视频编码标准编码效率对比分析

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"这篇论文是IEEE Transactions on Circuits and Systems for Video Technology在2012年12月发表的一篇文章，对比了多个视频编码标准的编码效率，包括H.262/MPEG-2、H.263、MPEG-4 Visual、H.264/MPEG-4 AVC以及最新的High Efficiency Video Coding (HEVC)。通过峰值信噪比(PSNR)和主观测试结果，对这些标准进行了统一的分析评估。" 本文的核心在于对比不同视频编码标准的编码效率，特别是HEVC（高效视频编码）与前几代标准如H.264/MPEG-4 AVC之间的差异。HEVC被证明在保持与H.264/MPEG-4 AVC同等主观质量的情况下，平均只需要大约50%的比特率，特别是在低比特率、高分辨率视频内容和低延迟通信应用中表现尤为出色。在进行编码效率的比较时，PSNR是一个关键指标，它衡量了压缩后的视频信号与原始无损信号之间的质量差距。较高的PSNR值通常意味着更好的图像质量。而主观测试结果则更关注人类视觉系统对视频质量的实际感知，这对于评价编码效率的实用性至关重要。对于WVGA（宽视场图形阵列）和高清(HD)序列的主观测试显示，HEVC编码器能够在保持观看体验相当的情况下显著减少比特率。这表明HEVC在处理高分辨率视频时，能更有效地利用带宽，为用户提供与H.264/MPEG-4 AVC相媲美的画质，同时降低了数据传输需求。在低延迟通信应用中，HEVC的效率提升尤为重要。在实时视频通信如视频会议或在线游戏等场景下，快速且高效的数据编码是保证用户体验的关键，HEVC的低延迟性能使得它成为这类应用的理想选择。这篇论文揭示了HEVC编码标准在视频编码效率上的显著进步，尤其是在节约带宽和提高低比特率视频质量方面，这为视频编码技术的发展和应用提供了重要的参考依据。随着技术的不断演进，HEVC的高效特性使得它在移动设备、网络流媒体服务等领域有着广泛的应用前景。

1672 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS FOR VIDEO TECHNOLOGY, VOL. 22, NO. 12, DECEMBER 2012

differences from older standards is its increased ﬂexibility for

inter coding. For the purpose of motion-compensated predic-

tion, an MB can be partitioned into square and rectangular

block shapes with sizes ranging from 4 × 4to16× 16

luma samples. H.264/MPEG-4 AVC also supports multiple

reference pictures. Similarly to annex U of H.263, motion

vectors are associated with a reference picture index for

specifying the employed reference picture. The motion vectors

are transmitted using quarter-sample precision relative to the

luma sampling grid. Luma prediction values at half-sample

locations are generated using a 6-tap interpolation ﬁlter and

prediction values at quarter-sample locations are obtained by

averaging two values at integer- and half-sample positions.

Weighted prediction can be applied using a scaling and offset

for the prediction signal. For the chroma components, a

bilinear interpolation is applied. In general, motion vectors

are predicted by the component-wise median of the motion

vectors of three neighboring previously decoded blocks. For

16 ×8 and 8 ×16 blocks, the predictor is given by the motion

vector of a single already decoded neighboring block, where

the chosen neighboring block depends on the location of the

block inside an MB. In contrast to prior coding standards, the

concept of B pictures is generalized and the picture coding

type is decoupled from the coding order and the usage as a

reference picture. Instead of I, P, and B pictures, the standard

actually speciﬁes I, P, and B slices. A picture can contain slices

of different types and a picture can be used as a reference

for inter prediction of subsequent pictures independently of

its slice coding types. This generalization allowed the usage

of prediction structures such as hierarchical B pictures [17]

that show improved coding efﬁciency compared to the IBBP

coding typically used for H.262/MPEG-2 Video.

H.264/MPEG-4 AVC also includes a modiﬁed design for

intra coding. While in previous standards some of the DCT

coefﬁcients can be predicted from neighboring intra blocks, the

intra prediction in H.264/MPEG-4 AVC is done in the spatial

domain by referring to neighboring samples of previously

decoded blocks. The luma signal of an MB can be either

predicted as a single 16 × 16 block or it can be partitioned

into 4 × 4or8× 8 blocks with each block being predicted

separately. For 4 ×4 and 8 ×8 blocks, nine prediction modes

specifying different prediction directions are supported. In the

intra 16 ×16 mode and for the chroma components, four intra

prediction modes are speciﬁed.

