Theora

Theora is a royalty-free, open standard, lossy video compression technology being developed by the Xiph.Org Foundation alongside their other open media efforts, most notably the Vorbis audio codec and the Ogg container.

libtheora is a reference implementation of the Theora video compression format being developed by the Xiph.Org Foundation.^[4][5]

Theora is derived from the proprietary VP3 codec, released into the public domain by On2 Technologies. It is broadly comparable firstly in design and bitrate efficiency to MPEG-4 Part 2, early versions of Windows Media Video, and RealVideo, and secondly in open standards philosophy to the BBC's Dirac codec.

Theora is named for Theora Jones, Edison Carter's Controller on the Max Headroom television program.^[6]

Technical details

Theora is a variable-bitrate, DCT-based video compression scheme. Like most common video codecs, Theora also uses chroma subsampling, block based motion compensation and an 8 by 8 DCT block. It supports intra-coded frames and forward predictive frames, but not bi-predictive frames which are found in H.264 and VC-1. Theora also does not currently support interlacing, variable frame rates, or bit-depths larger than 8 bits per component.^[2]

Theora video streams can be stored in any suitable container format. Most commonly it is found in the Ogg container with Vorbis audio streams which provides a completely open, royalty-free multimedia format, but can also be used with the Matroska container.^[7]

Theora video compression format is essentially compatible with the VP3 video compression format, consisting of a backward-compatible superset.^[8][9] Theora is a superset of VP3 and VP3 streams (with some minor syntactic modifications) can be changed into Theora streams without recompression (but not vice versa).^[9] VP3 video compression can be decoded using Theora implementations, but Theora video compression usually cannot be decoded using old VP3 implementations.

History

VP3 was originally a proprietary and patented video codec developed by On2 Technologies. On2 TrueMotion VP3.1 was introduced in May 2000 followed three months later by the VP3.2 release.^[10][11] Later that year, On2 announced VP3 plugins for QuickTime and RealPlayer.^[12][13] In May 2001, On2 released the beta version of its new VP4 proprietary codec.^[14][15] In June 2001, On2 also released a VP3 codec implementation for Microsoft Windows^[16] where the encoder was priced at $39.95 for personal use, and $2,995 for limited commercial use.^[17] In August 2001, On2 Technologies announced that they will be releasing an open source version of their VP3.2 video compression algorithm.^[18][19] In September 2001 they published the source code and open source license for VP3.2 video compression algorithm at www.vp3.com.^{[20][21][22][23][24]} The VP3.2 Public License 0.1 granted the right to modify the source code only if the resulting larger work continued to support playback of VP3.2 data.^[25][20][21] In March 2002, On2 altered licensing terms required to download the source code for VP3 to LGPL.^[26] In June 2002 On2 donated VP3 to the Xiph.Org Foundation under a BSD-like open source license.^[27][28][29] On2 also made an irrevocable, royalty-free license grant for any patent claims it might have over the software and any derivatives^[2], allowing anyone to use any VP3-derived codec for any purpose.^[30][8] In August 2002, On2 entered into an agreement with the Xiph.Org Foundation to make VP3 the basis of a new, free video codec, called Theora.^[31] On2 declared Theora to be the successor in VP3's lineage. On October 3, 2002 On2 and Xiph announced the completion and availability of the initial Alpha code release of Theora (libtheora).^[32] The libtheora reference implementation has reached its alpha 2 milestone on June 9, 2003^[33] and alpha 3 on March 20, 2004.^[34]

There is no formal specification for the VP3 bitstream format beyond the VP3 source code published by On2 Technologies. In 2003, Mike Melanson created an uncomplete description of the VP3 bitstream format and decoding process at a higher level than source code, with some help from On2 and Xiph.Org Foundation. The Theora specification adopted some portions of this VP3 description.^[35][2]

The Theora I bitstream format was frozen in June 2004 after the libtheora 1.0alpha3 release.^[1] Videos encoded with any version of the libtheora since the alpha3 will be compatible with any future player.^[1][36] This is also true for videos encoded with any implementation of the Theora I specification since the format freeze. The Theora I Specification was completely published in 2004.^[37] Any later changes in the specification are minor updates.

