Game Development Reference
In-Depth Information
Fig. 7.2
The in-loop filters, i.e., deblocking filter, SAO, and ALF, in AVS-2
In the following subsections, the three in-loop filters are introduced in detail based
on AVS-2, respectively.
7.2 Deblocking Filter
7.2.1 Overview of Deblocking Filter
Blocking artifact is one of thewell-known annoying artifacts, which appears as visible
artificial block boundaries between adjacent blocks. Many video coding standards
adopt deblocking filter in coding loop to reduce them, e.g., AVC/H.264, HEVC, and
AVS-1/2. The general deblocking filtering is a low pass operation on the samples
around block boundaries, e.g., 4
×
4 transform block boundaries in AVC/H.264, and
8
8 transform block boundaries in AVS. However, the deblocking filtering also may
smooth out image details or blurring edges, which are around of block boundaries.
Therefore, an effective deblocking filter to achieve significant coding gain should be
able to distinguish blocking artifacts and image edges, and apply different filtering
strength to them.
The widely used deblocking filters in video coding standards usually adjust the
filtering strength and filter taps not only according to the quantization parameters,
but also to the coding modes and sample correlations around the block boundaries.
As a consequence, it requires lots of conditional processing on the block edges and
sample levels, which increase the computational complexity significantly. In addition,
the deblocking filtering also increases the buffer size in hardware implementation.
Therefore, most deblocking filters are mainly used out of the coding loop only to
improve the subjective quality.
Thanks to the development of computer techniques and many efforts in opti-
mal design of deblocking filters, most of the popular video coding standards adopt
deblocking filters, and achieve significant video coding gain and video quality
improvement. In the following subsection, a detailed description of the deblocking
filtering process in AVS-2 is introduced (He et al.
2013
).
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