H.264 or MPEG-4 Part 10, Advanced Video Coding (MPEG-4 AVC) is a block-oriented motion-compensation-based video compression standard. As of 2014 it is one of the most commonly used formats for the recording, compression, and distribution of video content.^[1]

The intent of the H.264/AVC project was to create a standard capable of providing good video quality at substantially lower bit rates than previous standards (i.e., half or less the bit rate of MPEG-2, H.263, or MPEG-4 Part 2), without increasing the complexity of design so much that it would be impractical or excessively expensive to implement. An additional goal was to provide enough flexibility to allow the standard to be applied to a wide variety of applications on a wide variety of networks and systems, including low and high bit rates, low and high resolution video, broadcast, DVD storage, RTP/IP packet networks, and ITU-T multimedia telephony systems. The H.264 standard can be viewed as a "family of standards" composed of a number of different profiles. A specific decoder decodes at least one, but not necessarily all profiles. The decoder specification describes which profiles can be decoded. H.264 is typically used for lossy compression, although it is also possible to create truly lossless-coded regions within lossy-coded pictures or to support rare use cases for which the entire encoding is lossless.

H.264 was developed by the ITU-T Video Coding Experts Group (VCEG) together with the ISO/IEC JTC1 Moving Picture Experts Group (MPEG). The project partnership effort is known as the Joint Video Team (JVT). The ITU-T H.264 standard and the ISO/IEC MPEG-4 AVC standard (formally, ISO/IEC 14496-10 – MPEG-4 Part 10, Advanced Video Coding) are jointly maintained so that they have identical technical content. The final drafting work on the first version of the standard was completed in May 2003, and various extensions of its capabilities have been added in subsequent editions. High Efficiency Video Coding (HEVC), a.k.a. H.265 and MPEG-H Part 2 is a successor to H.264/MPEG-4 AVC developed by the same organizations, while earlier standards are still in common use.

H.264 is perhaps best known as being one of the video encoding standards for Blu-ray Discs; all Blu-ray Disc players must be able to decode H.264. It is also widely used by streaming internet sources, such as videos from Vimeo, YouTube, and the iTunes Store, web software such as the Adobe Flash Player and Microsoft Silverlight, and also various HDTV broadcasts over terrestrial (Advanced Television Systems Committee standards, ISDB-T, DVB-T or DVB-T2), cable (DVB-C), and satellite (DVB-S and DVB-S2).

H.264 is protected by patents owned by various parties. A license covering most (but not all) patents essential to H.264 is administered by patent pool MPEG LA.^[2]Commercial use of patented H.264 technologies requires the payment of royalties to MPEG LA 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 Systems pays royalties to MPEG LA on behalf of the users of binaries for its open source H.264 encoder.

Naming[edit]

The H.264 name follows the ITU-T naming convention, where the standard is a member of the H.26x line of VCEG video coding standards; 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.^[3]) Some software programs (such as VLC media player) internally identify this standard as AVC1.

History[edit]

In early 1998, the Video Coding Experts Group (VCEG – ITU-T SG16 Q.6) issued a call for proposals on a project called H.26L, with the target to double the coding efficiency (which means halving the bit rate necessary for a given level of fidelity) in comparison to any other existing video coding standards for a broad variety of applications. VCEG was chaired by Gary Sullivan (Microsoft, formerly PictureTel, U.S.). The first draft design for that new standard was adopted in August 1999. In 2000, Thomas Wiegand (Heinrich Hertz Institute, Germany) became VCEG co-chair.

In December 2001, VCEG and the Moving Picture Experts Group (MPEG – ISO/IEC JTC 1/SC 29/WG 11) formed a Joint Video Team (JVT), with the charter to finalize the video coding standard.^[4] Formal approval of the specification came in March 2003. The JVT was (is) chaired by Gary Sullivan, Thomas Wiegand, and Ajay Luthra (Motorola, U.S.: later Arris, U.S.). In June 2004, the Fidelity range extensions (FRExt) project was finalized. From January 2005 to November 2007, the JVT was working on an extension of H.264/AVC towards scalability by an Annex (G) called Scalable Video Coding (SVC). The JVT management team was extended by Jens-Rainer Ohm (Aachen University, Germany). From July 2006 to November 2009, the JVT worked on Multiview Video Coding (MVC), an extension of H.264/AVC towards free viewpoint television and 3D television. That work included the development of two new profiles of the standard: the Multiview High Profile and the Stereo High Profile.

