H.264与MPEG-4视频编码技术解析

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"H.264 and MPEG-4 Video Compression - Video Coding for Next-generation Multimedia" H.264和MPEG-4是两种广泛应用于视频压缩的编码标准，它们在现代多媒体领域扮演着至关重要的角色。这两项技术的出现极大地提高了视频数据的压缩效率，使得在有限带宽下传输高质量视频成为可能，对于互联网、移动通信和数字电视等领域的发展产生了深远影响。 H.264，也称为AVC（Advanced Video Coding），是由ITU-T的VCEG（Video Coding Experts Group）和ISO/IEC的MPEG（Moving Picture Experts Group）联合开发的。该标准采用了更高级的压缩算法，包括运动补偿、熵编码、多参考帧、宏块预测等技术，实现了更高的压缩比，同时保持了良好的视频质量。例如，与MPEG-2相比，H.264可以在相同的视频质量下减少50%的数据量，这对于网络视频流、高清电视和移动设备上的视频播放非常有利。 MPEG-4则是一个更为广泛的多媒体编码标准，它不仅包含视频压缩，还包括音频编码、对象编码和交互性等。MPEG-4的一大特点是支持基于内容的编码，可以对视频中的独立对象进行编码，这允许了更灵活的解码和重用。例如，MPEG-4支持3D对象、面部动画和视频剪辑，为互动式多媒体应用提供了基础。此外，MPEG-4还引入了形状预测和空间预测等技术，提高了编码效率。 H.264与MPEG-4的比较： 1. **压缩效率**：H.264在压缩效率上优于MPEG-4，尤其在高清晰度视频上。 2. **复杂度**：H.264的编码和解码过程相对更复杂，需要更多的计算资源，而MPEG-4相对简单一些。 3. **功能**：MPEG-4的多媒体特性更加丰富，适合于交互式应用和个性化媒体内容的处理，而H.264主要关注视频压缩。 4. **广泛应用**：两者都广泛应用，但H.264在高清视频流、DVD和蓝光光盘等领域更常见；MPEG-4则在早期的流媒体服务、移动设备和一些互动应用中广泛采用。随着技术的发展，尽管H.265（HEVC）和VP9等新一代视频编码标准已经推出，H.264和MPEG-4仍因其广泛的设备兼容性和稳定的性能，继续在许多场景中占据重要地位。了解并掌握这些视频压缩技术对于理解现代多媒体系统的工作原理以及优化视频内容的传输和存储至关重要。

Foreword

Work on the emerging “Advanced Video Coding” standard now known as ITU-T Recom-

mendation H.264 and as ISO/IEC 14496 (MPEG-4) Part 10 has dominated the video coding

standardization community for roughly the past three years. The work has been stimulating,

intense, dynamic, and all consuming for those of us most deeply involved in its design. The

time has arrived to see what has been accomplished.

Although not a direct participant, Dr Richardson was able to develop a high-quality,

up-to-date, introductory description and analysis of the new standard. The timeliness of this

book is remarkable, as the standard itself has only just been completed.

The new H.264/AVC standard is designed to provide a technical solution appropriate

for a broad range of applications, including:

Broadcast over cable, satellite, cable modem, DSL, terrestrial.

Interactive or serial storage on optical and magnetic storage devices, DVD, etc.

Conversational services over ISDN, Ethernet, LAN, DSL, wireless and mobile networks,

modems.

Video-on-demand or multimedia streaming services over cable modem, DSL, ISDN, LAN,

wireless networks.

Multimedia messaging services over DSL, ISDN.

The range of bit rates and picture sizes supported by H.264/AVC is correspondingly broad,

addressing video coding capabilities ranging from very low bit rate, low frame rate, “postage

stamp” resolution video for mobile and dial-up devices, through to entertainment-quality

standard-deﬁnition television services, HDTV, and beyond. A ﬂexible system interface for the

coded video is speciﬁed to enable the adaptation of video content for use over this full variety

of network and channel-type environments. However, at the same time, the technical design

is highly focused on providing the two limited goals of high coding efﬁciency and robustness

to network environments for conventional rectangular-picture camera-view video content.

Some potentially-interesting (but currently non-mainstream) features were deliberately left out

(at least from the ﬁrst version of the standard) because of that focus (such as support of

arbitrarily-shaped video objects, some forms of bit rate scalability, 4:2:2 and 4:4:4 chroma

formats, and color sampling accuracies exceeding eight bits per color component).

Foreword

•

xvi

In the work on the new H.264/AVC standard, a number of relatively new technical

developments have been adopted. For increased coding efﬁciency, these include improved

prediction design aspects as follows:

Variable block-size motion compensation with small block sizes,

Quarter-sample accuracy for motion compensation,

Motion vectors over picture boundaries,

Multiple reference picture motion compensation,

Decoupling of referencing order from display order,

Decoupling of picture representation methods from the ability to use a picture for reference,

Weighted prediction,

Improved “skipped” and “direct” motion inference,

Directional spatial prediction for intra coding, and

In-the-loop deblocking ﬁltering.

In addition to improved prediction methods, other aspects of the design were also enhanced

for improved coding efﬁciency, including:

Small block-size transform,

Hierarchical block transform,

Short word-length transform,

Exact-match transform,

Arithmetic entropy coding, and

Context-adaptive entropy coding.

And for robustness to data errors/losses and ﬂexibility for operation over a variety of network

environments, some key design aspects include:

Parameter set structure,

NAL unit syntax structure,

Flexible slice size,

Flexible macroblock ordering,

Arbitrary slice ordering,

Redundant pictures,

Data partitioning, and

SP/SI synchronization switching pictures.

