VersatileVideoCoding(VVC)算法与VTM11详解

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"Versatile Video Coding (VVC) 代码算法文档，由 JVET 官方发布，包括 VTM11 测试模型的详细描述，作者包括 Jianle Chen, Yan Ye 和 SeungHwan Kim 等，适用于视频编码领域的专业人士。" 本文档详细介绍了 Versatile Video Coding (VVC)，这是一种先进的视频编码标准，旨在提供比前一代标准如 H.265/HEVC 更高的压缩效率。VVC 是由 ISO/IEC JTC1/SC29/WG11（MPEG）和 ITU-T SG16 Q6（VCEG）组成的 Joint Video Experts Team (JVET) 共同开发的。JVET 在其第20次会议上（2020年10月7日至16日，以电话会议形式举行）确立了 VVC 工作草案11和VTM11算法描述及编码方法。 VTM11是VVC的测试模型，用于在实际编码器和解码器开发之前评估和优化编码性能。这个文档不仅提供了关于VVC设计的一般教程信息，还给出了VTM11的编码侧描述。这包括了编码流程、熵编码、运动估计与补偿、变换与量化、环路滤波等关键步骤的详细说明。在视频编码中，每个步骤都至关重要。例如，运动估计是通过比较不同时间帧间的相似性来减少视频数据量的关键技术；变换和量化则将像素数据转换到频率域，并进行非线性处理，以利于进一步压缩；熵编码则负责有效地编码这些处理后的数据，以便在传输或存储时占用更少的空间。 VVC 的目标是通过采用更复杂的编码策略，如多规模分析、增强的块划分结构和新的预测技术，来提高压缩效率，同时保持或提高视频质量。这些改进使得 VVC 特别适合于高分辨率视频、VR 内容以及网络视频流等应用，它们需要在有限带宽下传输高质量视频。文档中可能还包括了关于测试模型的实现细节，比如编码工具的参数设置、优化策略以及性能评估方法。对于开发者和研究人员来说，这份文档是理解和实现 VVC 标准的重要参考资料，有助于他们在实际项目中应用 VVC 技术，优化视频编码性能，以满足不断增长的高清视频需求。

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block by applying the prediction process based on the PU splitting type, a leaf

CU can be partitioned into

transform units (Tus) according to another quaternary-tree structure similar to the coding tree for the CU.

One of

key feature of

the HE

VC structure is that it has the multiple partition conceptions including CU,

PU, and TU.

In VVC,

quadtree with nested multi-type tree using binary and ternary splits segmentation structure

replaces the concepts of

multiple partition unit types, i.e.

removes the separation of

the CU, PU and TU

concepts except as needed for CUs that have

size too large for the maximum transform length, and

supports more flexibility

for

CU partition shapes. In the coding tree structure,

CU can have either a

square or rectangular shape. A coding tree unit (CTU) is first partitioned by a quaternary tree (a.k.a.

quadtree) structure. Then the quaternary tree leaf nodes can be further partitioned by a multi-type tree

structure. As shown in Figure 7, there are

four

splitting types in multi-type tree structure, vertical binary

splitting (SPLIT BT VER), horizontal binary splitting (SPLIT BT HOR), vertical ternary splitting

(SPLIT TT VER), and horizontal ternary splitting (SPLIT TT HOR). The multi-type tree leaf

nodes are

called coding units (CUs), and unless the CU is too large for the maximum transform length, this

segmentation is used

for

prediction and transform processing without

any

further partitioning. This means

that, in most cases, the CU, PU and TU have the same block size in the quadtree with nested multi-type

tree coding block structure. The exception occurs when maximum supported transform length is smaller

than the width or height of

the colour component of

the CU.

