HEVC编码效率对比：低码率与高清视频的卓越表现

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本文《视频编码标准的编码效率比较：包括高效视频编码（HEVC）》发表在2012年12月的IEEE Transactions on Circuits and Systems for Video Technology上。作者们对H.262/MPEG-2 Video、H.263、MPEG-4 Visual、H.264/MPEG-4 AVC（高级视频编码）以及主角HEVC进行了深度分析。文章的核心焦点在于比较这些视频编码标准在压缩性能上的差异，使用了峰值信噪比（PSNR）和主观测试的结果作为评估指标。 HEVC（High-Efficiency Video Coding）是新一代的视频编码标准，旨在提高编码效率，降低比特率的同时保持或接近与H.264/MPEG-4 AVC相同的主观观看质量。实验结果显示，HEVC编码器在WVGA和高清（HD）序列上，平均使用大约50%的比特率就能实现与H.264/MPEG-4 AVC相当的观感效果。这表明HEVC特别适用于低比特率、高分辨率视频内容以及对延迟敏感的通信应用领域。 HEVC的设计优化体现在以下几个关键方面： 1. **码率效率提升**：通过采用更先进的编码技术，如高效帧内预测、多模式工具、变换块尺寸和编码结构的优化，HEVC能够以更低的比特率达到相同的图像质量，从而提升了编码效率。 2. **多层结构**：HEVC支持多层结构，允许在不同的分辨率和复杂度下编码，可以根据实际需求灵活调整，适应不同场景下的复杂性和实时性要求。 3. **高效编码工具**：HEVC引入了诸如循环外预测、深度分层编码等创新技术，减少了冗余，提高了编码效率。 4. **低延迟设计**：HEVC特别针对实时通信需求，如网络视频流和在线会议，通过减少编码和解码延迟，实现了更快的响应速度。 5. **适应性强**：HEVC能在各种视频内容和网络条件下表现出色，对于高分辨率和复杂纹理的场景有显著优势，如4K和8K视频。总结来说，这篇文章提供了深入的实证证据，展示了HEVC如何通过技术创新和优化在视频编码效率上超越前代标准，尤其是在处理高分辨率、低比特率和低延迟场景时的优势，对于视频编码技术的研究者和实践者具有重要的参考价值。

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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