The standardization of the first version of H.264/AVC was completed in May 2003. In the first project to extend the original standard, the JVT then developed what was called the Fidelity Range Extensions (FRExt). These extensions enabled higher quality video coding by supporting increased sample bit depth precision and higher-resolution color information, including sampling structures known as Y'CbCr 4:2:2 (=YUV 4:2:2) and Y'CbCr 4:4:4. Several other features were also included in the Fidelity Range Extensions project, such as adaptive switching between 4×4 and 8×8 integer transforms, encoder-specified perceptual-based quantization weighting matrices, efficient inter-picture lossless coding, and support of additional color spaces. The design work on the Fidelity Range Extensions was completed in July 2004, and the drafting work on them was completed in September 2004.

Further recent extensions of the standard then included adding five other new profiles^[which?] intended primarily for professional applications, 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^[which?] 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 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.

Versions[edit]

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.

• Version 1 (Edition 1): (May 30, 2003) First approved version of H.264/AVC containing Baseline, Main, and Extended profiles.^[5]
• Version 2 (Edition 1.1): (May 7, 2004) Corrigendum containing various minor corrections.^[6]
• Version 3 (Edition 2): (March 1, 2005) Major addition to H.264/AVC containing the first amendment providing Fidelity Range Extensions (FRExt) containing High, High 10, High 4:2:2, and High 4:4:4 profiles.^[7]
• Version 4 (Edition 2.1): (September 13, 2005) Corrigendum containing various minor corrections and adding three aspect ratio indicators.^[8]
• Version 5 (Edition 2.2): (June 13, 2006) Amendment consisting of removal of prior High 4:4:4 profile (processed as a corrigendum in ISO/IEC).^[9]
• Version 6 (Edition 2.2): (June 13, 2006) Amendment consisting of minor extensions like extended-gamut color space support (bundled with above-mentioned aspect ratio indicators in ISO/IEC).^[9]
• Version 7 (Edition 2.3): (April 6, 2007) Amendment containing the addition of High 4:4:4 Predictive and four Intra-only profiles (High 10 Intra, High 4:2:2 Intra, High 4:4:4 Intra, and CAVLC 4:4:4 Intra).^[10]
• Version 8 (Edition 3): (November 22, 2007) Major addition to H.264/AVC containing the amendment for Scalable Video Coding (SVC) containing Scalable Baseline, Scalable High, and Scalable High Intra profiles.^[11]
• Version 9 (Edition 3.1): (January 13, 2009) Corrigendum containing minor corrections.^[12]
• Version 10 (Edition 4): (March 16, 2009) Amendment containing definition of a new profile (the Constrained Baseline profile) with only the common subset of capabilities supported in various previously specified profiles.^[13]
• Version 11 (Edition 4): (March 16, 2009) Major addition to H.264/AVC containing the amendment for Multiview Video Coding (MVC) extension, including the Multiview High profile.^[13]
• Version 12 (Edition 5): (March 9, 2010) Amendment containing definition of a new MVC profile (the Stereo High profile) for two-view video coding with support of interlaced coding tools and specifying an additional SEI message (the frame packing arrangement SEI message).^[14]
• Version 13 (Edition 5): (March 9, 2010) Corrigendum containing minor corrections.^[14]
• Version 14 (Edition 6): (June 29, 2011) Amendment specifying a new level (Level 5.2) supporting higher processing rates in terms of maximum macroblocks per second, and a new profile (the Progressive High profile) supporting only the frame coding tools of the previously specified High profile.^[15]
• Version 15 (Edition 6): (June 29, 2011) Corrigendum containing minor corrections.^[15]
• Version 16 (Edition 7): (January 13, 2012) Amendment containing definition of three new profiles intended primarily for real-time communication applications: the Constrained High, Scalable Constrained Baseline, and Scalable Constrained High profiles.^[16]
• Version 17 (Edition 8): (April 13, 2013) Amendment with additional SEI message indicators.^[17]
• Version 18 (Edition 8): (April 13, 2013) Amendment to specify the coding of depth map data for 3D stereoscopic video, including a Multiview Depth High profile.^[17]
• Version 19 (Edition 8): (April 13, 2013) Corrigendum to correct an error in the sub-bitstream extraction process for multiview video.^[17]
• Version 20 (Edition 8): (April 13, 2013) Amendment to specify additional color space identifiers (including support of ITU-R Recommendation BT.2020 for UHDTV) and an additional model type in the tone mapping information SEI message.^[17]
• Version 21 (Edition 9): (February 13, 2014) Amendment to specify the Enhanced Multiview Depth High profile.^[18]
• Version 22 (Edition 9): (February 13, 2014) Amendment to specify the multi-resolution frame compatible (MFC) enhancement for 3D stereoscopic video, the MFC High profile, and minor corrections.^[18]
• Version 23 (Edition 10): (February 13, 2016) Amendment to specify MFC stereoscopic video with depth maps, the MFC Depth High profile, the mastering display color volume SEI message, and additional color-related video usability information codepoint identifiers.^[19]
• Version 24 (Edition 11): (October 14, 2016) Amendment to specify additional levels of decoder capability supporting larger picture sizes (Levels 6, 6.1, and 6.2), the green metadata SEI message, the alternative depth information SEI message, and additional color-related video usability information codepoint identifiers.^[20]
• Version 25 (Edition 12): (April 13, 2017) Amendment to specify the Progressive High 10 profile and additional color-related VUI code points and SEI messages^[21]