SPLIT BT VER SPLIT BT HOR SPLIT TT VER SPLIT TT HOR

Figure 7

Multi-type tree splitting modes

Figure 8 illustrates the signalling mechanism of

the partition splitting information in quadtree with nested

multi-type tree coding tree structure. A coding tree unit (CTU) is treated as the root of

quaternary tree

and is first partitioned by a quaternary tree structure. Each quaternary tree leaf node (when sufficiently

large to allow it) is then further partitioned by a multi-type tree structure. In quadtree with nested multi-

type tree coding tree structure,

for

each CU node,

first flag (split cu flag)

signalled to indicate

whether the node is further partitioned. If the current CU node is a quadtree CU node, a second flag

(split qt flag) whether

it's

QT partitioning or MTT partitioning mode. When

node is partitioned with

MTT partitioning mode,

third flag (mttsplitcuverticalflag) is signalled to indicate the splitting

direction, and then a fourth flag (mttsplitcubinaryflag) is signalled to indicate whether the split is a

binary split or a ternary split. Based on the values of mttsplitcuverticalflag and

mtt split cu binary flag, the multi-type tree slitting mode (MttSplitMode) of

CU is derived as shown

in Table 3- 1.

Date Saved: 2021-10-09

Page: 16

—

Figure

Example of

quadtree with nested multi-type tree coding block structure

The following parameters are defined

for

the quadtree with nested multi-type tree coding tree scheme.

These parameters are specified by SPS syntax elements and can be further refined by picture header

syntax elements.

CTU size: the root node size of

a quaternary tree

MinQTSize:

the minimum allowed quaternary tree leaf

node size

McixBtSize'.

the maximum allowed binary

tree root node size

MaxTtSize'.

the maximum allowed ternary tree root node size

MaxMttDepth'.

the maximum allowed hierarchy depth of

multi-type tree splitting from a quadtree leaf

MinCbSize'.

the minimum allowed coding block

node size

In one example of

the quadtree with nested multi-type tree coding tree structure, the CTU size

set

128x128 luma samples with two corresponding 64x64 blocks of

4:2:0 chroma samples, the MinQTSize is

set as 16x16, the McixBtSize

is set

128x128 and MaxTtSize is set as 64x64, the

MinCbsize

(for both

width and height) is set

4x4, and the MaxMttDepth is set as 4. The quaternary tree partitioning is

applied to the CTU first to generate quaternary tree leaf

nodes. The quaternary tree leaf

nodes may have

size from 16x16 (i.e., the MinQTSize} to 128x128 (i.e., the CTU size). If

the leaf

QT node is 128x128, it

will not be further split by the binary tree since the size exceeds the McixBtSize

and MaxTtSize (i.e.,

64x64). Otherwise, the leaf

qdtree node could be further partitioned by the multi-type tree. Therefore, the

quaternary tree leaf node is also the root node for the multi-type tree and it has multi-type tree depth

(mttDepth) as 0. When the multi-type tree depth reaches MaxMttDepth (i.e., 4), no further splitting is

considered. When the multi-type tree node has width equal to

MinCbsize,

no further horizontal splitting is

considered. Similarly, when the multi-type tree node has height equal to MinCbsize, no further vertical

splitting is considered.

In VVC, the coding tree scheme supports the ability for the luma and chroma to have a separate block tree

structure. For

and B slices, the luma and chroma CTBs in one CTU have to share the same coding tree

Date Saved: 2021-10-09

Page: 18

structure. However, for I slices, the luma and chroma can have separate block tree structures. When

separate block tree mode is applied, luma CTB is partitioned into CUs by one coding tree structure, and

the chroma CTBs are partitioned into chroma CUs by another coding tree structure. This means that a CU

in an I slice may consist of

coding block of the luma component or coding blocks of two chroma

components, and a CU in a

or B slice always consists of

coding blocks of

all three colour components

unless the video is monochrome.

3.2.4 CU splits on picture boundaries

Similar to HEVC, when a portion of

a tree node block exceeds the bottom or right picture boundary, the

tree node block is forced to be split until the all samples of

every coded CU are located inside the picture

boundaries. The following splitting rules are applied in the VVC:

any portion of

tree node block exceeds the bottom or the right picture boundaries, TT splitting

not allowed.

any

portion of

tree node block exceeds the bottom or the right picture boundaries, and any of

and BT splitting is not allowed due to block size restriction, the block is forced to be split with

split

mode.

