In-Loop Filter - Advanced Video Coding Systems - page 115

7.2.2 Deblocking Filter in AVS2

The deblocking filter is the first in-loop filter in AVS-2, which takes the 8 8 block

as the basic filtering unit. In order to avoid blurring the image edges, the deblocking
filter first judges the discontinuitieswhether they are blocking artifacts or image edges
according to sample levels, coding modes, and quantization steps. Different filtering
strength and filter taps are applied according the judgment. For some boundaries,
the deblocking filtering is skipped to preserve the image detail structures. Based on
some artful designs and optimization, the deblocking filter achieves better tradeoff
between performance and complexity according to rate-distortion cost.
Since the minimum size of transform unit is 8

×

× 8 in AVS-2, the deblocking filter

takes 8 8 block as the basic filtering unit just as illustrated in Fig. 7.3 . However,

not all the boundaries of filtering unit need to be filtered. Considering coding unit,
prediction unit, and transform unit all enlarged compared with previous coding stan-
dards, e.g., coding unit varying from 8

×

× 8to64 × 64 and transform unit varying

from 4 8

filtering unit when these boundaries also lie on the boundaries of coding unit, pre-
diction unit, or transform unit. If the boundaries of filtering unit lie inside the coding
unit, prediction unit, or transform unit, they are not filtered. Therefore, the number
of filtered boundaries and the computation complexity is reduced.
In order to avoid blurring true image edges, the filtering strength and filter taps need
be determined according to the coding modes, sample levels, and quantization steps.
The true edges should be left unfiltered or weakly filtered as much as possible while
filtering artificial ones. In order to efficiently select reasonable filtering strength, the
deblocking filter inAVS-2 first calculates a Boundary-Strength ( BS ) parameter, which
is an integer value from 0 to 4 used to direct the deblocking filtering strength based on
the sample correlations around boundaries of each 8

× 4to64 × 64, the filtering operation is only applied to boundaries of 8 ×

8 block. Figure 7.4 illustrates
six samples which may be modified in deblocking filtering process according to
BS. If the sample variations satisfy the following constraint in Eq. ( 7.1 ), BS value is

×

Fig. 7.3 Boundaries (Sampling format 4:2:0), the solid line is vertical filtering boundaries of
current block, the bold dash line is the horizontal filtering boundaries and the thin dash is the
filtering boundaries for next block. a Luma filtering block. b Chroma filtering block