Editing H.263

{{Short description|Video compression standard}}
{{Infobox technology standard
| title             = H.263
| long_name         = Video coding for low bit rate communication
| image             =
| caption           =
| status            = In force
| first_published   = {{Start date|1996|03|20|df=y}}
| version           = 3rd edition
| version_date      = {{Start date|2005|06|24|df=y}}
| organization      = [[ITU-T]]
| committee         = [[VCEG]]
| base_standards    = [[H.261]], [[H.262]] (aka MPEG-2 Video)
| related_standards = [[H.264]], [[H.265]], [[H.266]], [[H.323]], [[H.324]], [[ISO/IEC 14496-2]]
| predecessor       = [[H.262]]
| successor         = [[H.264]]
| abbreviation      =
| domain            = [[Video compression]]
| license           = [[Reasonable and non-discriminatory licensing|RAND]]<ref>{{cite web |url=https://www.itu.int/ITU-T/recommendations/related_ps.aspx?id_prod=4242 |title=ITU-T Rec. H.263 declared patent(s) |website=ITU |publisher=International Telecommunication Union |access-date=11 May 2022 |url-status=live |archive-url=https://web.archive.org/web/20220511134731/https://www.itu.int/ITU-T/recommendations/related_ps.aspx?id_prod=4242 |archive-date=11 May 2022}}</ref><ref>{{cite web |url=https://developer.mozilla.org/en-US/docs/Web/Media/Formats/Video_codecs |at=H.263 |title=Web video codec guide |website=MDN Web Docs |publisher=Mozilla Foundation |access-date=11 May 2022 |url-status=live |archive-url=https://web.archive.org/web/20220503075453/https://developer.mozilla.org/en-US/docs/Web/Media/Formats/Video_codecs |archive-date=3 May 2022}}</ref>
| website           = [https://www.itu.int/rec/T-REC-H.263 www.itu.int/rec/T-REC-H.263]
}}

'''H.263''' is a [[video compression standard]] originally designed as a low-bit-rate compressed format for [[videotelephony]]. It was standardized by the [[ITU-T]] [[Video Coding Experts Group]] (VCEG) 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 the previous H.26x standards, H.263 is a block-based hybrid video coding scheme using 16×16 [[macroblock]]s of [[YCbCr]] color sample arrays, [[motion compensation|motion-compensated prediction]], an 8×8 [[discrete cosine transform]] for prediction differences, zig-zag scanning of transform coefficients, [[scalar quantization]], [[run-length coding|run-length transform coefficient symbols]], and [[variable-length coding]] (basically like [[Huffman coding]] but with structured coding tables).<ref>{{cite news |last1=Davis |first1=Andrew |title=The H.320 Recommendation Overview |url=https://www.eetimes.com/document.asp?doc_id=1275886 |access-date=7 November 2019 |work=[[EE Times]] |date=13 June 1997}}</ref> The first (1995) version of H.263 included some optional features including [[overlapped block motion compensation]] and variable block-size motion compensation, and the spec 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 ([[Public Switched Telephone Network|PSTN]] and other [[Circuit switching|circuit-switched]] network [[videoconferencing]] and [[videotelephony]]), but it also found use in [[H.323]] ([[Real-time Transport Protocol|RTP]]/IP-based videoconferencing), [[H.320]] ([[Integrated Services Digital Network|ISDN]]-based videoconferencing, where it became the most widely used video compression standard),<ref>{{cite book |title=IEEE WESCANEX 97: communications, power, and computing : conference proceedings |date=May 22–23, 1997 |publisher=[[Institute of Electrical and Electronics Engineers]] |location=University of Manitoba, Winnipeg, Manitoba, Canada |isbn=9780780341470 |page=30 |url=https://books.google.com/books?id=8vhEAQAAIAAJ |quote=H.263 is similar to, but more complex than H.261. It is currently the most widely used international video compression standard for video telephony on ISDN (Integrated Services Digital Network) telephone lines.}}</ref> [[RTSP]] ([[streaming media]]) and [[Session Initiation Protocol|SIP]] (IP-based videoconferencing) solutions.

