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IN A NUTSHELL

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IEEE SIGNAL PROCESSING MAGAZINE [178] MARCH 2015

transform coefficient blocks [16]. A coeffi-

cient block is partitioned into 4 # 4 coef-

ficient groups (CGs), as shown in

Figure 7. Then zig-zag scanning and con-

text-adaptive binary arithmetic coding

(CABAC) is performed at both the CG

level and coefficient level. 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 non-

zero coefficients. At the coefficient level,

for each nonzero CG, the coefficients are

further scanned into the form of (run,

level) pair in zig-zag order. Level and run

refer to the magnitude of a nonzero coeffi-

cient and the number of zero coefficients

between two nonzero coefficients, respec-

tively. For the last CG, the coefficient posi-

tion that denotes the position of the last

nonzero coefficient in scan order is coded

first. For a nonlast CG, a last run is coded

that denotes number of zero coefficients

after the last nonzero coefficient in zig-

zag scan order. And then the (level, 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 [17]. In this context

design, 34 intraprediction modes are

classified into three prediction mode

sets: vertical, horizontal, and diagonal.

Depending on the prediction mode set,

each CG is divided to two regions, as

shown in Figure 8. The intraprediction

modes and CG regions are applied in the

context coding of syntax elements

including the last CG position, last coef-

ficient position, and run value.

IN-LOOP FILTERING

Artifacts such as blocking artifacts, ring-

ing artifacts, color biases, and blurring

artifacts are quite common in com-

pressed video, especially at medium and

low bit rate. To suppress those artifacts,

deblocking filtering, sample adaptive off-

set (SAO) filtering [18], and adaptive

loop filter (ALF) [19] are applied to the

reconstructed pictures sequentially.

(a) (b) (c)

[FIG7] A subblock scan for transform blocks of size (a) 8 # 8, (b) 16 # 16, and (c) 32 #

32 transform blocks; each subblock represents a 4 # 4 CG.

(a) (b) (c)

[FIG8]

A subblock region partitions of 4 # 4 CG in an intraprediction block.

2N × nU 2N × nD

2N × 2N

2N × 0.5N 0.5N × 2N

nL × 2NnR× 2N

Level 0

Level 1

Others

Split

2N × 2N

Split

2N × 2N

Split

[FIG6] A PU partition and two-level transform coding.

THE WORLD’S NEWSSTAND

THE WORLD’S NEWSSTAND