Encoder H.264

[Arnold Gilgen]

AdvancedVideo Coding (AVC), also referred to as H.264 or MPEG-4 Part 10, is a video compression standard based on block-oriented, motion-compensated coding.[2] It is by far the most commonly used format for the recording, compression, and distribution of video content, used by 91% of video industry developers as of September 2019[update].[3][4] It supports a maximum resolution of 8K UHD.[5][6]

H.264 is perhaps best known as being the most commonly used video encoding format on Blu-ray Discs. It is also widely used by streaming Internet sources, such as videos from Netflix, Hulu, Amazon Prime Video, Vimeo, YouTube, and the iTunes Store, Web software such as the Adobe Flash Player and Microsoft Silverlight, and also various HDTV broadcasts over terrestrial (ATSC, ISDB-T, DVB-T or DVB-T2), cable (DVB-C), and satellite (DVB-S and DVB-S2) systems.

H.264 is restricted by patents owned by various parties. A license covering most (but not all[citation needed]) patents essential to H.264 is administered by a patent pool formerly administered by MPEG LA. Via Licensing Corp acquired MPEG LA in April 2023 and formed a new patent pool administration company called Via Licensing Alliance.[8] The commercial use of patented H.264 technologies requires the payment of royalties to Via and other patent owners. MPEG LA has allowed the free use of H.264 technologies for streaming Internet video that is free to end users, and Cisco paid royalties to MPEG LA on behalf of the users of binaries for its open source H.264 encoder openH264.

The H.264 name follows the ITU-T naming convention, where Recommendations are given a letter corresponding to their series and a recommendation number within the series. H.264 is part of "H-Series Recommendations: Audiovisual and multimedia systems". H.264 is further categorized into "H.200-H.499: Infrastructure of audiovisual services" and "H.260-H.279: Coding of moving video".[9] The MPEG-4 AVC name relates to the naming convention in ISO/IEC MPEG, where the standard is part 10 of ISO/IEC 14496, which is the suite of standards known as MPEG-4. The standard was developed jointly in a partnership of VCEG and MPEG, after earlier development work in the ITU-T as a VCEG project called H.26L. It is thus common to refer to the standard with names such as H.264/AVC, AVC/H.264, H.264/MPEG-4 AVC, or MPEG-4/H.264 AVC, to emphasize the common heritage. Occasionally, it is also referred to as "the JVT codec", in reference to the Joint Video Team (JVT) organization that developed it. (Such partnership and multiple naming is not uncommon. For example, the video compression standard known as MPEG-2 also arose from the partnership between MPEG and the ITU-T, where MPEG-2 video is known to the ITU-T community as H.262.[10]) Some software programs (such as VLC media player) internally identify this standard as AVC1.

Throughout the development of the standard, additional messages for containing supplemental enhancement information (SEI) have been developed. SEI messages can contain various types of data that indicate the timing of the video pictures or describe various properties of the coded video or how it can be used or enhanced. SEI messages are also defined that can contain arbitrary user-defined data. SEI messages do not affect the core decoding process, but can indicate how the video is recommended to be post-processed or displayed. Some other high-level properties of the video content are conveyed in video usability information (VUI), such as the indication of the color space for interpretation of the video content. As new color spaces have been developed, such as for high dynamic range and wide color gamut video, additional VUI identifiers have been added to indicate them.

Five other new profiles (see version 7 below) intended primarily for professional applications were then developed, adding extended-gamut color space support, defining additional aspect ratio indicators, defining two additional types of "supplemental enhancement information" (post-filter hint and tone mapping), and deprecating one of the prior FRExt profiles (the High 4:4:4 profile) that industry feedback[by whom?] indicated should have been designed differently.