H.263 is a required video coding format in [[ETSI]] [[3GPP]] technical specifications for [[IP Multimedia Subsystem]] (IMS), [[Multimedia Messaging Service]] (MMS) and Transparent end-to-end Packet-switched Streaming Service (PSS).<ref>ETSI (2009-04) [http://www.3gpp.org/ftp/Specs/html-info/26234.htm ETSI TS 126 234 V8.2.0 (2009-04); 3GPP TS 26.234; Transparent end-to-end Packet-switched Streaming Service (PSS); Protocols and codecs] Retrieved on 2009-06-02.</ref><ref>ETSI (2009-01) [http://www.3gpp.org/ftp/Specs/html-info/26140.htm ETSI TS 126 140 V8.0.0 (2009-01); 3GPP TS 26.140; Multimedia Messaging Service (MMS); Media formats and codes] Retrieved on 2009-06-02.</ref><ref>ETSI (2009-01) [http://www.3gpp.org/ftp/Specs/html-info/26141.htm ETSI TS 126 141 V8.0.0 (2009-01); 3GPP TS 26.141; IP Multimedia System (IMS) Messaging and Presence; Media formats and codecs] Retrieved on 2009-06-02.</ref> In 3GPP specifications, H.263 video is usually used in [[3GP]] [[Container format (digital)|container format]].

H.263 also found many applications on the internet: much [[Flash Video]] content (as used on sites such as [[YouTube]], [[Google Video]], and [[MySpace]]) used to be encoded in [[Sorenson Spark#Sorenson Spark|Sorenson Spark]] format (an incomplete implementation of H.263<ref name="incomplete-h263">{{cite web|url=http://www.kaourantin.net/2005/08/quest-for-new-video-codec-in-flash-8.html|title=The quest for a new video codec in Flash 8|author=Kaourantin.net|date=2005-08-13|archive-url=https://web.archive.org/web/20090206142709/http://kaourantin.net/2005/08/quest-for-new-video-codec-in-flash-8.html|archive-date=2009-02-06|url-status=dead|access-date=2009-08-10|quote=We went this route before with Sorenson Spark which is an incomplete implementation of H.263 and it bit us badly when trying to implement certain solutions.}}</ref><ref name="almost">{{cite mailing list|url=http://lists.mplayerhq.hu/pipermail/ffmpeg-devel/2009-March/065410.html|title=h263-svq3 optimizations|date=2009-03-17|mailing-list=FFmpeg-devel|author=Benjamin Larsson|quote=Sorenson _Spark_ and H.263 are actually very similar. They differ mostly in header structure and ranges of the coefficients.|access-date=2009-08-09|url-status=dead|archive-url=https://web.archive.org/web/20090817080612/http://lists.mplayerhq.hu/pipermail/ffmpeg-devel/2009-March/065410.html|archive-date=2009-08-17}}</ref><ref>{{cite web|url=http://wiki.multimedia.cx/index.php?title=Sorenson_Spark|title=Sorenson Spark|publisher=MultimediaWiki|access-date=2009-11-03|quote=Video codec used in Flash Video files, based on H.263.}}</ref>). The original version of the [[RealVideo]] codec was based on H.263 until the release of RealVideo 8.<ref>{{cite web|url=http://wiki.multimedia.cx/index.php?title=RealVideo_G2|title=RealVideo G2|author=MultimediaWiki|access-date=2009-11-02}}</ref>

H.263 was developed as an evolutionary improvement based on experience from [[H.261]] and [[H.262]] (aka [[MPEG-2 Video]]), the previous ITU-T standards for video compression, and the [[MPEG-1]] standard developed in ISO/IEC. The first version of H.263 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 (also known as H.263+ or H.263 1998) and H.263v3 (also known as H.263++ or H.263 2000). It was also used as the basis for the development of [[MPEG-4 Part 2]].<ref name="end_of_the_ride">{{cite web|url=http://ride.chiariglione.org/end_of_the_ride_qm.htm|title=Riding the Media Bits, End of the Ride?|author=chiariglione.org|date=2006-08-10|archive-url=https://web.archive.org/web/20110122013232/http://ride.chiariglione.org/end_of_the_ride_qm.htm|archive-date=2011-01-22|url-status=dead|access-date=2010-03-10}}</ref><ref>{{cite web|url=http://mpeg.chiariglione.org/tutorials/papers/icj-mpeg4-si/02-overview_paper/2-overview_paper.htm|title=MPEG-4: Why, What, How and When?|author=Fernando Pereira|publisher=chiariglione.org|archive-url=https://web.archive.org/web/20111018071929/http://mpeg.chiariglione.org/tutorials/papers/icj-mpeg4-si/02-overview_paper/2-overview_paper.htm|archive-date=2011-10-18|url-status=dead|access-date=2010-03-10}}</ref><ref name="mpeg-4-video-faq">{{cite web|url=http://mpeg.chiariglione.org/faq/mp4-vid/mp4-vid.htm|title=MPEG-4 Video - Frequently Asked Questions|author=ISO/IEC JTC1/SC29/WG11|date=March 2000|publisher=chiariglione.org|access-date=2010-03-10}}</ref> 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.<ref name="end_of_the_ride" /><ref name="inside_MPEG-4_part_B">{{cite web|url=http://ride.chiariglione.org/inside_MPEG-4_part_B/inside_MPEG-4_part_B.htm|title=Riding the Media Bits, Inside MPEG-4 - Part B|author=chiariglione.org|date=2003-10-25|archive-url=https://web.archive.org/web/20110122013103/http://ride.chiariglione.org/inside_MPEG-4_part_B/inside_MPEG-4_part_B.htm|archive-date=2011-01-22|url-status=dead|access-date=2010-03-10}}</ref>

