Dirac бейне кодегі: ашық формат және тиімділік

Video compression format

Dirac (and Dirac Pro, a subset standardised as SMPTE VC 2) is an open and royalty free video compression format, specification and software video codec developed by BBC Research & Development. Dirac aimed to provide high quality video compression for Ultra HDTV and competed with existing formats such as H.264. Version 2.2.3 of the full Dirac specification, including motion compensation and inter frame coding, was issued a few days later. Dirac Pro was used internally by the BBC to transmit HDTV pictures at the Beijing Olympics in 2008. Two open source and royalty free video codec software implementations, libschrodinger and dirac research, were developed. The format implementations were named in honour of the theoretical physicists Paul Dirac and Erwin Schrödinger, who shared the 1933 Nobel Prize in physics.

Dirac supports resolutions of HDTV (1920×1080) and greater, and is claimed to provide significant savings in data rate and improvements in quality over video compression formats such as MPEG 2 Part 2, MPEG 4 Part 2 and its competitors such as Theora and WMV. Dirac's implementers made a preliminary claim of "a two fold reduction in bit rate over MPEG 2 for high definition video", which makes it comparable to VC 1 and simpler profiles of H.264. Dirac supports both constant bit rate and variable bit rate operation. When the low delay syntax is used, the bit rate will be constant for each area (Dirac slice) in a picture to ensure constant latency. Dirac supports lossy and lossless compression modes. Dirac can be used in Ogg and Matroska container formats and is also registered for use in the ISO base media (MP4) file format and MPEG transport streams.

Two software implementations were initially developed. The BBC's reference implementation, initially called Dirac but renamed dirac research to avoid confusion, was written in C++ and released under the Mozilla Public License, GNU GPL 2 and GNU LGPL free software licenses. Version 1.0.0 of this implementation was released on 17 September 2008 and defines the Dirac bitstream format. By the March 2010 release of Schrödinger version 1.0.9, it was outperforming dirac research "in most encoding situations, both in terms of encoding speed and visual quality". With that release, most of the encoding tools in dirac research were ported over to Schrödinger, giving Schrödinger the same as or better compression efficiency than dirac research. Development of Schrödinger ceased after the 1.0.11 release in 2012. After the standardisation of Dirac Pro as SMPTE VC 2, development began on an open source reference VC 2 encoder. The code is provided in a git repository by the BBC and is available on GitHub. An encoder quality testing system has been put in place at BBC to check how well new encoding tools work and to make sure bugs that affect quality are quickly fixed.

Dirac video playback is supported by VLC media player since version 0.9.2 (2008), and by applications using the GStreamer framework. Support has also been added to FFmpeg. Applications which can encode to Dirac include FFmpeg, MediaCoder, LiVES and OggConvert.

The algorithms in the original Dirac specification were intended to provide compression performance comparable to mainstream video compression standards of the time. A 2009 comparison of the Dirac and H.264 codecs, which used implementations from the second quarter of 2008, showed x264 scoring slightly higher than Dirac. Another 2009 comparison found similar results for standard definition content, but did not compare high definition (HD) video content. These studies show that Dirac compression performance is close to that of MPEG 4 Part 2 Advanced Simple Profile (ASP, popularised as DivX). While also approaching low complexity H.264 encodes, a High Profile H.264 encoded video will have better compression for the same perceived quality. Since 2010, royalty free, open source video codecs such as VP8, VP9, and AV1 have been developed with better compression performance and more widespread adoption, including dominant streaming services such as YouTube and Netflix.