Game Development Reference
In-Depth Information
(
,
)
The CG level coding is first invoked, followed by the
Level
Run
coding within one
CG which is similar to CBAC in AVS1.
6.3.4.1 Scan Order
In CBAC for AVS2, the coefficient coding for a transform block (TB) is decoupled
into two levels, i.e., CG level coding and coefficient level coding. In both levels,
the coding follows the reverse zig-zag scan order. Figure
6.6
shows the zig-zag scan
pattern in an 8
8 TB, which is split into four sub-blocks. The scan order of CGs is
indicated by the red dashed lines while the scan order within one CG is indicated by
the black dashed lines. The CG-based coding methods have two main advantages:
(1) allowing formodular processing, that is, for harmonized sub-block-based process-
ing across all block sizes, (2) with much lower implementation complexity compared
to that of a scan for the entire TB, both in software implementations and hardware.
×
6.3.4.2 First Level Coding
At CG level, the position of the last CG is signaled, where the last CG is the CG
that contains the last nonzero coefficient in the transform block in the scan order.
Different ways are used to signal the position of the last CG which is dependent on
the TB sizes. For an 8
8 block, a syntax element
LastCGPos
is coded, which is the
scan position of the last CG. For larger TBs, such as 16
×
×
×
32 TBs, one flag
LastCG
0
flag
is first coded first to indicate whether the last CG is at position (0; 0).
In the case that
lastCG
0
flag
is equal to one, two more syntax elements
LastCGX
and
LastCGY
are coded to signal the (x; y) coordinates of the last CG position. Note that,
(
16 and 32
LastCGY
−
1
)
is coded instead of
LastCGY
when
LastCGX
is zero since
lastCG
0
flag
is equal to 1.
For each bin of
LastCGPos
, the context is derived based on the bin index. In
addition, to further improve the coding efficiency for intramodes, an intraprediction
mode-based context design is employed (Wang et al.
2013b
). The rationale behind
Fig. 6.6
Zig-zag scan
patternin8
×
8TB:the
zig-zag scan of a 4
×
4TBis
used within each CG of
larger blocks
