H.263


H.263 is a video compression standard originally designed as a low-bit-rate compressed format for videoconferencing. It was standardized by the ITU-T Video Coding Experts Group in a project ending in 1995/1996. It is a member of the H.26x family of video coding standards in the domain of the ITU-T.
Like previous H.26x standards, H.263 is based on discrete cosine transform video compression. H.263 was later extended to add various additional enhanced features in 1998 and 2000. Smaller additions were also made in 1997 and 2001, and a unified edition was produced in 2005.

History and background

The H.263 standard was first designed to be utilized in H.324 based systems, but it also found use in H.323, H.320, RTSP and SIP solutions.
H.263 is a required video coding format in ETSI 3GPP technical specifications for IP Multimedia Subsystem, Multimedia Messaging Service and Transparent end-to-end Packet-switched Streaming Service. In 3GPP specifications, H.263 video is usually used in 3GP container format.
H.263 also found many applications on the internet: much Flash Video content used to be encoded in Sorenson Spark format. The original version of the RealVideo codec was based on H.263 until the release of RealVideo 8.
H.263 was developed as an evolutionary improvement based on experience from H.261 and H.262, the previous ITU-T standards for video compression, and the MPEG-1 standard developed in ISO/IEC. Its first version was completed in 1995 and provided a suitable replacement for H.261 at all bit rates. It was further enhanced in projects known as H.263v2 and H.263v3. It was also used as the basis for the development of MPEG-4 Part 2. MPEG-4 Part 2 is H.263 compatible in the sense that basic "baseline" H.263 bitstreams are correctly decoded by an MPEG-4 Video decoder.
The next enhanced format developed by ITU-T VCEG after H.263 was the H.264 standard, also known as AVC and MPEG-4 part 10. As H.264 provides a significant improvement in capability beyond H.263, the H.263 standard is now considered a legacy design. Most new videoconferencing products now include H.264 as well as H.263 and H.261 capabilities. An even-newer standard format, HEVC, has also been developed by VCEG and MPEG, and has begun to emerge in some applications.

Versions

Since the original ratification of H.263 in March 1996, there have been two subsequent additions which improved on the original standard by additional optional extensions.

Version 1 and Annex I

The original version of H.263 specified the following annexes:
The first version of H.263 supported a limited set of picture sizes:
In March 1997, an informative Appendix I describing Error Tracking – an encoding technique for providing improved robustness to data losses and errors, was approved to provide information for the aid of implementers having an interest in such techniques.

H.263v2 (H.263+)

H.263v2 is the informal name of the second edition of the ITU-T H.263 international video coding standard. It retained the entire technical content of the original version of the standard, but enhanced H.263 capabilities by adding several annexes which can substantially improve encoding efficiency and provide other capabilities. The H.263+ project was ratified by the ITU in February 1998. It added the following Annexes:
H.263v2 also added support for flexible customized picture formats and custom picture clock frequencies. As noted above, the only picture formats previously supported in H.263 had been Sub-QCIF, QCIF, CIF, 4CIF, and 16CIF, and the only picture clock frequency had been 30000/1001 clock ticks per second.
H.263v2 specified a set of recommended modes in an informative appendix :
Level 1Level 2Level 3
Advanced INTRA Coding
Deblocking Filter
Supplemental Enhancement Information
Modified Quantization
Unrestricted Motion Vectors
Slice Structured Mode
Reference Picture Resampling
Advanced Prediction
Improved PB-frames
Independent Segment Decoding
Alternate INTER VLC
Level 1Level 2Level 3

H.263v3 (H.263++) and Annex X

The definition of H.263v3 added three annexes. These annexes and an additional annex that specified profiles were originally published as separate documents from the main body of the standard itself. The additional annexes specified are:
The prior informative Appendix II was obsoleted by the creation of the normative Annex X.
In June 2001, another informative appendix was approved. It describes techniques for encoding and for error/loss concealment by decoders.
In January 2005, a unified H.263 specification document was produced.
In August 2005, an implementors guide was approved to correct a small error in the seldom-used Annex Q reduced-resolution update mode.

Open-source implementation

In countries without software patents, H.263 video can be legally encoded and decoded with the free LGPL-licensed libavcodec library which is used by programs such as ffdshow, VLC media player and MPlayer.