Graphics Reference
In-Depth Information
7.2.2.3
Strong Filtering Mode
The strong deblocking filter in HEVC is applied to smooth flat areas, where block
artifacts are more visible. This filtering mode modifies three samples from the block
boundary and enables strong low-pass filtering. The HEVC strong filter is similar
to the strong filter used by the H.264/AVC video standard [
21
] except the clipping
operation, which is not present in the H.264/AVC strong filter. The reason for the
clipping operation in HEVC deblocking filter is that the strong filtering decision is
made based on the sample values in only two of the lines in the block, corresponding
to
i
D
0and
i
D
3. The clipping operation limits the amount of filtering in order to
make sure that there is no excessive filtering on the lines which were not evaluated
in the filtering decisions [
19
]. The filtering for the samples in block P is performed
using the following equations if the clipping operation is neglected:
p
0
0
D
.p
2
C
2p
1
C
2p
0
C
2q
0
C
q
1
C
4/ >> 3;
(7.17)
p
1
0
D
.p
2
C
p
1
C
p
0
C
q
0
C
2/ >> 2;
(7.18)
p
2
0
D
.2p
3
C
3p
2
C
p
1
C
p
0
C
q
0
C
4/ >> 3;
(7.19)
where
p
0
0
,
p
1
0
,and
p
2
0
are modified values of samples
p
0
,
p
1
,and
p
2
respectively.
The modified sample values are then clipped to the range [
p
i
2
t
C
,
p
i
C
2
t
C
]. The
equations for modification of samples
q
0
,
q
1
,and
q
2
can be obtained by replacing
p
with
q
in (
7.17
)-(
7.19
).
7.2.2.4
Chroma Deblocking
As mentioned in Sect.
7.2.2.1
, the deblocking is only applied to the chroma block
boundaries which have boundary strength
Bs
equal to 2, i.e. when one of the
adjacent blocks is intra-predicted. The block boundary should also be a CU, TU or
a prediction partition boundary and be aligned with the 8
8 chroma sample grid.
No further evaluation on the signal is done for chroma block boundaries. Chroma
deblocking only modifies one sample from each side of the block boundary. The
following expression is used to obtain the modification offset for the chroma block
boundary.
c
D
Clip3.
t
C
;t
C
; ...q
0
p
0
/<<2/
C
p
1
q
1
C
4/ >> 3/
(7.20)
The value of
4
c
is used for modification of the chroma samples
p
0
and
q
0
similarly to luma samples as in (
7.7
)and(
7.8
).
