Open 263 file

The 263 file extension is associated with H.263 is a provisional ITU-T standard, it is due to be published sometime in 1995/1996. It was designed for low bitrate communication, early drafts specified datarates less than 64 Kbits/s, however this limitation has now been removed. It is expected that the standard will be used for a wide range of bitrates, not just low bitrate applications. It is expected that H.263 will replace H.261 in many applications.

The coding algorithm of H.263 is similar to that used by H.261, however with some improvements and changes to improve performance and error recovery. The differences between the H.261 and H.263 coding algorithms are listed below. Half pixel precision is used for motion compensation whereas H.261 used full pixel precision and a loop filter. Some parts of the hierarchical structure of the datastream are now optional, so the codec can be configured for a lower datarate or better error recovery. There are now four optional negotiable options included to improve performance: Unrestricted Motion Vectors, Syntax-based arithmetic coding, Advance prediction, and forward and backward frame prediction similar to MPEG called P-B frames.

H.263 supports five resolutions. In addition to QCIF and CIF that were supported by H.261 there is SQCIF, 4CIF, and 16CIF. SQCIF is approximately half the resolution of QCIF. 4CIF and 16CIF are 4 and 16 times the resolution of CIF respectively. The support of 4CIF and 16CIF means the codec could then compete with other higher bitrate video coding standards such as the MPEG standards.