HEVC测试模型16.14软件更新：高效视频编码与范围扩展

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"Joint Collaborative Team on Video Coding (JCT-VC) 是由ITU-T SG16 WP3和ISO/IEC JTC1/SC29/WG11共同成立的团队，专注于视频编码标准的研发。该团队的工作成果体现在HEVC（High Efficiency Video Coding）即H.265标准中，其版本1和版本2的Range Extension (RExt) 都在HEVC测试模型软件中得到了实现。这份文档概述了HEVC编码工具，以及HM编码器使用的算法和用户配置选项。" 正文: JCT-VC，全称为Joint Collaborative Team on Video Coding，是由国际电信联盟ITU-T的SG16工作组第三分组和国际标准化组织ISO/IEC的JTC1/SC29/WG11联合组建的合作团队，其主要任务是开发和改进视频编码技术，以提高视频压缩效率和图像质量。这个团队的工作成果体现在了HEVC（High Efficiency Video Coding）中，这是继H.264/AVC之后的一个重要的视频编码标准。 HEVC，也被称为H.265，是一种旨在提高视频数据压缩比的编码标准，允许在相同的带宽下传输更高质量的视频。在HEVC Version 1中，JCT-VC引入了一系列创新的编码工具，包括更精细的块划分结构、预测模式的扩展、熵编码的优化等，这些工具显著提高了编码效率，使得视频在保持画质的同时，所需的比特率显著降低。随着技术的发展，HEVC Version 2的Range Extension (RExt) 进一步扩展了标准的性能，支持了更高的动态范围和广色域视频，以及高比特率的视频编码。RExt引入了新的编码工具，如像素级别的动态范围转换、高动态范围色彩空间的处理，以及针对4:2:0、4:2:2和4:4:4采样格式的支持。 HM (High Efficiency Video Coding Test Model) 是JCT-VC团队开发的一系列测试模型软件，用于验证和优化HEVC标准的编码算法。HM16.14是其中的一个版本，它包含了HEVC Version 1的基本功能和Version 2的Range Extension特性。此软件提供了丰富的编码控制选项，允许用户根据特定应用场景和需求进行参数设置，例如码率控制、质量优先或延迟优化等。在编码控制方面，HM编码器有多种配置选项，比如通过配置文件指定编码参数，包括分辨率、帧率、编码模式、比特率控制策略等。这些配置选项使得编码过程可定制化，满足不同应用场景的需求，如在线流媒体、视频存储或广播传输。 JCT-VC的工作对视频编码领域产生了深远影响，HEVC及其Range Extension技术不仅提升了视频编码的效率，还推动了高动态范围和广色域视频的普及，而HM测试模型软件则为开发者和研究人员提供了评估和优化编码算法的实用工具。

• Picture parameter set (PPS): Contains information which may vary on a per-picture basis. Examples of

information contained in the PPS are: quantization parameter and flags indicating the use of particular coding

tools (e.g. Transform Skip).

Parameter sets may reference other parameter sets, specifically, a PPS has an ID indicating the associated SPS and an

SPS has an ID indicating the associated VPS. Regardless of these associations, to facilitate parsing robustness, each

parameter set can be parsed independently, i.e. there is no conditional dependency in the syntax parsing on information

present in any associated parameter set.

4.1.3 Picture types

Random access functionality is provided using intra random access point (IRAP) pictures. An IRAP picture can only

contain one or more I-slices. HEVC defines three types of IRAP. These three picture types are:

• Instantaneous decoding refresh (IDR)

• Clean random access (CRA)

• Broken link access (BLA)

An IDR picture, when encountered, results in flushing of the decoded picture buffer (DPB). IDR pictures provide RAP

functionality but sacrifice coding performance because frames decoded prior to an IDR picture are no longer available

for reference in inter coding. In order to allow random access to the content and maintain the coding performance, HEVC

defines CRA pictures. CRA pictures are intra coded but, when encountered, do not empty the DPB. Consequently,

pictures following a CRA picture in decoding order can still use reference pictures that precede the CRA picture in

decoding order. Leading pictures may follow a CRA picture; leading pictures can either be decoded or skipped. Pictures

following a CRA in decoding order and correctly decodable are called random access decodable leading (RADL)

pictures. Pictures that follow a CRA picture in decoding order but cannot be correctly decoded without preceding

reference frames having also been decoded are called random access skipped leading (RASL) pictures.

One example use case for this functionality is splicing bitstreams to insert advertisements in a television programme.

Consider the case when Bitstream 1 (B1) and Bitstream 2 (B2) are concatenated as B1·B2 and the picture which starts

the segment associated to B2 is a CRA. All RASL pictures following the CRA picture in decoding order in B2 cannot be

correctly decoded because their associated reference pictures are not present in the DPB. These RASL pictures should be

discarded from the decoder output and this is accomplished by the splicing operation declaring the CRA picture in B2 to

be a BLA picture. In this case the decoder knows that all RASL associated with this BLA picture will not be displayed.

