Graphics Reference
In-Depth Information
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Fig. 7.10
Dependency of “ on QP. Reproduced with permission from [ 37 ], © 2012 IEEE
7.2.2.5
Deblocking Adaption
Clipping operations are used in deblocking to avoid excessive filtering. The clipping
parameter t C is derived from a look-up table and depends on the average of the
quantization parameter QP of the two adjacent blocks [ 44 ], which determines how
coarse the quantization is. Variable t C is obtained from a table as t C (QP) when both
adjacent blocks are inter-predicted ( Bs D 1) and t C (QP C 2) when at least one of
the adjacent blocks is intra-predicted ( Bs D 2). The deblocking parameter “,which
mainly determines which block boundaries are modified by the deblocking filtering,
also depends on the quantization parameter QP. One can see the dependency of
the parameter “ on the QP in Fig. 7.10 and parameter t C on QP in Fig. 7.11 .The
higher the QP, the higher are the values of “ and t C and therefore the more often
the deblocking filtering is applied, and more samples from the block boundary are
modified. In addition, greater modifications to the sample values are allowed for
higher QP. For low QP values, when a reconstructed picture has higher quality, the
deblocking is essentially turned off by setting “ and t C to zero.
The deblocking strength can be further adjusted on a slice or picture level
by sending parameters tc_offset_div2 and beta_offset_div2 in the
slice header or picture parameter set (PPS) [ 46 ]. These parameters specify offsets
(divided by two) that are added to a QP value before determining “ and t C from the
look-up tables. This gives an encoder a possibility to adapt the deblocking strength
depending on the sequence characteristics, the encoding mode, and other factors.
Changing the deblocking parameters on a picture level can be used to improve
the subjective quality when using a hierarchical coding structure. An example
of a hierarchical-B GOP8 coding structure, similar to the one used in the HM
common test conditions random-access mode [ 6 ] is shown in Fig. 7.12 .Inthis
structure, the encoder can use QP cascading in order to improve the compression
 
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