View source for H.263

==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.