Otherwise if

portion of

tree node block exceeds both the bottom and the right

picture boundaries,

the block is a QT node and the size of

the block is larger than the minimum QT size, the block

is forced

to be split

with QT split mode.

Otherwise, the block is forced to be split with SPLITBTHOR

mode

Otherwise if

portion of

tree node block exceeds the bottom picture boundaries,

the block is a QT node, and the size of

the block is larger than the minimum

size, and the

size of the block is larger than the maximum BT size, the block is forced to be split with

split mode.

Otherwise, if

the block is a QT node, and the size of

the block is larger than the minimum

size

and the size of

the block is smaller than or equal to the maximum BT size, the block is forced to

be split with QT split mode or SPLIT BT HOR mode and a flag split

Jlag

is signalled to

specify the partitioning mode for this case.

Otherwise (the block is a BTT node or the size of the block is smaller than or equal to the

minimum

size), the block is forced to be split

with SPLIT BT HOR mode.

Otherwise if

portion of

tree node block exceeds the right

picture boundaries,

the block is a QT node, and the size of

the block is larger than the minimum

size, and the

size of the block is larger than the maximum BT size, the block is forced to be split with

split mode.

Otherwise, if

the block is a QT node, and the size of

the block is larger than the minimum

size

and the size of

the block is smaller than or equal to the maximum BT size, the block is forced to

be split with QT split mode or SPLITBTVER

mode and

flag split

Jlag

is signalled to

specify the partitioning mode for this case.

Otherwise (the block is a BTT node or the size of the block is smaller than or equal to the

minimum

size), the block is forced to be split

with SPLIT BT VER

mode.

3.2.5 Restrictions on redundant CU splits

The quadtree with nested multi-type tree coding block structure provides

highly flexible block

partitioning structure. Due to the types of splits supported the multi-type tree, different splitting patterns

could potentially result in the same coding block structure. In VVC, some of these redundant splitting

patterns are disallowed.

Date Saved: 2021-10-09

Page: 19

Figure 10 illustrates the redundant splitting patterns of

binary tree splits and ternary tree splits. As shown

in Figure 10, two levels of consecutive binary splits in one direction could have the same coding block

structure as a ternary tree split followed by a binary tree split of the central partition. In this case, the

binary tree split (in the given direction)

for

the central partition of

a ternary tree split is prevented by the

syntax. This restriction applies for CUs in

all

pictures.

Figure 10-Redundant splitting patterns of

binary tree split and ternary tree split cases

When the splits are prohibited as described above, signalling of the corresponding syntax elements is

modified to account for the prohibited cases. For example, when any case in Figure 10 is identified (i.e.

the binary split is prohibited for

CU of

a central partition), the syntax element mtt

split

binary flag

which

specifies

whether the split is

binary split or a ternary split is

not

signalled and is instead inferred

to be equal to 0 by the decoder.

3.2.6 Virtual pipeline data units (VPDUs)

Virtual pipeline data units (VPDUs) are defined as non-overlapping units in a picture. In hardware

decoders, successive VPDUs are processed by multiple pipeline stages at the same time. The VPDU size

is roughly proportional to the buffer size in most pipeline stages, so it is important to keep the VPDU size

small. In most hardware decoders, the VPDU size can be set to maximum transform block (TB) size.

However, in VVC, ternary tree (TT) and binary tree (BT) partition may lead to the increasing of

VPDUs

size.

In order to keep the VPDU size as 64x64 luma samples, the following normative partition restrictions

(with syntax signaling modification) are applied in VTM,

shown in Figure 1

TT split is not allowed

for

a CU with either width or height, or both width and height equal to

128.

For a 128xN CU with N < 64 (i.e. width equal to

128

and height smaller than 128),

horizontal BT is not allowed.

For an Nxl28 CU with N < 64 (i.e. height equal

128 and width smaller than 128), vertical

BT is not allowed.

128 (luma)

128

Figure 11

Examples of

disallowed TT and BT partitioning in VTM

Date Saved: 2021-10-09

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