Applications[edit]

The H.264 video format has a very broad application range that covers all forms of digital compressed video from low bit-rate Internet streaming applications to HDTV broadcast and Digital Cinema applications with nearly lossless coding. With the use of H.264, bit rate savings of 50% or more compared to MPEG-2 Part 2 are reported. For example, H.264 has been reported to give the same Digital Satellite TV quality as current MPEG-2 implementations with less than half the bitrate, with current MPEG-2 implementations working at around 3.5 Mbit/s and H.264 at only 1.5 Mbit/s.^[22] Sony claims that 9 Mbit/s AVC recording mode is equivalent to the image quality of the HDV format, which uses approximately 18–25 Mbit/s.^[23]

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 3 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.^[24][25] 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.^[26]

The CCTV (Closed Circuit TV) and Video Surveillance markets have included the technology in many products.

Many common DSLRs use H.264 video wrapped in QuickTime MOV containers as the native recording format.

Derived formats[edit]

AVCHD is a high-definition recording format designed by Sony and Panasonic that uses H.264 (conforming to H.264 while adding additional application-specific features and constraints).

AVC-Intra is an intraframe-only compression format, developed by Panasonic.

XAVC is a recording format designed by Sony that uses level 5.2 of H.264/MPEG-4 AVC, which is the highest level supported by that video standard.^[27][28] XAVC can support 4K resolution (4096 × 2160 and 3840 × 2160) at up to 60 frames per second (fps).^[27][28] Sony has announced that cameras that support XAVC include two CineAlta cameras—the Sony PMW-F55 and Sony PMW-F5.^[29][30] The Sony PMW-F55 can record XAVC with 4K resolution at 30 fps at 300 Mbit/s and 2K resolution at 30 fps at 100 Mbit/s.^[31] XAVC can record 4K resolution at 60 fps with 4:2:2 chroma subsampling at 600 Mbit/s.^[32][33]

Design[edit]

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Features[edit]

 

Block diagram of H.264

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:

• Multi-picture inter-picture prediction including the following features:
  • Using previously encoded pictures as references in a much more flexible way than in past standards, allowing up to 16 reference frames (or 32 reference fields, in the case of interlaced encoding) to be used in some cases. In profiles that support non-IDR frames, most levels specify that sufficient buffering should be available to allow for at least 4 or 5 reference frames at maximum resolution. This is in contrast to prior standards, where the limit was typically one; or, in the case of conventional "B pictures" (B-frames), two. This particular feature usually allows modest improvements in bit rate and quality in most scenes.^{[citation needed]} But in certain types of scenes, such as those with repetitive motion or back-and-forth scene cuts or uncovered background areas, it allows a significant reduction in bit rate while maintaining clarity.
  • Variable block-size motion compensation (VBSMC) with block sizes as large as 16×16 and as small as 4×4, enabling precise segmentation of moving regions. The supported luma prediction block sizes include 16×16, 16×8, 8×16, 8×8, 8×4, 4×8, and 4×4, many of which can be used together in a single macroblock. Chroma prediction block sizes are correspondingly smaller according to the chroma subsampling in use.
  • The ability to use multiple motion vectors per macroblock (one or two per partition) with a maximum of 32 in the case of a B macroblock constructed of 16 4×4 partitions. The motion vectors for each 8×8 or larger partition region can point to different reference pictures.
  • The ability to use any macroblock type in B-frames, including I-macroblocks, resulting in much more efficient encoding when using B-frames. This feature was notably left out from MPEG-4 ASP.
  • Six-tap filtering for derivation of half-pel luma sample predictions, for sharper subpixel motion-compensation. Quarter-pixel motion is derived by linear interpolation of the halfpel values, to save processing power.
  • Quarter-pixel precision for motion compensation, enabling precise description of the displacements of moving areas. For chroma the resolution is typically halved both vertically and horizontally (see 4:2:0) therefore the motion compensation of chroma uses one-eighth chroma pixel grid units.
  • Weighted prediction, allowing an encoder to specify the use of a scaling and offset when performing motion compensation, and providing a significant benefit in performance in special cases—such as fade-to-black, fade-in, and cross-fade transitions. This includes implicit weighted prediction for B-frames, and explicit weighted prediction for P-frames.
• Spatial prediction from the edges of neighboring blocks for "intra" coding, rather than the "DC"-only prediction found in MPEG-2 Part 2 and the transform coefficient prediction found in H.263v2 and MPEG-4 Part 2. This includes luma prediction block sizes of 16×16, 8×8, and 4×4 (of which only one type can be used within each macroblock).
• Lossless macroblock coding features including:
  • A lossless "PCM macroblock" representation mode in which video data samples are represented directly,^[34] allowing perfect representation of specific regions and allowing a strict limit to be placed on the quantity of coded data for each macroblock.
  • An enhanced lossless macroblock representation mode allowing perfect representation of specific regions while ordinarily using substantially fewer bits than the PCM mode.
• Flexible interlaced-scan video coding features, including:
  • Macroblock-adaptive frame-field (MBAFF) coding, using a macroblock pair structure for pictures coded as frames, allowing 16×16 macroblocks in field mode (compared with MPEG-2, where field mode processing in a picture that is coded as a frame results in the processing of 16×8 half-macroblocks).
  • Picture-adaptive frame-field coding (PAFF or PicAFF) allowing a freely selected mixture of pictures coded either as complete frames where both fields are combined together for encoding or as individual single fields.
• New transform design features, including:
  • An exact-match integer 4×4 spatial block transform, allowing precise placement of residual signals with little of the "ringing" often found with prior codec designs. This design is conceptually similar to that of the well-known discrete cosine transform (DCT), introduced in 1974 by N. Ahmed, T.Natarajan and K.R.Rao, which is Citation 1 in Discrete cosine transform. However, it is simplified and made to provide exactly specified decoding.
  • An exact-match integer 8×8 spatial block transform, allowing highly correlated regions to be compressed more efficiently than with the 4×4 transform. This design is conceptually similar to that of the well-known DCT, but simplified and made to provide exactly specified decoding.
  • Adaptive encoder selection between the 4×4 and 8×8 transform block sizes for the integer transform operation.
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