H.264/AVC可伸缩视频编码标准概述

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"这篇文章是对H.264/AVC标准可伸缩视频编码（Scalable Video Coding, SVC）扩展的概述，主要讨论了SVC在视频压缩能力上的显著提升以及它在不同应用场景中的优势。" 正文: H.264/AVC标准的引入，标志着视频编码领域取得了重大进步，显著提升了视频压缩的效率。这个标准是由国际电信联盟电信标准分部视频编码专家组(ITU-T VCEG)和国际标准化组织/国际电工委员会动态图像专家组(MPEG)联合制定的。随着技术的发展，他们又进一步标准化了SVC扩展，使得视频编码变得更加灵活和适应性更强。 SVC的核心理念是允许传输和解码部分比特流，从而在保持相对较高重建质量的同时，提供低时间分辨率、空间分辨率或降低的保真度的视频服务。这意味着，即使在有损传输环境中，SVC也能实现平滑的降级，保证视频服务的连续性。此外，SVC还支持比特率、格式和功率的自适应，这些特性对于传输和存储应用具有极大的增强作用。与之前视频编码标准的可伸缩性相比，SVC实现了编码效率的显著提高。它提供了更广泛的支持，包括时间、空间和质量等多维度的伸缩性。这种增强的伸缩性使得SVC在处理各种网络条件和设备性能差异时，能够更好地调整编码策略，确保视频质量和带宽使用之间的平衡。 SVC在实时通信、在线视频流媒体、移动通信和多媒体存储等领域有着广泛的应用。例如，在无线网络环境下，SVC可以自动适应网络状况，保证视频流的稳定播放；在多屏互动场景中，同一视频源可以根据不同的设备屏幕大小和性能进行优化编码，确保用户在各种设备上都能获得良好的观看体验。 H.264/AVC的SVC扩展不仅提高了编码效率，还增强了视频服务的适应性和鲁棒性，是现代视频编码技术的一个重要里程碑。随着技术的不断进步，SVC将在未来的视频编码和传输中发挥更大的作用，持续推动视频通信行业的创新和发展。

To appear in IEEE Transactions on Circuits and Systems for Video Technology, September 2007.

B. Video Coding Layer (VCL)

The VCL of H.264/AVC follows the so-called block-based

hybrid video coding approach. Although its basic design is

very similar to that of prior video coding standards such as

H.261, MPEG-1 Video, H.262 | MPEG-2 Video, H.263, or

MPEG-4 Visual, H.264/AVC includes new features that en-

able it to achieve a significant improvement in compression

efficiency relative to any prior video coding standard [14]. The

main difference to previous standards is the largely increased

flexibility and adaptability of H.264/AVC.

The way pictures are partitioned into smaller coding units in

H.264/AVC, however, follows the rather traditional concept of

subdivision into macroblocks and slices. Each picture is parti-

tioned into macroblocks that each covers a rectangular picture

area of 16×16 luma samples and, in the case of video in 4:2:0

chroma sampling format, 8×8 samples of each of the two

chroma components. The samples of a macroblock are either

spatially or temporally predicted, and the resulting prediction

residual signal is represented using transform coding. The

macroblocks of a picture are organized in slices, each of which

can be parsed independently of other slices in a picture. De-

pending on the degree of freedom for generating the prediction

signal, H.264/AVC supports three basic slice coding types:

– I slice: intra-picture predictive coding using spatial

prediction from neighboring regions,

– P slice: intra-picture predictive coding and inter-picture

predictive coding with one prediction signal for each

predicted region,

– B slice: intra-picture predictive coding, inter-picture

predictive coding, and inter-picture bi-predictive cod-

ing with two prediction signals that are combined with a

weighted average to form the region prediction.

