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IEEE SIGNAL PROCESSING MAGAZINE [175] MARCH 2015

digital media content description project in

which visual objects in the images or videos

are described with multilevel features for

facilitating visual object based storage,

retrieval, and interactive applications, etc.

This column will provide a short

overview of AVS2 video coding technol-

ogy and a performance comparison with

other video coding standards.

TECHNOLOGY AND KEY FEATURES

Similar to previous coding standards,

AVS2 adopts the traditional prediction/

transform hybrid coding framework, as

shown in Figure 1. Within the framework,

a more flexible coding structure is

adopted for efficient high-resolution video

coding, and more efficient coding tools

are developed to make full use of the tex-

tual information and temporal redundan-

cies. These tools can be classified into

four categories: 1) prediction coding

(including intraprediction and interpre-

diction), 2) transform, 3) entropy coding,

and 4) in-loop filtering. We will give a

brief introduction to the coding frame-

work and coding tools.

CODING FRAMEWORK

In AVS2, a coding unit (CU)-, prediction

unit (PU)-, and transform unit (TU)-based

coding/prediction/transform structure is

adopted to represent and organize the

encoded data [3]. First, pictures are split

into largest coding units (LCUs), which

consist of

NN22# samples of a lumi-

nance component and associated chromi-

nance samples with

,,N 816= or 32. One

LCU can be a single CU or can be split into

four smaller CUs with a quad-tree parti-

tion structure; a CU can be recursively

split until it reaches the smallest CU size

limit, as shown in Figure 2(a). Once the

splitting of the CU hierarchical tree is

finished, the leaf node CUs can be further

split into PUs. PU is the basic unit for

intra- and interprediction and allows mul-

tiple different shapes to encode irregular

image patterns, as shown in Figure 2(b).

The size of a PU is limited to that of a CU

with various square or rectangular shapes.

More specifically, both intra- and interpre-

diction partitions can be symmetric or

asymmetric. Intraprediction partitions

vary in the set

{,,NNNNN22 2###

.,. },NNN05 05 2# while inter-prediction

partitions vary in the set {,NNN222##

,, , , ,NN N N nU N nDnL N22 2 2### #

},nR N2# where

,,,UDL

and R are the

abbreviations of “Up,” “Down,” “Left,” and

“Right,” respectively. Besides CU and PU,

TU is also defined to represent the basic

unit for transform coding and quantiza-

tion. The size of a TU cannot exceed that

of a CU, but it is independent of the

PU size.

CU Partition

CU Depth,

d = 0

= 32

CU Depth,

d = 1

= 16

CU Depth,

d = 2

= 8

CU Depth,

d = 3

= 4

Split Flag = 0 Split Flag = 1

CU0

Split Flag = 0 Split Flag = 1

CU1

Split Flag = 0

Last Depth: No Splitting Flag

Split Flag = 1

CU2

CU3

d = 0

d = 1

d = 2

d = 3

× 2N

× 0.5N

0.5N

× 2N

× N

× 2N

× N

× nU nL × 2N

nR × 2N

× nD

PU_Skip

/Direct

PU_Intra

PU_Inter

n = 0.25N

PU Partition

[FIG2]

(a) The maximum possible recursive CU structure in AVS2. (LCU size = 64, maximum hierarchical depth = 4). (b) Possible

PU splitting for skip, intramodes, and intermodes in AVS2, including symmetric and asymmetric prediction (d=1, 2 for

intraprediction, and d= 0,1,2 for interprediction).

THE WORLD’S NEWSSTAND

THE WORLD’S NEWSSTAND