Graphics Reference
In-Depth Information
Tabl e 8. 7
Derivation of context increment (ctxInc) for
split_transform_flag
,
cbf_luma
,
cbf_cb
,and
cbf_cr
for the example in Fig.
8.11
RQT
Transform
cbf_cb
,
cbf_cr
split_transform_flag
cbf_luma
depth
Size
(ctxInc)
(ctxInc)
(ctxInc)
0
n/a
n/a
1
0
1
32 32
0
0
1
2
16 16
1
0
2
3
8
8
2
0
3
4
4
4
n/a
0
n/a
Fig. 8.12
Context index
assignment for
sig_coeff_flag
in
4
0
1
4
5
2
3
4
5
4 TBs
6
6
8
8
7
7
8
8
it is the last non-zero SIG; this prevents unnecessary SIG from being signaled.
Different contexts are used depending on the position within the 4
4 and 8
8
transform blocks, and whether the bin represents an SIG or LAST. Since SIG
and LAST are interleaved, the context selection of the current bin depends on the
immediate preceding bin. The dependency of LAST on SIG results in a strong bin
to bin dependency for context selection of significance map entries in H.264/AVC
as illustrated in Fig.
8.3
.
8.6.4.1
sig_coeff_flag
(SIG)
While in HEVC position based context assignment for coding of
sig_coeff_
flag
(SIG) is used for 4
4 TBsasshowninFig.
8.12
, new forms of context
assignment for larger transforms were needed. In HM1.0, additional dependencies
were introduced in the context selection of SIG for 16
16 and 32
32 TBs to
improve coding efficiency. Specifically, the context selection of SIG was calculated
based on a local template using 10 (already decoded) SIG neighbors as shown in
Fig.
8.13
a[
57
,
102
]. By using this template-based context selection bit rate savings
of 1.4-2.8 % were reported [
57
].
To reduce context selection dependencies and storage costs, Sze and Budagavi
[
85
] proposed using fewer neighbors and showed that this could be done without
severely sacrificing coding efficiency. For instance, using only a maximum of 8
neighbors (removing neighbors A and D as shown in Fig.
8.13
b) had negligible
impact on coding efficiency, while using only six neighbors (removing neighbors
A, B, D, E and H as shown in Fig.
8.13
c) results in a coding efficiency loss of
only 0.2 %. This was further extended in [
18
] for HM2.0, where only a maximum
of five neighbors was used by removing dependencies on positions G and K, as
