Game Development Reference
In-Depth Information
8
×
8 block
16
×
16 block
32
×
32 block
Fig. 3.7
Sub-block scan (each sub-block represents a 4
×
4CG)
4
4 coefficient groups (CGs), as shown in Fig.
3.7
. Then zig-zag scanning and
context-adaptive binary arithmetic coding (CABAC) is performed at both the CG
and coefficient levels. At the CG level for a TU, the CGs are scanned in zig-zag
order, and the CG position indicating the position of the last nonzero CG is coded
first, followed by a bin string of significant CG flags indicating whether the CG
scanned in zig-zag order contains nonzero coefficients. At the coefficient level, for
each nonzero CG, the coefficients are further scanned into the form of (
run
×
el
)
pair in zig-zag order. Level and run refer to the magnitude of a nonzero coefficient
and the number of zero coefficients between two nonzero coefficients respectively.
For the last CG, the coefficient position which denotes the position of the last nonzero
coefficient in scan order, is coded first. For a non-last CG, a last run is coded which
denotes number of zero coefficients after the last nonzero coefficient in zig-zag scan
order. And then the (
le
,
le
v
run
) pairs in a CG are coded in reverse zig-zag scan order.
For the context modeling used in the CABAC, AVS2 employs a mode depen-
dent context selection design for intra prediction blocks (Wang et al.
2013
). In this
context design, 34 intra prediction modes are classified into three prediction mode
sets: vertical, horizontal, and diagonal. Depending on the prediction mode set, each
v
el
,
Fig. 3.8
Sub-block regions (each sub-block represents a 4
×
4CG)
