Graphics Reference
In-Depth Information
where
t
C
is a clipping parameter dependent on the QP, and Clip3(
a
,
b
,
x
) function
clips the variable
x
to the range (a, b), i.e.
Clip3.a;b;x/
D
Max .a; Min .b; x// ;
(7.10)
and • is determined as
•
D
.9
.q
0
p
0
/
3
.q
1
p
1
/
C
8/ >> 4:
(7.11)
Neglecting the clipping operation, the impulse response of the filter is (3, 7, 9,
3)/16. The value of • is proportional to the deviation of the signal at the sides
of the block boundary from a ramp and is equal to zero when the signal across the
boundary has the form of a perfect ramp across the block boundary [
35
].
Deblocking filtering is only applied to a line of samples across the block
boundary if the absolute value of • is below
t
C
,i.e.
j
•
j
<10t
C
:
(7.12)
Expression (
7.12
) evaluates whether the discontinuity at the block boundary is
likely to be a natural edge or caused by a block artifact.
If two samples are modified in block P, i.e. condition in (
7.5
) is true, the sample
p
1
is modified as
p
1
0
D
p
1
C
p
1
;
(7.13)
and if condition in (
7.6
)istrue,sample
q
1
is modified as
q
1
0
D
q
1
C
q
1
;
(7.14)
where the
p
1
0
and
q
1
0
are new values of samples
p
1
and
q
1
respectively, and the
values of
4
p
1
and
4
q
1
are obtained as follows:
p
1
D
Clip3.-t
C
=2; t
C
=2; ....p
2
C
p
0
C
1/ >> 1/
p
1
C
0
/>>1//;
(7.15)
q
1
D
Clip3.-t
C
=2; t
C
=2; ....q
2
C
q
0
C
1/ >> 1/
q
1
0
/>>1//: (7.16)
The impulse response of the filter is (8, 19, -1, 9, -3)/32 if the clipping operation
is neglected. One can see that the value of the offset obtained in (
7.9
)isusedin
calculation of
4
p
1
and
4
q
1
. The filtering operations at positions
p
0
,
p
1
,
q
0
,and
q
1
do not modify the signal that has a form of a perfect ramp across the block boundary.
The deblocking filter decisions done for each line of a four-sample segment of a
block boundary are summarized in a flowchart in Fig.
7.8
.
An example of modifications to the block boundary samples in the normal
filtering mode is shown in Fig.
7.9
.
