Graphics Reference
In-Depth Information
8.4.4
Signaling of Block-Based Quantization Parameter
Change
In the regular, i.e., lossy residual coding process, a different quantizer step size
can be used for each CU to improve bit allocation, rate control, or both. Rather
than sending the absolute quantization parameter (QP), the difference in QP steps
relative to the slice QP is sent in the form of a so-called delta QP. This functionality
can be enabled in the picture parameter set (PPS) by using the syntax element
cu
_qp_delta_enabled_flag
.
In H.264/AVC,
mb_qp_delta
is used to provide the same instrument of delta
QP at the macroblock level. The value of
mb_qp_delta
can range from
.26
C
QpBdOffset
Y
=2)to25
C
QpBdOffset
Y
=2. For 8-bit video, this is
26 to 25, while
for 10-bit video this is
32 to 31.
mb_qp_delta
is unary coded and thus requires
up to 53 bins for 8-bit video and 65 bins for 10-bit video. All bins are regular coded.
In HEVC, delta QP is represented by the two syntax elements
cu_qp_delta
_abs
and
cu_qp_delta_sign_flag
,if
cu_qp_delta_enabled_flag
in the PPS indicates so. The sign is sent separately from the absolute value, which
reduces the average number of bins by half [
23
].
cu_qp_delta_sign_flag
is only sent if the absolute value is non-zero. The absolute value is binarized with
TrU (cMax=5) as the prefix and EG0 as the suffix [
98
]. The prefix is regular coded
and the suffix is bypass coded. The first bin of the prefix uses a different context
than the other four bins in the prefix (which share the same context) to capture the
probability of having a zero-valued delta QP. Note that syntax elements for delta
QP are only signaled for CUs that have non-vanishing prediction errors (i.e., at least
one non-zero transform coefficient). Conceptually, the delta QP is an element of
the transform coding part of HEVC and hence, can also be interpreted as a syntax
element that is always signaled at the root of the RQT, regardless which transform
block partitioning is given by the RQT structure. Table
8.3
shows examples of how
delta QP is signaled for H.264/AVC and HEVC.
8.4.5
Signaling of SAO Parameters
SAO is a form of in-loop filtering that was introduced in HEVC. It is used to process
the output of samples from the deblocking filter process and is the last step of the
decoding process. SAO involves sample based processing rather than block based
processing. There are two types of filtering: edge offset and band offset.
Edge offset (EO) involves comparing the sample and its neighboring sample
values in one of four angular directions (horizontal, vertical, 45
ı
, 135
ı
).
13
The
sample is compared to its neighbors in the selected direction (e.g., the sample has
a lower value than both its neighbors); based on the comparison, the sample is
13
Direction is also referred to as class in the HEVC specification.
