Graphics Reference
In-Depth Information
preserving the subjective quality on other types of video content [
33
,
38
,
39
]. An
example of deblocking parameters improving the subjective quality on chaotic con-
tent in the hierarchical-B GOP8 coding structure is setting the
tc_offset_div2
to values 0, 3, 3, and 5 for pictures at hierarchy levels 0, 1, 2, and 3 respectively,
while the
beta_offset_div2
is set to zero for all levels.
7.3
Sample Adaptive Offset (SAO)
7.3.1
Motivation and Overview of SAO
The key function of the sample adaptive offset is to attenuate ringing artifacts,
which are more likely to appear when larger transform sizes are used. The SAO
reduces sample distortion by first classifying the samples in the region into multiple
categories with as selected classifier and adding a specific offset to each sample
depending on its category. The classifier index and the offsets for each region
are signaled in the bitstream [
7
,
8
]. The SAO encoder is not standardized. It may
minimize the average sample rate-distortion cost, as done in the HM reference
software, or may use another criterion to generate SAO parameters. SAO can use
different sample offsets in a region depending on the sample classification, and SAO
parameters can change from one region of a picture to another. The HEVC uses two
SAO types:
edge offset
(EO) and
band offset
(BO) [
11
]. In EO, the classification of
a sample is based on its neighborhood, i.e. on the comparison between the current
sample and its neighboring samples. In BO, the classification is based on the sample
value.
The best coding efficiency could be achieved by a picture-based region partition-
ing method [
11
], which would, however, introduce additional encoding latency. In
order to achieve low encoding latency and reduce the buffer requirement, the size
of the region can be fixed and set as small as one coding tree unit (CTU). Multiple
CTUs can share the same SAO parameters by region merging [
14
] to reduce side
information. In HEVC, a CTU consists of three coding tree blocks (CTBs) of color
components, and each color component can have its own SAO offsets and share the
same EO/BO type for chroma components [
10
].
7.3.2
Edge Offset
Figure
7.13
shows the Gibbs phenomenon, which can be used to explain the
appearance of ringing artifacts in image and video coding. The horizontal axis shows
the sample position along a 1-D line and the vertical axis denotes the sample value.
The dotted curve represents the original samples while the solid curve represents
the reconstructed samples when the highest frequencies in the signal are discarded
due to quantization of transform coefficients. Local peaks, convex edges/corners,