Prior to the H.264/AVC project, the big recent video coding activity was the MPEG-4 Part 2

(Visual) coding standard. That speciﬁcation introduced a new degree of creativity and ﬂex-

ibility to the capabilities of the representation of digital visual content, especially with its

coding of video “objects”, its scalability features, extended N-bit sample precision and 4:4:4

color format capabilities, and its handling of synthetic visual scenes. It introduced a number

of design variations (called “proﬁles” and currently numbering 19 in all) for a wide variety

of applications. The H.264/AVC project (with only 3 proﬁles) returns to the narrower and

more traditional focus on efﬁcient compression of generic camera-shot rectangular video pic-

tures with robustness to network losses – making no attempt to cover the ambitious breadth of

MPEG-4 Visual. MPEG-4 Visual, while not quite as “hot off the press”, establishes a landmark

in recent technology development, and its capabilities are yet to be fully explored.

Foreword

•

xvii

Most people ﬁrst learn about a standard in publications other than the standard itself.

My personal belief is that if you want to know about a standard, you should also obtain a

copy of it, read it, and refer to that document alone as the ultimate authority on its content,

its boundaries, and its capabilities. No tutorial or overview presentation will provide all of the

insights that can be obtained from careful analysis of the standard itself.

At the same time, no standardized speciﬁcation document (at least for video coding), can

be a complete substitute for a good technical book on the subject. Standards speciﬁcations are

written primarily to be precise, consistent, complete, and correct and not to be particularly

readable. Standards tend to leave out information that is not absolutely necessary to comply

with them. Many people ﬁnd it surprising, for example, that video coding standards say almost

nothing about how an encoder works or how one should be designed. In fact an encoder is

essentially allowed to do anything that produces bits that can be correctly decoded, regardless

of what picture quality comes out of that decoding process. People, however, can usually only

understand the principles of video coding if they think from the perspective of the encoder, and

nearly all textbooks (including this one) approach the subject from the encoding perspective.

A good book, such as this one, will tell you why a design is the way it is and how to make

use of that design, while a good standard may only tell you exactly what it is and abruptly

(deliberately) stop right there.

In the case of H.264/AVC or MPEG-4 Visual, it is highly advisable for those new to the

subject to read some introductory overviews such as this one, and even to get a copy of an

older and simpler standard such as H.261 or MPEG-1 and try to understand that ﬁrst. The

principles of digital video codec design are not too complicated, and haven’t really changed

much over the years – but those basic principles have been wrapped in layer-upon-layer of

technical enhancements to the point that the simple and straightforward concepts that lie at

their core can become obscured. The entire H.261 speciﬁcation was only 25 pages long, and

only 17 of those pages were actually required to fully specify the technology that now lies at

the heart of all subsequent video coding standards. In contrast, the H.264/AVC and MPEG-4

Visual and speciﬁcations are more than 250 and 500 pages long, respectively, with a high

density of technical detail (despite completely leaving out key information such as how to

encode video using their formats). They each contain areas that are difﬁcult even for experts

to fully comprehend and appreciate.

Dr Richardson’s book is not a completely exhaustive treatment of the subject. However,

his approach is highly informative and provides a good initial understanding of the key con-

cepts, and his approach is conceptually superior (and in some aspects more objective) to other

treatments of video coding publications. This and the remarkable timeliness of the subject

matter make this book a strong contribution to the technical literature of our community.

Gary J. Sullivan

Biography of Gary J. Sullivan, PhD

Gary J. Sullivan is the chairman of the Joint Video Team (JVT) for the development of the latest

international video coding standard known as H.264/AVC, which was recently completed as a

joint project between the ITU-T video coding experts group (VCEG) and the ISO/IEC moving

picture experts group (MPEG).

Preface

With the widespread adoption of technologies such as digital television, Internet streaming

video and DVD-Video, video compression has become an essential component of broad-

cast and entertainment media. The success of digital TV and DVD-Video is based upon the

10-year-old MPEG-2 standard, a technology that has proved its effectiveness but is now

looking distinctly old-fashioned. It is clear that the time is right to replace MPEG-2 video

compression with a more effective and efﬁcient technology that can take advantage of recent

progress in processing power. For some time there has been a running debate about which

technology should take up MPEG-2’s mantle. The leading contenders are the International

Standards known as MPEG-4 Visual and H.264.

This book aims to provide a clear, practical and unbiased guide to these two standards

to enable developers, engineers, researchers and students to understand and apply them effec-

tively. Video and image compression is a complex and extensive subject and this book keeps

an unapologetically limited focus, concentrating on the standards themselves (and in the case

of MPEG-4 Visual, on the elements of the standard that support coding of ‘real world’ video

material) and on video coding concepts that directly underpin the standards. The book takes an

application-based approach and places particular emphasis on tools and features that are help-

ful in practical applications, in order to provide practical and useful assistance to developers

and adopters of these standards.

I am grateful to a number of people who helped to shape the content of this book. I

received many helpful comments and requests from readers of my book Video Codec Design.

Particular thanks are due to Gary Sullivan for taking the time to provide helpful and detailed

comments, corrections and advice and for kindly agreeing to write a Foreword; to Harvey

Hanna (Impact Labs Inc), Yafan Zhao (The Robert Gordon University) and Aitor Garay for

reading and commenting on sections of this book during its development; to members of the

Joint Video Team for clarifying many of the details of H.264; to the editorial team at John

Wiley & Sons (and especially to the ever-helpful, patient and supportive Kathryn Sharples);

to Phyllis for her constant support; and ﬁnally to Freya and Hugh for patiently waiting for the

long-promised trip to Storybook Glen!

I very much hope that you will ﬁnd this book enjoyable, readable and above all useful.

Further resources and links are available at my website, http://www.vcodex.com/. I always

appreciate feedback, comments and suggestions from readers and you will ﬁnd contact details

at this website.

Iain Richardson

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