H.264/AVC视频编码标准概览
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本文档是对H.264/AVC视频编码标准的概述，适合初学者了解这一编码技术。H.264/AVC是由国际电信联盟（ITU-T）视频编码专家小组（VCEG）和国际标准化组织/国际电工委员会（ISO/IEC）动态图像专家组（MPEG）共同制定的新一代视频编码标准。其主要目标是在提高压缩效率的同时，提供一种“网络友好”的视频表示形式，适用于对话式（如视频电话）和非对话式（如存储、广播或流媒体）的应用场景。相比现有的视频编码标准，H.264/AVC在率失真效率方面实现了显著提升。
一、引言
H.264/AVC是最新发布的国际视频编码标准。在发布时，预计已经被ITU-T采纳为H.264建议，并由ISO/IEC批准为国际标准14496-10（MPEG-4第10部分）高级视频编码。这个标准的制定是为了满足不断增长的多媒体通信需求，特别是在带宽有限的情况下，确保高质量的视频传输。
二、技术特点
H.264/AVC引入了许多创新技术，包括：
1. 分层编码：通过使用多级编码，允许编码器根据网络条件自适应地调整视频质量。
2. 高效的运动估计与补偿：采用更精确的运动矢量预测，减少冗余信息，提高压缩效率。
3. 熵编码：结合了上下文自适应二进制算术编码（CABAC）和上下文自适应变长编码（CAVLC），优化编码过程中的信息表示。
4. 块划分多样性：支持多种块大小和形状，适应不同类型的图像结构。
5. 亚像素运动估计：增加运动估计的精度，进一步减小码率。
6. 基于宏块的多参考帧：使用多个参考帧进行预测，改善运动补偿的效果。
7. 帧内预测：预测像素值基于同一帧内的其他像素，降低传输的数据量。
8. 变分量化：量化参数可以根据图像内容和网络状况灵活调整，以达到最佳的编码效率。
三、应用和配置文件
H.264/AVC标准支持多种配置文件，以适应不同的应用场景。例如，基本配置文件（Baseline Profile）适合简单的流媒体服务，而主配置文件（Main Profile）则支持更复杂的编码特性，适用于高清电视和DVD。高级配置文件（High Profile）则提供了更高的压缩效率，适合蓝光光盘和高清电视广播。
四、标准化过程
H.264/AVC的制定过程中，两大标准组织VCEG和MPEG进行了紧密合作，成立了联合视频团队（Joint Video Team, JVT），确保了技术的统一和兼容性。这一合作模式也对后来的标准制定产生了深远影响。
总结：
H.264/AVC视频编码标准凭借其高效压缩、网络适应性和广泛的应用支持，成为多媒体通信领域的关键技术。其技术特点如分层编码、熵编码和亚像素运动估计等，使得它在保持高画质的同时，能够有效地降低数据传输的需求，从而在各种网络环境中实现流畅的视频播放。无论是在实时视频通话还是在大规模的流媒体服务中，H.264/AVC都扮演着至关重要的角色。
WIEGAND et al.: OVERVIEW OF THE H.264/AVC VIDEO CODING STANDARD 4
Robustness to data errors/losses and flexibility for operation
over a variety of network environments is enabled by a
number of design aspects new to the H.264/AVC standard
including the following highlighted features.
• Parameter set structure: The parameter set
design provides for robust and efficient
conveyance header information. As the loss of a
few key bits of information (such as sequence
header or picture header information) could have a
severe negative impact on the decoding process
when using prior standards, this key information
was separated for handling in a more flexible and
specialized manner in the H.264/AVC design.
• NAL unit syntax structure: Each syntax structure
in H.264/AVC is placed into a logical data packet
called a NAL unit. Rather than forcing a specific
bitstream interface to the system as in prior video
coding standards, the NAL unit syntax structure
allows greater customization of the method of
carrying the video content in a manner appropriate
for each specific network.
• Flexible slice size: Unlike the rigid slice structure
found in MPEG-2 (which reduces coding
efficiency by increasing the quantity of header data
and decreasing the effectiveness of prediction),
slice sizes in H.264/AVC are highly flexible, as
was the case earlier in MPEG-1.
• Flexible macroblock ordering (FMO): A new
ability to partition the picture into regions called
slice groups has been developed, with each slice
becoming an independently-decodable subset of a
slice group. When used effectively, flexible
macroblock ordering can significantly enhance
robustness to data losses by managing the spatial
relationship between the regions that are coded in
each slice. (FMO can also be used for a variety of
other purposes as well.)
• Arbitrary slice ordering (ASO): Since each slice
of a coded picture can be (approximately) decoded
independently of the other slices of the picture, the
H.264/AVC design enables sending and receiving
the slices of the picture in any order relative to
each other. This capability, first found in an
optional part of H.263+, can improve end-to-end
delay in real-time applications, particularly when
used on networks having out-of-order delivery
behavior (e.g., internet protocol networks).
• Redundant pictures: In order to enhance
robustness to data loss, the H.264/AVC design
contains a new ability to allow an encoder to send
redundant representations of regions of pictures,
enabling a (typically somewhat degraded)
representation of regions of pictures for which the
primary representation has been lost during data
transmission.
• Data Partitioning: Since some coded information
for representation of each region (e.g., motion
vectors and other prediction information) is more
important or more valuable than other information
for purposes of representing the video content,
H.264/AVC allows the syntax of each slice to be
separated into up to three different partitions for
transmission, depending on a categorization of
syntax elements. This part of the design builds
further on a path taken in MPEG-4 Visual and in
an optional part of H.263++. Here the design is
simplified by having a single syntax with
partitioning of that same syntax controlled by a
specified categorization of syntax elements.
• SP/SI synchronization/switching pictures: The
H.264/AVC design includes a new feature
consisting of picture types that allow exact
synchronization of the decoding process of some
decoders with an ongoing video stream produced
by other decoders without penalizing all decoders
with the loss of efficiency resulting from sending
an I picture. This can enable switching a decoder
between representations of the video content that
used different data rates, recovery from data losses
or errors, as well as enabling trick modes such as
fast-forward, fast-reverse, etc.
In the following two sections, a more detailed description of
the key features is given.
III. N
ETWORK ABSTRACTION LAYER
The network abstraction layer (NAL) is designed in order to
provide "network friendliness" to enable simple and
effective customization of the use of the VCL for a broad
variety of systems.
The NAL facilitates the ability to map H.264/AVC VCL
data to transport layers such as
• RTP/IP for any kind of real-time wire-line and wireless
Internet services (conversational and streaming)
• File formats, e.g. ISO MP4 for storage and MMS
• H.32X for wireline and wireless conversational
services
• MPEG-2 systems for broadcasting services, etc.
The full degree of customization of the video content to fit
the needs of each particular application is outside the scope
of the H.264/AVC standardization effort, but the design of
the NAL anticipates a variety of such mappings. Some key
concepts of the NAL are NAL units, byte stream, and
packet format uses of NAL units, parameter sets, and access
units. A short description of these concepts is given below
whereas a more detailed description including error
resilience aspects is provided in [6] and [7].
A. NAL units
The coded video data is organized into NAL units, each of
which is effectively a packet that contains an integer
number of bytes. The first byte of each NAL unit is a
header byte that contains an indication of the type of data in
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