Graphics Reference
In-Depth Information
A transform unit (TU) aggregates the transform blocks (TBs) of luma and chroma
samples as well as the syntax elements used to represent the associated transform
coefficient levels. Each TU and the related luma and two chroma TBs are determined
as a leaf of the corresponding RQT. Supported TB sizes for both luma and chroma
are in the range from 4
4 to 32
32 samples, where the corresponding core
transforms are separable applications of a fixed-point approximation of the 1-D
Discrete Cosine Transform (DCT) for dyadically increasing lengths from 4 to 32
points [
26
]. An exception is given for 4
4 luma TBs of residual signals resulting
from intra-picture predicted blocks, where instead of the DCT-like core transform
a separable fixed-point approximation of the 1-D Discrete Sine Transform (DST) is
used [
100
].
Note that a prediction unit (PU) aggregates the prediction blocks (PBs) of
luma and chroma samples and the associated syntax elements like motion data.
A coding unit (CU) encapsulates the luma and chroma coding block (CB) samples
and the so-called prediction mode, i.e., the decision whether the corresponding
samples are coded using intra-picture or inter-picture prediction, as well as some
additional syntax elements. On the top level of the hierarchy, a coding tree unit
(CTU) comprises the CTBs of luma and chroma samples, the associated CQT syntax
structure and all CUs at the CQT leaves.
8.3.1.1
Coefficient Grouping into Subblocks
Given the larger variety of TB sizes, one of the primary goals of CABAC entropy
coding for transform coefficient data in HEVC was to achieve a design that uses
for all block sizes as much of the same logic and the same procedures as possible.
Although at first glance this objective seems to be somehow unrelated to coding
efficiency, it turns out that at least one particular element leading to such a unified
design is also crucial for achieving high coding efficiency. This coding element is
given by the grouping of coefficients into so-called subblocks of size 4
4 for
transform blocks with size greater than 4
4: Subblocks were first proposed in
[
49
,
55
,
102
] and became part of HM1.0. In the subsequent HEVC development
process, their use was iteratively refined and extended in a way as will be explained
in more detail in Sect.
8.6
.
8.3.1.2
Hierarchy of Significance Flags
Since for most common coding conditions, a large portion of transform coefficients
is quantized to zero, or equivalently, the representation of the residual signal in the
DCT-/DST-like basis functions is supposed to be sparse, a hierarchical structured set
