Graphics Reference
In-Depth Information
of four different significance flags
4
is introduced in HEVC to reduce the number of
individual significance flags to be transmitted. This hierarchy of syntax elements
also reflects the hierarchical processing of TBs within the RQT as well as the
processing of subblocks within a given TB.
The use of so-called
coded block flags
(CBF), indicating the occurrence of
significant, i.e., nonzero transform coefficients in a TB, was already part of
H.264/AVC CABAC-based residual coding. In HEVC, this concept was extended
to also cover the RQT root on the top level of the hierarchy as well as the subblock
on a lower level of the hierarchy. Consequently, there are a
rqt_root_cbf
,at
least for RQT roots in inter-predicted CUs,
cbf_luma
,
cbf_cb
,and
cbf_cr
for
the visited TBs of the three color components, and a
coded_sub_block_flag
(CSBF) for each visited subblock in a TB. On the lowest level of the hierarchy, for
each visited subblock a so-called
significance map
indicates the location of nonzero
coefficients for each scan position in a subblock.
This hierarchy of significance flags is complemented by the syntax elements
indicating the last significant scan position in a TB, which somehow serve as
an entry point into each significant TB and which is equivalent to signaling the
insignificance of a partial area of a TB. The latter concept differs from H.264/AVC,
where for each
significant_coeff_flag
(SIG) with a value of one, a
last
_significant_coefficient_flag
(LAST) is signaled indicating if the
current scan position is the last nonzero coefficient inside the TB. Note that this
latter signaling scheme is equivalent to using a TrU binarization (with inverted bin
values) for the number of nonzero coefficients in a TB, such that each bin of the
resulting bin sequence is intertwined with the corresponding nonzero significance
flag. This design aspect of mixing two flags on a bin level in H.264/AVC was later
found to be critical in terms of throughput, as will be discussed in Sect.
8.6
.
8.3.1.3
Context Modeling for Coding of Significance Flags
Particular care has been taken to properly specify the context models for coding
of significance flags. For instance, modeling of the CBF is based on the RQT
depth, while that for the CSBF is using neighboring CSBF information. For coding
of the significance map, which typically consumes most of the bits in HEVC
transform coding, additional dependencies between neighboring elements have been
exploited, at least for TBs larger than 4
4: Initially, for that purpose a local
template was proposed [
49
,
55
,
102
] and adopted for HM1.0. Although this design
provides high coding efficiency, it introduces some critical data dependencies.
As a solution to this problem, a combination of position-based information (as
used in H.264/AVC) and template-based neighborhood information was finally
adopted for context modeling of significance map entries [
41
,
77
]. This particular
4
Note that the term “significance flag” is interpreted here and in the following in a much broader
sense than originally used in the context of H.264/AVC.