The next enhanced format developed by ITU-T [[VCEG]] (in partnership with [[MPEG]]) after H.263 was the [[H.264/MPEG-4 AVC|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 that is only used for compatibility with older products. Newer videoconferencing products now include H.264 as well as, or instead of, H.263 and [[H.261]] capabilities. Even newer standard formats, [[HEVC]] and [[Versatile Video Coding|VVC]], have also been developed by VCEG and MPEG, and have begun to replace H.264 in some applications.

==Versions==

Since the original ratification of H.263 in March 1996<ref name="itu-t-h263">{{cite web | url=http://www.itu.int/rec/T-REC-H.263/ | title=H.263 : Video coding for low bit rate communication | author=ITU-T | access-date=2009-11-02}}</ref> (approving a document that was produced in November 1995), there have been two subsequent additions which improved on the original standard by additional optional extensions (for example, the H.263v2 project added a [[deblocking filter (video)|deblocking filter]] in its Annex J).

=== Version 1 and Annex I ===
The original version of H.263 specified the following annexes:
* Annex A – Inverse transform accuracy specification
* Annex B – Hypothetical Reference Decoder
* Annex C – Considerations for Multipoint
* Annex D – Unrestricted Motion Vector mode
* Annex E – Syntax-based Arithmetic Coding mode
* Annex F – Advanced Prediction mode
* Annex G – PB-frames mode
* Annex H – Forward Error Correction for coded video signal

The first version of H.263 supported a limited set of picture sizes:
* 128×96 (a.k.a. Sub-QCIF)
* 176×144 (a.k.a. QCIF)
* 352×288 (a.k.a. [[Common Intermediate Format|CIF]])
* 704×576 (a.k.a. 4CIF)
* 1408×1152 (a.k.a. 16CIF)

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 (also known as ''H.263+'', or as ''the 1998 version of H.263'') 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 (such as enhanced robustness against data loss in the transmission channel). The H.263+ project was ratified by the ITU in February 1998.  It added the following Annexes:
* Annex I – Advanced INTRA Coding mode
* Annex J – [[Deblocking filter (video)|Deblocking Filter mode]]
* Annex K – Slice Structured mode
* Annex L – Supplemental Enhancement Information Specification
* Annex M – Improved PB-frames mode
* Annex N – Reference Picture Selection mode
* Annex O – Temporal, SNR, and Spatial Scalability mode
* Annex P – Reference picture resampling
* Annex Q – Reduced-Resolution Update mode (see implementors' guide correction as noted below)
* Annex R – Independent Segment Decoding mode
* Annex S – Alternative INTER VLC mode
* Annex T – Modified Quantization mode
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, [[Common Intermediate Format|CIF]], 4CIF, and 16CIF, and the only picture clock frequency had been 30000/1001 (approximately 29.97) clock ticks per second.

