Graphics Reference
In-Depth Information
requirements for future video codecs. Furthermore, since high throughput can
be traded-off for power savings using voltage scaling [
14
], the serial nature
of CABAC may limit the battery life for video codecs that reside on mobile
devices. This limitation is a critical concern, as a significant portion of video
codecs today are running on battery-operated devices. Accordingly, both coding
efficiency and throughput improvement tools as well as the trade-off between these
two requirements were investigated in the standardization of entropy coding for
HEVC. The trade-off between coding efficiency and throughput comes from the
fact that, in general, dependencies are a result of removing redundancy which, in
turn, improves coding efficiency; however, increasing dependencies usually makes
parallel processing more difficult which, as a consequence, may degrade throughput.
This section describes the various techniques used to improve both coding efficiency
and throughput of CABAC entropy coding for HEVC.
8.3.1
Brief Summary of HEVC Block Structures and CABAC
Coding Efficiency Improvements
In the evolutionary process from H.264/AVC to HEVC, improved coding efficiency
for CABAC entropy coding was addressed in a number of proposals, such as
[
24
,
102
,
106
]. The majority of coding-efficiency related CABAC proposals in the
HEVC standardization process was oriented towards transform coefficient coding,
since at medium to high bit rates the dominant part of bits is consumed by syntax
elements related to residual coding. As a consequence, this subsection will focus on
considerations that were made with regards to the specific CABAC design for those
syntax elements. Note, however, that due to the more consistent design of HEVC
in terms of tree structures for both partitioning of prediction blocks and transform
blocks, special care has also been taken to ensure an efficient modeling and coding
of the corresponding tree structuring elements. In addition, for new coding tools in
HEVC, such as block merging and sample adaptive offset (SAO) in-loop filtering,
additional assignments of binarization and context modeling schemes were needed.
Transform coding in HEVC is based on a tree-structured variable block size
approach with the corresponding quadtree structure referred to as
residual quadtree
(RQT) [
49
,
102
]. RQTs are nested into the leaves of another quadtree, the so-
called
coding quadtree
(CQT), which determines the subdivision of each block of
2
N
2
N
luma samples, referred to as a coding tree block (CTB) [
49
,
102
]. The
block partitioning for both prediction and transform coding is the same for luma
and chroma picture component samples,
3
and hence, a common coding and residual
quadtree syntax is used to signal the partitioning. As a result, the blocks of luma and
chroma samples and associated syntax elements are grouped together in a so-called
unit
.
3
There is one exception to this general rule in HEVC, which is discussed in more detail in Chap.
3
.