The next major feature added to the standard was Scalable Video Coding (SVC). Specified in Annex G of H.264/AVC, SVC allows the construction of bitstreams that contain layers of sub-bitstreams that also conform to the standard, including one such bitstream known as the "base layer" that can be decoded by a H.264/AVC codec that does not support SVC. For temporal bitstream scalability (i.e., the presence of a sub-bitstream with a smaller temporal sampling rate than the main bitstream), complete access units are removed from the bitstream when deriving the sub-bitstream. In this case, high-level syntax and inter-prediction reference pictures in the bitstream are constructed accordingly. On the other hand, for spatial and quality bitstream scalability (i.e. the presence of a sub-bitstream with lower spatial resolution/quality than the main bitstream), the NAL (Network Abstraction Layer) is removed from the bitstream when deriving the sub-bitstream. In this case, inter-layer prediction (i.e., the prediction of the higher spatial resolution/quality signal from the data of the lower spatial resolution/quality signal) is typically used for efficient coding. The Scalable Video Coding extensions were completed in November 2007.

The next major feature added to the standard was Multiview Video Coding (MVC). Specified in Annex H of H.264/AVC, MVC enables the construction of bitstreams that represent more than one view of a video scene. An important example of this functionality is stereoscopic 3D video coding. Two profiles were developed in the MVC work: Multiview High profile supports an arbitrary number of views, and Stereo High profile is designed specifically for two-view stereoscopic video. The Multiview Video Coding extensions were completed in November 2009.

Additional extensions were later developed that included 3D video coding with joint coding of depth maps and texture (termed 3D-AVC), multi-resolution frame-compatible (MFC) stereoscopic and 3D-MFC coding, various additional combinations of features, and higher frame sizes and frame rates.

Versions of the H.264/AVC standard include the following completed revisions, corrigenda, and amendments (dates are final approval dates in ITU-T, while final "International Standard" approval dates in ISO/IEC are somewhat different and slightly later in most cases). Each version represents changes relative to the next lower version that is integrated into the text.

To ensure compatibility and problem-free adoption of H.264/AVC, many standards bodies have amended or added to their video-related standards so that users of these standards can employ H.264/AVC. Both the Blu-ray Disc format and the now-discontinued HD DVD format include the H.264/AVC High Profile as one of three mandatory video compression formats. The Digital Video Broadcast project (DVB) approved the use of H.264/AVC for broadcast television in late 2004.

The Advanced Television Systems Committee (ATSC) standards body in the United States approved the use of H.264/AVC for broadcast television in July 2008, although the standard is not yet used for fixed ATSC broadcasts within the United States.[35][36] It has also been approved for use with the more recent ATSC-M/H (Mobile/Handheld) standard, using the AVC and SVC portions of H.264.[37]

H.264/AVC/MPEG-4 Part 10 contains a number of new features that allow it to compress video much more efficiently than older standards and to provide more flexibility for application to a wide variety of network environments. In particular, some such key features include:

Like other ISO/IEC MPEG video standards, H.264/AVC has a reference software implementation that can be freely downloaded.[50] Its main purpose is to give examples of H.264/AVC features, rather than being a useful application per se. Some reference hardware design work has also been conducted in the Moving Picture Experts Group.The above-mentioned aspects include features in all profiles of H.264. A profile for a codec is a set of features of that codec identified to meet a certain set of specifications of intended applications. This means that many of the features listed are not supported in some profiles. Various profiles of H.264/AVC are discussed in next section.

The standard defines several sets of capabilities, which are referred to as profiles, targeting specific classes of applications. These are declared using a profile code (profile_idc) and sometimes a set of additional constraints applied in the encoder. The profile code and indicated constraints allow a decoder to recognize the requirements for decoding that specific bitstream. (And in many system environments, only one or two profiles are allowed to be used, so decoders in those environments do not need to be concerned with recognizing the less commonly used profiles.) By far the most commonly used profile is the High Profile.

For camcorders, editing, and professional applications, the standard contains four additional Intra-frame-only profiles, which are defined as simple subsets of other corresponding profiles. These are mostly for professional (e.g., camera and editing system) applications:

As a result of the Scalable Video Coding (SVC) extension, the standard contains five additional scalable profiles, which are defined as a combination of a H.264/AVC profile for the base layer (identified by the second word in the scalable profile name) and tools that achieve the scalable extension:

As the term is used in the standard, a "level" is a specified set of constraints that indicate a degree of required decoder performance for a profile. For example, a level of support within a profile specifies the maximum picture resolution, frame rate, and bit rate that a decoder may use. A decoder that conforms to a given level must be able to decode all bitstreams encoded for that level and all lower levels.

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