Graphics Reference
In-Depth Information
SIG(i)
SIG(i+1)
SIG(i+2)
SIG(i+3)
EOB
Fig. 8.26
No context speculation is required to achieve 5
parallelism when processing the
4
4 significance map in HEVC. i = coefficient position; EOB = end of block; SIG =
sig_coeff_flag
8.8.2.2
Group Bypass Coded Bins
Grouping bypass bins together into longer chains increases the number of bins
processed per cycle and reduces the number of cycles required to process a single
bypass bin. This is a technique used in coding of syntax elements related to motion
vector difference, intra mode, last position, and coefficient levels. For instance, for
the Kimono sequence, encoded using the RandomAccess configuration, grouping
bypass bins increases the average bypass bin run length from 2.1 to 6.4. In HEVC,
under common test conditions, up to a 30 % reduction in number of cycles can be
achieved compared to the case of no grouping [
89
].
The benefit of bypass grouping can also be seen in the example of Figs.
8.8
and
8.9
. If bypass grouping was not used, it would take five cycles to process the 5 sign
bypass bins. Assuming the architecture of [
103
], where 4 bypass bins are processed
per cycle, only two cycles are required to process the 5 sign bins.
8.8.2.3
Group Bins with Same Context
Grouping bins with same context together is done for motion vector difference,
significance map and coefficient level. As a results, fewer speculative calculations
are needed to decode multiple bins per cycle since all bins that use the same logic
for context selection are grouped together.
Figure
8.3
showed the speculation required when
significant_coeff
_flag
and
last_significant_coeff_flag
are interleaved in H.264/AVC.
In HEVC, no speculation is required for significance map as shown in Fig.
8.26
.
Thus for this example, the number of operations are reduced from 14 to 5.
8.8.2.4
Reduce Context Selection Dependencies
Context selection dependencies were reduced such that coding gains could be
achieved without significant penalty to throughput. For instance, the last significant
coefficient position information is sent before the SIG flag to remove a tight bin
to bin data dependency. Relative to HM1.0, the neighboring dependencies for SIG
were reduced from 10 to 5 neighboring SIG bins, and then further modified to only
depend on neighboring 4
4 subblocks. The remaining context selection for SIG
is only based on its position within the block as in H.264/AVC.