Finally, HEVC also defines two additional types of pictures to support temporal scalability:

• Temporal sublayer access (TSA)

• Step-wise temporal sub-layer access (STSA)

These pictures impose restrictions on the reference used between different temporal layers so that temporal down-

switching and up-switching operations can be made possible (see Figure 5 in [5] for an example on the use of TSA and

STSA pictures).

4.1.4 Reference picture set

The reference picture set (RPS) has been introduced in HEVC to handle reference pictures in the DPB. In fact, when a

picture is no longer used for reference by other pictures, it should be discarded from the DPB. If instead a picture is used

as reference for future pictures it must be kept in the DPB to correctly decode the bitstream. The RPS contains

information on the status of the DPB and may be signalled in the SPS, and additionally signalled, or overridden, in the

slice header. The signalling is absolute, i.e. each RPS describes the DPB status and does not refer to any previous status

for its description. In this way, bitstream error resilience is improved even when some NAL units are lost.

4.2 Picture partitioning

4.2.1 Coding tree unit (CTU) partitioning

Pictures are divided into a sequence of coding tree units (CTUs), all being the same size, and each covering a square

pixel region of the picture. An example of a picture divided into CTUs is shown in Figure 4-1. The size of a CTU is

specified with respect to the luma channel, to prevent ambiguity when considering chroma formats.

The size of the CTU is configured as one of 16×16, 32×32 or 64×64 luma samples.

Figure 4-1. Example of a picture divided into CTUs.

4.2.2 Slice and tile structures

A slice is a data structure that can be decoded independently from other slices of the same picture, in terms of entropy

coding, signal prediction, and residual signal reconstruction. A slice can either be the entire picture or a region of a

picture, which is not necessarily rectangular. A slice consists of a sequence of one or more slice segments starting with

an independent slice segment and containing all subsequent dependent slice segments (if any) that precede the next

independent slice segment (if any) within the same access unit.

A slice segment consists of a sequence of CTUs. An independent slice segment is a slice segment for which the values of

the syntax elements of the slice segment header are not inferred from the values for a preceding slice segment. A

dependent slice segment is a slice segment for which the values of some syntax elements of the slice segment header are

inferred from the values for the preceding independent slice segment in decoding order. For dependent slice segments,

prediction can be performed across dependent slice segment boundaries, and entropy coding is not initialized at the

starting of the dependent slice segment parsing process.

An example of picture with 11 by 9 CTUs that is partitioned into two slices is shown in Figure 4-2, below. In this

example, the first slice is composed of an independent slice segment containing 4 CTUs, a dependent slice segment

containing 32 CTUs, and another dependent slice segment containing 24 CTUs. The second slice consists of a single

independent slice segment containing the remaining 39 CTUs of the picture.

Figure 4-2. Example of slices and slice segments.

A tile is a rectangular region containing an integer number of CTUs. CTUs are ordered in raster scan within a tile, and

tiles in a picture are ordered consecutively in a raster scan of the tiles of the picture. This defines the coding order of

CTUs, which is referred to as the tile scan order.

slice segment

boundary

slice boundary

independent

slice segment

dependent

slice segment

A tile may consist of CTUs contained in more than one slice. Similarly, a slice may consist of CTUs contained in more

than one tile. Note that within the same picture, there may be both slices that contain multiple tiles and tiles that contain

multiple slices. However, one or both of the following conditions must be fulfilled for each slice and tile:

– All coding tree units in a slice belong to the same tile.

– All coding tree units in a tile belong to the same slice.

In addition, one or both of the following conditions must be fulfilled for each slice segment and tile:

– All coding tree units in a slice segment belong to the same tile.

– All coding tree units in a tile belong to the same slice segment.

Two examples of possible slice and tile structures for a picture with 11 by 9 coding tree units are shown in Figure 4-3,

below. In both examples, the picture is partitioned into two tiles, separated by a vertical tile boundary. The left-hand

example shows a case in which the picture only contains one slice, starting with an independent slice segment and

followed by four dependent slice segments. The right-hand example illustrates an alternative case in which the picture

contains two slices in the first tile and one slice in the second tile.

Figure 4-3. Examples of tiles and slices.

4.2.3 Coding unit (CU) and coding tree structure

The coding unit (CU) is a square region (in terms of pixels/luma-samples), and is a node of the quadtree partitioning of

the CTU. The quadtree partitioning structure allows recursive splitting into four equally sized nodes, starting from the

CTU and stopping when no further splitting is signalled in the bit stream (as determined by an encoder) or when the

minimum CU size is reached. The minimum CU size is configured in the SPS to be 32×32, 16×16, or 8×8 luma samples;

8×8 is used in the common test conditions [3] for HEVC development. Each leaf node CU is configured to use a

particular prediction mode, that being either intra prediction or inter-prediction. Figure 4-4 shows a CTU divided into

multiple CUs.

tile

boundary

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