Graphics Reference
In-Depth Information
Tabl e 8. 6
Distribution of bins in CABAC for HEVC and H.264/AVC under common test
conditions [
6
,
101
] and for the worst case
HEVC
H.264/AVC
Common
conditions
AI LP LB RA worst worst
MAIN MAIN MAIN MAIN case HierB HierP case
CTU/CU bins 5.4% 15.8% 16.7% 11.7% 1.4% 27.0% 34.0% 0.5%
PU bins 9.2% 20.6% 19.5% 18.8% 5.0% 23.4% 26.3% 15.8%
TU bins 85.4% 63.7% 63.8% 69.4% 94.0% 49.7% 39.7% 83.7%
Generated bins are discriminated along the HEVC categories CTU/CU, PU, and TU as well as
their corresponding counterparts in H.264/AVC
from spatial to frequency domain, thereby decorrelating the residual samples and
performing an energy compaction in the sense that, after quantization, the signal can
be represented in terms of a few non-vanishing coefficients. The method of signaling
the quantized values and frequency positions of these coefficients is referred to as
transform coefficient coding
.
Syntax elements related to transform coefficient coding account for a significant
portion of the bin workload as shown in Table
8.6
. At the same time, those
syntax elements also account for a significant portion of the total number of bits
for a compressed video, and as a result the compression of quantized transform
coefficients significantly impacts the overall coding efficiency. Thus, transform
coefficient coding with CABAC must be carefully designed in order to balance
coding efficiency and throughput demands. Accordingly, as part of the HEVC
standardization process, a core experiment on coefficient scanning and coding was
established to investigate tools related to transform coefficient coding [
97
].
This section describes how transform coefficient coding evolved from
H.264/AVC to the first test model of HEVC (HM1.0) to the Final Draft International
Standard (FDIS) of HEVC (HM10.0), and discusses the reasons behind design
choices that were made. Many of the throughput improvement techniques were
applied, and new tools for improved coding efficiency were simplified. As a
reference for the beginning and end points of the development, Figs.
8.8
and
8.9
show examples of transform coefficient coding for 4
4 blocks in H.264/AVC and
HEVC, respectively.
8.6.1
Transform Block Structure
As already discussed in Sect.
8.3.1
, transform coding in HEVC involves a tree-
structured variable block-size approach with supported transform block sizes of
4
4; 8
8; 16
16; and 32
32: This means that the actual transform block
sizes, used to code the prediction error of a given CU, can be selected based on
the characteristics of the residual signal by using a quadtree-based partitioning,
also known as residual quadtree (RQT), as illustrated in Fig.
8.10
. While this larger