For transform coding, H.264/MPEG-4 AVC speciﬁes a 4×4

and an 8×8 transform. While chroma blocks are always coded

using the 4 × 4 transform, the transform size for the luma

component can be selected on an MB basis. For intra MBs,

the transform size is coupled to the employed intra prediction

block size. An additional 2×2 Hadamard transform is applied

to the four DC coefﬁcients of each chroma component. For the

intra 16×16 mode, a similar second-level Hadamard transform

is also applied to the 4 × 4 DC coefﬁcients of the luma

signal. In contrast to previous standards, the inverse transforms

are speciﬁed by exact integer operations, so that, in error-

free environments, the reconstructed pictures in the encoder

and decoder are always exactly the same. The transform

coefﬁcients are represented using a uniform reconstruction

quantizer, that is, without the extra-wide dead-zone that is

found in older standards. Similar to H.262/MPEG-2 Video and

MPEG-4 Visual, H.264/MPEG-4 AVC also supports the usage

of quantization weighting matrices. The transform coefﬁcient

levels of a block are generally scanned in a zig–zag fashion.

For entropy coding of all MB syntax elements, H.264/

MPEG-4 AVC speciﬁes two methods. The ﬁrst entropy coding

method, which is known as context-adaptive variable-length

coding (CAVLC), uses a single codeword set for all syntax

elements except the transform coefﬁcient levels. The approach

for coding the transform coefﬁcients basically uses the concept

of run-level coding as in prior standards. However, the efﬁ-

ciency is improved by switching between VLC tables depend-

ing on the values of previously transmitted syntax elements.

The second entropy coding method speciﬁes context-adaptive

binary arithmetic coding (CABAC) by which the coding

efﬁciency is improved relative to CAVLC. The statistics of

previously coded symbols are used for estimating conditional

probabilities for binary symbols, which are transmitted using

arithmetic coding. Inter-symbol dependencies are exploited

by switching between several estimated probability models

based on previously decoded symbols in neighboring blocks.

Similar to annex J of H.263, H.264/MPEG-4 AVC includes

a deblocking ﬁlter inside the motion compensation loop. The

strength of the ﬁltering is adaptively controlled by the values

of several syntax elements.

The High proﬁle (HP) of H.264/MPEG-4 AVC includes

all tools that contribute to the coding efﬁciency for 8-bit-per-

sample video in 4:2:0 format, and is used for the comparison

in this paper. Because of its limited beneﬁt for typical video

test sequences and the difﬁculty of optimizing its parameters,

the weighted prediction feature is not applied in the testing.

E. HEVC (Draft 9 of October 2012)

High Efﬁciency Video Coding (HEVC) [4] is the name of

the current joint standardization project of ITU-T VCEG and

ISO/IEC MPEG, currently under development in a collabora-

tion known as the Joint Collaborative Team on Video Coding

(JCT-VC). It is planned to ﬁnalize the standard in early 2013.

In the following, a brief overview of the main changes relative

to H.264/MPEG-4 AVC is provided. For a more detailed

description, the reader is referred to the overview in [2].

In HEVC, a picture is partitioned into coding tree blocks

(CTBs). The size of the CTBs can be chosen by the encoder

according to its architectural characteristics and the needs of

its application environment, which may impose limitations

such as encoder/decoder delay constraints and memory re-

quirements. A luma CTB covers a rectangular picture area of

N ×N samples of the luma component and the corresponding

chroma CTBs cover each (N/2) × (N/2) samples of each of

the two chroma components. The value of N is signaled inside

the bitstream, and can be 16, 32, or 64. The luma CTB and the

two chroma CTBs, together with the associated syntax, form a

coding tree unit (CTU). The CTU is the basic processing unit

of the standard to specify the decoding process (conceptually

corresponding to an MB in prior standards).

The blocks speciﬁed as luma and chroma CTBs can be

further partitioned into multiple coding blocks (CBs). The

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