The Theora reference implementation libtheora spent several years in alpha and beta status.^[36] The last alpha version was libtheora 1.0alpha7 released on June 20, 2006. It was followed by libtheora 1.0 beta1 on September 22, 2007. The last beta version was libtheora 1.0 beta3 released on April 16, 2008.^[36] The first stable release of libtheora as version 1.0 was made in November 2008.^[38][39] Work then focused on improving the codec performance in the "Thusnelda" branch, which was released as version 1.1 in September 2009 as the second stable libtheora release.^[40][36] This release brought some technical improvements and new features, e.g. the new rate control module and the new two-pass rate control.

Theora is well established as a video format in open source applications and as the video format of choice for Wikipedia's video content. However, the proposed adoption of Theora as part of the baseline video support in HTML5 resulted in controversy.

Performance

Encoding performance

Evaluations of the VP3^[41] and early Theora encoders^{[42] [43] [44]} found their subjective visual quality was inferior to contemporary video codecs. More recently however, Xiph developers have compared the 1.1 Theora encoder to YouTube's H.264 and H.263+ encoders, in response to concerns about Theora's inferior performance from Chris DiBona, a Google employee.^[45] They found the results from Theora to be nearly the same as YouTube's H.264 output, and much better than the H.263+ output.^[46][47]

The performance characteristics of the Theora 1.0 reference implementation are dominated mostly by implementation issues inherited from the original VP3 code base.^[48] Work leading up to the 1.1 stable release was focused on improving on or eliminating these implementation problems. A May 2009 review of this work shows a considerable improvement in quality, both subjectively and as measured by PSNR, just by improving the forward DCT and quantisation matrices.^[49] A flaw in the version of FFmpeg used in the test initially led to incorrect reports of Theora PSNR surpassing that of H.264. Although not achieving this goal, the improvement in the measured PSNR and the perceived quality is considerable. Further work on adaptive quantization, as well as overall detailed subjective tuning of the codec, is still to come.

Playback performance

There is an open source VHDL code base for a hardware Theora decoder in development.^[50] It began as a 2006 Google Summer of Code project, and it has been developed on both the Nios II and LEON processors.^[51] However there are currently no Theora decoder chips in production, and iPods and similar devices with limited computing power rely on such chips to provide efficient playback. However since decoding Theora is less CPU intensive than decoding H.264, hardware acceleration may not be necessary in all devices.

Playback

Embedded in HTML 5

As originally recommended by HTML 5, these browsers support Theora when embedded by the video element:

• Mozilla Firefox 3.5 and later versions^[52][53]
• Google Chrome as of version 3.0.182.2 ^[54][55]
• SeaMonkey as of version 2.0^[56]
• Opera experimental video build^[57][58]

Browser plugins

• Annodex plugin via OggPlay
• Cortado, a Java based applet
  • Itheora, a PHP wrapper for Cortado
• Mv Embed HTML 5 video tag wrapper for numerous plugin types.
• VLC media player browser plugin for IE or Firefox

Supporting media frameworks

• DirectShow with use of DirectShow filters
• GStreamer supported via Theora module, supports GStreamer based applications eg. Totem and Songbird
• Phonon
• QuickTime with use of Xiph QuickTime Components

Supporting applications

• FFmpeg (own implementation)
• Helix Player
• Miro Media Player (formerly known as Democracy Player)
• MPlayer and front-ends
• Songbird
• VLC (native support)
• xine and all libxine-based players like Kaffeine

Encoding

There are several third-party programs that support encoding through libtheora:

The libtheora library contains the reference implementation of the Theora specification for encoding and decoding. libtheora is still under development by the Xiph.Org Foundation. The library is released under the terms of a BSD-style license.

Also, several media frameworks have support for Theora.

• The open-source ffdshow audio/video decoder is capable of encoding Theora videos using its Video for Windows (VFW) multi-codec interface within popular AVI editing programs^[59][60][61] such as VirtualDub.^{[citation needed]} It supports both encoding and decoding Theora video streams and uses Theora's alpha 4 libraries. However, many of the more refined features of Theora aren't available to the user in ffdshow's interface.
• The GStreamer framework has support for parsing raw Theora streams, encoding and decoding raw Theora streams to/from YUV video^[62][63]

Editing

Streaming

The following streaming media servers are capable of streaming Theora video:

Theora Streaming Studio is a complete client to connect to an Icecast server.

See also

Free software portal

• Dirac (codec)
• Video editing software
• Comparison of video codecs
• Comparison of video encoders

References

External links

• Theora.org
• Examples of Theora-encoded videos
• Why Theora Matters for Internet TV
• Theora user manual
• RTP Payload Format for Theora Encoded Video - Xiph.Org, IETF Internet-Draft