H.263v2 specified a set of recommended modes in an informative appendix (Appendix II, since deprecated):
{| class="wikitable" style="font-size: 100%; text-align: center;"
|-
!
!Level 1
!Level 2
!Level 3
|-
! Advanced INTRA Coding
| {{yes}}
| {{yes}}
| {{yes}}
|-
! Deblocking Filter
| {{yes}}
| {{yes}}
| {{yes}}
|-
! Supplemental Enhancement Information (Full-Frame Freeze Only)
| {{yes}}
| {{yes}}
| {{yes}}
|-
! Modified Quantization
| {{yes}}
| {{yes}}
| {{yes}}
|-
! Unrestricted Motion Vectors
| {{no}}
| {{yes}}
| {{yes}}
|-
! Slice Structured Mode
| {{no}}
| {{yes}}
| {{yes}}
|-
! Reference Picture Resampling (Implicit Factor-of-4 Mode Only)
| {{no}}
| {{yes}}
| {{yes}}
|-
! Advanced Prediction
| {{no}}
| {{no}}
| {{yes}}
|-
! Improved PB-frames
| {{no}}
| {{no}}
| {{yes}}
|-
! Independent Segment Decoding
| {{no}}
| {{no}}
| {{yes}}
|-
! Alternate INTER VLC
| {{no}}
| {{no}}
| {{yes}}
|-
!
!Level 1
!Level 2
!Level 3
|}

=== H.263v3 (H.263++) and Annex X ===
The definition of H.263v3 (also known as H.263++ or as the 2000 version of H.263) added three annexes.  These annexes and an additional annex that specified profiles (approved the following year) were originally published as separate documents from the main body of the standard itself. The additional annexes specified are:
* Annex U – Enhanced reference picture selection mode
* Annex V – Data-partitioned slice mode
* Annex W – Additional supplemental enhancement information specification
* Annex X (originally specified in 2001) – Profiles and levels definition

The prior informative Appendix II (recommended optional enhancement) was obsoleted by the creation of the normative Annex X.

In June 2001, another informative appendix (Appendix III, Examples for H.263 encoder/decoder implementations) 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 (with the exception of Appendix III, which remains as a separately-published document).

In August 2005, an implementors' guide was approved to correct a small error in the seldom-used Annex Q reduced-resolution update mode.

==Patent rights and open-source implementation==
H.263 was developed under the "[[reasonable and non-discriminatory]]" patent licensing policy of ITU-T, although in practice, the licensing for patent rights on the standard did not become the subject of substantial litigation. Due to the age of the standard, most or all relevant patents would currently be expired{{snd}} at least for patents that would apply to the early versions of the standard.{{cn|date=May 2022}}

Open-source implementations include the [[LGPL]]-licensed [[libavcodec]] library (part of the [[FFmpeg]] project) which is used by programs such as [[ffdshow]], [[VLC media player]] and [[MPlayer]].<ref>{{Cite web|title=7.3. Encoding with the libavcodec codec family|url=http://www.mplayerhq.hu/DOCS/HTML/en/menc-feat-enc-libavcodec.html|access-date=2021-05-08|website=www.mplayerhq.hu}}</ref>

==See also==
* [[H.262/MPEG-2 Part 2]]
* [[MPEG-4 Part 2]] (MPEG-4 Visual)

==References==
{{Reflist|30em}}

== External links ==
*  [http://www.itu.int/rec/T-REC-H.263/ The ITU-T specification for H.263]
*  [https://web.archive.org/web/20011030094328/http://www.ietf.org/html.charters/avt-charter.html IETF AVT Working Group] - Group that reviews codec packetizations for [[Real-time Transport Protocol|RTP]]
** {{IETF RFC|4629|link=no}} - RTP Payload Format for ITU-T Rec. H.263 Video
** {{IETF RFC|2429|link=no}} - RTP Payload Format for the 1998 Version of ITU-T Rec. H.263 Video (H.263+) (Obsolete, upgraded spec in RFC 4629)
** {{IETF RFC|2190|link=no}} - RTP Payload Format for H.263 Video Streams (Historic)
* [http://wiki.multimedia.cx/index.php?title=H.263 H.263 - MultimediaWiki]
* [http://www.intel.com/cd/software/products/asmo-na/eng/perflib/ipp/302910.htm Intel Integrated Performance Primitives]
* [http://www4.informatik.uni-erlangen.de/Projects/ScalVico/vic/ H.263 implementation in vic (source code available)]

{{Compression formats}}
{{Authority control}}

[[Category:H.26x]]
[[Category:ITU-T H Series Recommendations]]
[[Category:ITU-T recommendations]]
[[Category:Open standards covered by patents]]
[[Category:Video codecs]]
[[Category:Videotelephony]]