Graphics Reference
In-Depth Information
Tabl e 8. 4
Differences in signaling between CTU/CU layer in HEVC and MB layer in H.264/AVC
HEVC
H.264/AVC
Prediction and Coding
Block Structure and
Prediction Mode
cu_skip_flag
,
split_cu_flag
,
pred_mode_flag
,
part_mode
mb_skip_flag
,
mb_type
,
sub_mb_type
Maximum number of
bins for delta QP
5 (regular), 10 (bypass)
53 (regular)
Maximum number of
bins for SAO
parameters
4 (regular), 113 (bypass)
n/a
For the band offset, the
sao_offset_sign
is signaled only when the offset is
non-zero to reduce the total number of bins [
36
], while for edge offset the sign is
inferred from the category [
40
].
To leverage the spatial correlation across CTBs,
sao_merge_left_flag
and
sao_merge_up_flag
are used to indicate if SAO parameters can be inherited
from neighboring CTBs, which reduces signaling overhead. Both of these flags are
regular coded using separate context models.
Significant effort was made to reduce the number of regular coded bins required
to represent SAO filter syntax elements. As a result, the only regular coded bins are
the merge flags and the first bin of the
sao_type_idx_luma
and
sao_type
_idx_chroma
with the latter indicating whether SAO is enabled for luma and
chroma CTBs, respectively.
8.4.6
Comparison of HEVC and H.264/AVC
Tab le
8.4
highlights the differences in signaling between the CTU/CU layer in
HEVC and the macroblock (MB) layer in H.264/AVC, when processing 8-bit video.
For a comparable block partitioning, HEVC typically produces fewer regular coded
bins than H.264/AVC. At the same time, some of those regular coded bins in
addition to those of the skip flag are adaptively selected based on CU depth, size
and neighbors in HEVC, which improves coding efficiency relative to H.264/AVC.
In general, however, the total amount of bits spent for signaling at the CTU/CU or
MB layer is lower by more than an order of magnitude compared to the total amount
of bits spent for transform coefficient level coding. As already discussed above and
summarized in Table
8.4
, the majority of bypass bins for the SAO parameters are due
to the signaling of the offsets, while for H.264/AVC an excessive number of bins is
only generated in the rare cases where large delta QP values have to be transmitted.
