Graphics Reference
In-Depth Information
Tabl e 8. 10
Context selection for regular coded prefix bins of the
coordinates of the last position
last_sig_coeff_x_prefix
and
last_sig_coeff_y_prefix
Bin Index
0
1
2
3
4
5
6
7
8
4 4 luma TB
0
1
2
8
8 luma TB
3
3
4
4
5
16
16 luma TB
6
6
7
7
8
8
9
32
32 luma TB
10
10
11
11
12
12
13
13
14
4
4 chroma TB
15
16
17
8
8 chroma TB
15
15
16
16
17
16
16 chroma TB
15
15
15
15
16
16
16
coded_sub_block_flag
sig_coeff_flag
last position
(
sig_coeff_flag
inferred)
1 1 0
1 0 0
0 0 0
0 0 1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
0
0
inferred
(DC)
inferred
(last)
1
1
subblock with all
zeros not signaled
0 1 0
0 0 0
1
0
0
0
0
0
0
0
0
1
0
0
0 0 0
0 0 0
0
0
0
0
0
0
0
0
0
0
Fig. 8.17
Example of the hierarchical signaling of an 8
8 significance map
8.6.4.3
coded_sub_block_flag
(CSBF)
As already explained in Sect.
8.3.1
, the number of bins to be transmitted for signal-
ing the significance map is considerably reduced by using a hierarchical signaling
scheme of significance flags. Part of this hierarchy is the
coded_sub_block
_flag
(CSBF) that indicates for each 4
4 subblock of a TB whether there are
non-zero coefficients in the subblock [
56
,
60
]. If CSBF is equal to 1, the subblock
contains at least one non-zero transform coefficient level and, consequently, SIGs
within the subblock are signaled. No SIGs are signaled for a 4
4 subblock that
contains all vanishing transform coefficients, since this information is signaled by a
CSBF equal to 0. For large TB sizes, a reduction in SIG bins of up to a 30 % can
be achieved by the use of CSBFs, which corresponds to an overall bin reduction of
3-4 % under common test conditions. To avoid signaling of redundant information,
the CSBF for the subblocks containing the DC and the last position are inferred to
be equal to 1. Figure
8.17
shows an example of the hierarchical signaling of an 8
8
significance map.
In HM7.0, the CSBF was additionally used to further reduce dependencies in the
context selection of SIG for 16
16 and 32
32 TBs. Specifically, the neighboring
subblocks and their corresponding CSBFs (Fig.
8.18
) are used for context selection
rather than the individual SIG neighbors, as shown in Fig.
8.13
e[
41
]. This context
selection scheme was extended to 8
8 TBs in HM8.0 [
77
]. According to this