For I slices, H.264/AVC provides several directional spatial

intra prediction modes, in which the prediction signal is gener-

ated by using neighboring samples of blocks that precede the

block to be predicted in coding order. For the luma compo-

nent, the intra prediction is either applied to 4×4, 8×8, or

16×16 blocks, whereas for the chroma components, it is al-

ways applied on a macroblock basis

For P and B slices, H.264/AVC additionally permits vari-

able block size motion-compensated prediction with multiple

reference pictures [27]. The macroblock type signals the parti-

tioning of a macroblock into blocks of 16×16, 16×8, 8×16, or

8×8 luma samples. When a macroblock type specifies parti-

tioning into four 8×8 blocks, each of these so-called sub-

macroblocks can be further split into 8×4, 4×8, or 4×4 blocks,

which is indicated through the sub-macroblock type. For P

slices, one motion vector is transmitted for each block. In addi-

tion, the used reference picture can be independently chosen

for each 16×16, 16×8, or 8×16 macroblock partition or 8×8

sub-macroblock. It is signaled via a reference index parameter,

Some details of the profiles of H.264/AVC that were designed primarily

to serve the needs of professional application environments are neglected in

this description, particularly in relation to chroma processing and range of

step sizes.

which is an index into a list of reference pictures that is repli-

cated at the decoder.

In B slices, two distinct reference picture lists are utilized,

and for each 16×16, 16×8, or 8×16 macroblock partition or

8×8 sub-macroblock, the prediction method can be selected

between list 0, list 1, or bi-prediction. While list 0 and list 1

prediction refer to unidirectional prediction using a reference

picture of reference picture list 0 or 1, respectively, in the bi-

predictive mode, the prediction signal is formed by a weighted

sum of a list 0 and list 1 prediction signal. In addition, special

modes as so-called direct modes in B slices and skip modes in

P and B slices are provided, in which such data as motion vec-

tors and reference indices are derived from previously trans-

mitted information.

For transform coding, H.264/AVC specifies a set of integer

transforms of different block sizes. While for intra macro-

blocks the transform size is directly coupled to the intra pre-

diction block size, the luma signal of motion-compensated

macroblocks that do not contain blocks smaller than 8×8 can

be coded by using either a 4×4 or 8×8 transform. For the

chroma components a two-stage transform, consisting of 4×4

transforms and a Hadamard transform of the resulting DC co-

efficients is employed

. A similar hierarchical transform is also

used for the luma component of macroblocks coded in intra

16×16 mode. All inverse transforms are specified by exact

integer operations, so that inverse-transform mismatches are

avoided. H.264/AVC uses uniform reconstruction quantizers.

One of 52 quantization step sizes

can be selected for each

macroblock by the quantization parameter QP. The scaling

operations for the quantization step sizes are arranged with

logarithmic step size increments, such that an increment of the

QP by 6 corresponds to a doubling of quantization step size.

For reducing blocking artifacts, which are typically the most

disturbing artifacts in block-based coding, H.264/AVC speci-

fies an adaptive deblocking filter, which operates within the

motion-compensated prediction loop.

H.264/AVC supports two methods of entropy coding, which

both use context-based adaptivity to improve performance

relative to prior standards. While CAVLC (context-based adap-

tive variable-length coding) uses variable-length codes and its

adaptivity is restricted to the coding of transform coefficient

levels, CABAC (context-based adaptive binary arithmetic cod-

ing) utilizes arithmetic coding and a more sophisticated

mechanism for employing statistical dependencies, which

leads to typical bit rate savings of 10-15% relative to CAVLC.

In addition to the increased flexibility on the macroblock

level, H.264/AVC also allows much more flexibility on a pic-

ture and sequence level compared to prior video coding stan-

dards. Here we mainly refer to reference picture memory con-

trol. In H.264/AVC, the coding and display order of pictures is

completely decoupled. Furthermore, any picture can be

marked as reference picture for use in motion-compensated

prediction of following pictures, independent of the slice cod-

ing types. The behavior of the decoded picture buffer (DPB),

which can hold up to 16 frames (depending on the used con-

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