transform, which could be interpreted as a low-complexity variant of
a 16
16 transform. In all three intra macroblock modes, the entire 8
8 chroma
blocks are predicted and the residual is coded using a two-stage variant of an 8
8
transform. The partitioning of the luma component of a macroblock for intra-picture
coding is illustrated in the bottom of Fig.
3.3
.
For motion-compensated prediction, H.264
j
MPEG-4 AVC supports four parti-
tioning modes, which are referred to as Inter-16
16,Inter-16
8,Inter-8
16,
and Inter-8
8 coding mode and are illustrated in the top of Fig.
3.3
.IntheInter-
16
16 mode, the motion-compensated prediction for all luma and chroma samples
is done with the same set of motion parameters. For the Inter-16
8 and Inter-8
16
modes, the luma and chroma blocks of a macroblock are horizontally and vertically
split into two rectangles of the same size, respectively, and each of the two resulting
blocks is associated with a separate set of motion parameters. When the Inter-8
8
mode is used, the macroblock is split into four sub-macroblocks, each with a size of
4
H.264
MPEG-4 AVC additionally supports a so-called PCM mode, in which the macroblock
samples are directly written into the bitstream.
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