CABAC 上下文自适应算术编码

context adaptive binary arithmetic coding ,是HEVC里面的熵编码工具,比H.264里面的算术编码实现效率更高

1778IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS FOR VIDEO TECHNOLOGY,VOL.22,NO.12,DECEMBER2012 High Throughput CABAC Entropy

Coding in HEVC

Vivienne Sze,Member,IEEE,and Madhukar Budagavi,Senior Member,IEEE

Abstract—Context-adaptive binary arithmetic coding(CAB-AC)is a method of entropy coding?rst introduced in H.264/A VC and now used in the newest standard High Ef?ciency Video Coding(HEVC).While it provides high coding ef?ciency,the data dependencies in H.264/A VC CABAC make it challenging to parallelize and thus,limit its throughput.Accordingly,during the standardization of entropy coding for HEVC,both coding ef?ciency and throughput were considered.This paper highlights the key techniques that were used to enable HEVC to potentially achieve higher throughput while delivering coding gains relative to H.264/A VC.These techniques include reducing context coded bins,grouping bypass bins,grouping bins with the same context, reducing context selection dependencies,reducing total bins, and reducing parsing dependencies.It also describes reductions to memory requirements that bene?t both throughput and implementation costs.Proposed and adopted techniques up to draft international standard(test model HM-8.0)are discussed. In addition,analysis and simulation results are provided to quantify the throughput improvements and memory reduction compared with H.264/A VC.In HEVC,the maximum number of context-coded bins is reduced by8×,and the context memory and line buffer are reduced by3×and20×,respectively.This paper illustrates that accounting for implementation cost when designing video coding algorithms can result in a design that enables higher processing speed and lowers hardware costs,while still delivering high coding ef?ciency.

Index Terms—Context-adaptive binary arithmetic coding (CABAC),entropy coding,high-ef?ciency video coding(HEVC), video coding.

I.Introduction

H IGH EFFICIENCY Video Coding(HEVC)is currently

being developed by the Joint Collaborative Team for Video Coding(JCT-VC).It is expected to deliver up to a 50%higher coding ef?ciency compared to its predecessor H.264/A VC.HEVC uses several new tools for improving coding ef?ciency,including larger block and transform sizes, additional loop?lters,and highly adaptive entropy coding. While high coding ef?ciency is important for reducing the transmission and storage cost of video,processing speed and area cost also need to be considered in the development of next-generation video coding to handle the demand for higher resolution and frame rates.

Manuscript received April16,2012;revised July7,2012;accepted August 21,2012.Date of publication October2,2012;date of current version January 8,2013.This paper was recommended by Associate Editor F.Wu.

The authors are with Texas Instruments,Dallas,TX75243USA(e-mail: sze@alum.mit.edu;madhukar@http://www.77cn.com.cn).

Color versions of one or more of the?gures in this paper are available online at http://www.77cn.com.cn.

Digital Object Identi?er10.1109/TCSVT.2012.2221526

Context-adaptive binary arithmetic coding(CABAC)[1]is a form of entropy coding used in H.264/A VC[2]and also in HEVC[3].While CABAC provides high coding ef?ciency, its data dependencies cause it to be a throughput bottleneck for H.264/A VC video codecs[4].This makes it dif?cult to support the growing throughput requirements for next-generation video codecs.Furthermore,since high throughput can be traded off for power savings using voltage scaling [5],the serial nature of CABAC limits the battery life for video codecs that reside on mobile devices.This limitation is a critical concern,as a signi?cant portion of the video codecs today are battery operated.Accordingly,both coding ef?ciency and throughput improvement tools,and the tradeoff between these requirements,were investigated in the standard-ization of entropy coding for HEVC.The tradeoff between coding ef?ciency and throughput exists,since dependencies are a result of removing redundancy,which improves coding ef?ciency;however,dependencies make parallel processing dif?cult,which degrades throughput.

This paper describes how CABAC entropy coding has evolved from H.264/A VC to HEVC(Draft International Stan-dard,HM-8.0)[3],[6].While both coding ef?ciency and throughput improvement tools are discussed,the focus of this paper will be on tools that increase throughput while maintaining coding ef?ciency.Section II provides an overview of CABAC entropy coding.Section III explains the cause of the throughput bottleneck.Section IV describes several key techniques used to improve the throughput of the CABAC engine.Sections V and VI describe how these techniques are applied to prediction unit(PU)coding and transform unit(TU) coding,respectively.Section VII compares the overall CABAC throughput and memory requirements of H.264/A VC and HEVC for both common conditions and worst-case conditions.

II.CABAC Entropy Coding

Entropy coding is a form of lossless compression used at the last stage of video encoding(and the?rst stage of video decoding),after the video has been reduced to a series of syntax elements.Syntax elements describe how the video sequence can be reconstructed at the decoder.This includes the method of prediction(e.g.,spatial or temporal prediction,intra prediction mode,and motion vectors)and prediction error,also referred to as residual.Table I shows the syntax elements used in HEVC and H.264/A VC.These syntax elements describe properties of the coding unit(CU),

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context adaptive binary arithmetic coding ,是HEVC里面的熵编码工具,比H.264里面的算术编码实现效率更高

SZE AND BUDAGA VI:HIGH THROUGHPUT CABAC ENTROPY CODING IN HEVC1779

TABLE I

CABAC Coded Syntex Elements in HEVC and H.264/AVC

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