Graphics Reference
In-Depth Information
deblocking uses decisions for a four-sample segment of the block boundary based
on two lines crossing the block boundary. In contrast, the decisions in H.264/AVC
deblocking are done for every line. The complexity of chroma deblocking filtering
in HEVC has also been reduced compared to H.264/AVC since only chroma block
boundaries with
Bs
equal to 2 are filtered. Therefore, only block boundaries adjacent
to the intra-predicted blocks are filtered in the chroma components in contrast to
H.264/AVC, where the chroma deblocking is also applied to the block boundaries
between the inter-predicted blocks.
7.4.1.2
Deblocking Filter Parallelization Aspects
HEVC deblocking filter allows easy parallelization on several levels. First, paral-
lelization is possible on the color component level. In HEVC, filtering decisions
for chroma components are only based on the block boundary strength. Therefore,
the only data to be shared between the luma and the chroma deblocking is the
Bs
, which depends on the prediction type [
43
]. This makes it possible to process
chroma components independently of the luma component unlike in H.264/AVC,
where chroma deblocking uses the decisions made for luma deblocking.
The vertical and horizontal block boundaries in HEVC are processed in a
different order than in H.264/AVC. In HEVC, all the vertical block boundaries in
the picture are filtered first, and then all the horizontal block boundaries are filtered
[
20
,
37
]. Since the minimum distance between two parallel block boundaries in
HEVC is eight samples, and HEVC deblocking modifies at most three samples from
the block boundary and uses four samples from the block boundary for deblocking
decisions, filtering of one vertical boundary does not affect filtering of any other
vertical boundary. This means there are no deblocking dependencies across the
block boundaries. In principle, any vertical block boundary can be processed
in parallel to any other vertical boundary. The same holds for the horizontal
boundaries, although the modified samples from filtering the vertical boundaries
are used as the input to filtering the horizontal boundaries.
The deblocking in HEVC can also be performed on the 8
8blockbasis
[
32
,
37
,
47
]. Figure
7.20
illustrates how the deblocking (both for vertical and hori-
zontal boundaries) can be performed independently for each 8
8 block of samples.
The deblocking is performed on the 8
8 luma sample grid and decisions are done
separately for each four-sample segment of the block boundary, which means that
two parts of the eight-sample block boundary are deblocked independently of each
other [
34
]. Therefore, the deblocking of the 8
8 sample square with the crossing
of vertical and horizontal lines on the 8
8 sample filtering grid in the middle of the
block is not dependent on the deblocking in the other parts of the picture. Basically,
the whole picture can be split into such 8
8 sample blocks (4
8and8
4blocks
at the picture boundaries), which can all be processed independently of other blocks.
Since all vertical block boundaries in HEVC are processed before the horizontal
