HEVC视频编码中四叉树分区的块合并技术

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"Block Merging for Quadtree-Based Partitioning in HEVC" 在视频编码领域，高效率视频编码（High Efficiency Video Coding，简称HEVC）是一个重大的技术进步，由国际电信联盟电信标准化部门（ITU-T）视频编码专家小组和国际标准化组织/国际电工委员会（ISO/IEC）运动图像专家组联合开发。这篇论文发表在2012年12月的《IEEE Transactions on Circuits and Systems for Video Technology》上，由Philipp Helle、Simon Oudin、Benjamin Bross等人撰写，他们提出了一个针对HEVC的块合并算法，旨在解决基于四叉树的块分区所带来的冗余问题。 HEVC标准采用了一种混合视频编码方法，其中的关键特征是基于四叉树的块分割与运动补偿预测相结合。四叉树结构允许编码器根据视频内容自适应地调整编码块的大小，从而提高压缩效率。然而，尽管这种自适应性带来了诸多优点，但其内在的缺点也显而易见：可能会导致传输多余的一组运动参数。论文指出，这些冗余可以通过合并四叉树结构中的叶子节点来有效去除。作者提出的块合并算法就是针对这一问题的解决方案。该算法能够分析四叉树结构，并识别出可以合并的相邻块，以减少冗余的运动参数，从而优化编码过程，提高编码效率，降低码流中的数据量，同时保持视频质量。块合并算法的具体实施可能涉及以下几个步骤： 1. 分析四叉树结构：对四叉树的每个叶节点进行检查，寻找相邻并且具有相似运动信息的块。 2. 运动参数合并：找到可以合并的块后，将它们的运动参数进行整合，以减少传输的数据量。 3. 冗余检测：评估合并对视频质量和压缩效率的影响，确保优化过程中不会引入过多的编码失真。 4. 算法优化：通过迭代或基于概率模型的方法，进一步改进合并策略，以达到最佳的压缩性能。这个块合并算法的引入，不仅减少了编码复杂性，还提高了HEVC编码器的性能，为视频编码标准的发展提供了新的思路。通过减少传输的冗余信息，它有助于在有限的带宽下提供更高质量的视频流，这对现代通信和多媒体应用具有重要意义。

1722 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS FOR VIDEO TECHNOLOGY, VOL. 22, NO. 12, DECEMBER 2012

Fig. 2. Different partitions allowed for inter-picture prediction in a CB used

by HEVC.

better capturing the distinct types of motion in the scene.

Ideally, the boundary of each region should coincide with

the motion discontinuities in the given video signal. Using

quadtree-based block partitioning combined with merging, the

picture can be subdivided into smaller and smaller blocks,

thereby approximating the motion boundary and minimizing

the size of partitions containing the boundary. Subsequently,

each created quadtree leaf block can be merged into a region

on either side of the boundary.

It should be noted that a scheme merging spatially neighbor-

ing blocks is conceptually similar to spatial prediction modes

as, e.g., the spatial direct mode in H.264/AVC [16]. This

mode also tries to reduce coding cost by using redundancies

of motion parameters in neighboring blocks. However, the

improvements over H.264/AVC shown in previous work [4],

[6] suggest that the merging concept is superior in exploiting

these redundancies. This is also conﬁrmed by the experiments

presented in this paper, where the proposed block merging

algorithm is compared to a direct mode similar to that of

H.264/AVC, which we have reintegrated into HEVC just for

the purpose of analysis.

III. Quadtree-Based Partitioning in HEVC

The ﬁrst part of this section gives an overview of the

quadtree-based partitioning in HEVC and introduces terminol-

ogy used throughout the rest of this paper. As we are particu-

larly concerned about the motion model and its parameters, we

also introduce the prediction employed for differential coding

of motion vectors (MVs). The following description of HEVC

is based on the Main proﬁle, which is the only proﬁle deﬁned

in the DIS.

A. Quadtree Structure

For HEVC, a quadtree-based coding approach was intro-

duced such that each picture is divided into square coding tree

blocks (CTBs). Each CTB is the root of a coding tree, which

is used to further divide the CTB into coding blocks (CBs).

Their size can be adaptively chosen by using a quadtree-based

partitioning with the leaves of the quadtree representing the

CBs [7]. Each CB is a root for a prediction and a transforma-

tion tree. The prediction tree has only one level and describes

Fig. 3. HEVC quadtree structures. (a) CTB (solid block) partitioned into

CB (solid) and transform blocks (dashed) of variable size. (b) Corresponding

nested quadtree structure.

how a CB can be further split into so-called prediction blocks

(PBs), for each of which prediction parameters are speciﬁed.

Fig. 2 depicts all different ways allowed by the current Main

proﬁle to split a CB into inter-PBs. For transform coding of

the prediction residual signal, each CB can also be split into

smaller transform blocks (TBs) using another quadtree called

the residual quadtree (RQT) [7], [17]. Fig. 3 illustrates this

nested quadtree structure, i.e., the coding quadtree with the

CTB as root (solid bold line) and the CBs as leaves (solid

lines), each of which is the root of the nested RQT with the

TBs as leaves (dashed lines).

All the blocks in different trees (coding, prediction, or trans-

form tree) correspond to speciﬁc sample arrays with different

sizes. Depending on which tree they are related to, these

blocks are associated with a speciﬁc syntax structure and form

together the so-called units. The TB luma and chroma sample

arrays and associated syntax elements, e.g., coded block ﬂags

or transform coefﬁcient levels, are grouped together in a trans-

form unit (TU). A prediction unit (PU) encapsulates everything

that is related to prediction, i.e., the PB sample arrays and as-

sociated syntax elements, e.g., MVs or intra-picture prediction

modes. The CB sample arrays, the associated syntax elements

like the mode information whether intra- or inter-picture pre-

diction are used and the associated PUs and TUs are grouped

together in a coding unit (CU). Consequently, the CTB sample

arrays, associated coding tree syntax and associated CUs are

considered as coding tree unit (CTU). Thus, it can be said that

the CTU generalizes the concept of a macroblock as the basic

processing unit in standardized video coding.

B. Prediction of MVs

The H.264/AVC standard has only one single MV predictor

to differentially code the MVs, computed as the median of

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