Theora

Theora is a free lossy video compression format.[6] It is developed by the Xiph.Org Foundation and distributed without licensing fees alongside their other free and open media projects, including the Vorbis audio format and the Ogg container.

For the genus of clams, see Theora (bivalve).

Theora
Filename extension
.ogv, .ogg
Internet media type
video/ogg
Developed byXiph.org
Initial release1 June 2004 (2004-06-01)[1]
Latest release
Theora I
(16 March 2011[2])
Type of formatCompressed video
Contained byOgg, Matroska
Extended fromVP3
StandardSpecification
Open format?Yes[3]
Websitetheora.org
libtheora
Developer(s)Xiph.org
Initial release3 November 2008 (2008-11-03) (1.0)
Stable release
1.1.1 / 1 October 2009 (2009-10-01)[4]
Preview release
1.2.0 Alpha 1 / 24 September 2010 (2010-09-24)[5]
Written inC
Operating systemUnix-like (incl Linux, Mac OS X), Windows
TypeVideo codec, reference implementation
License3-clause BSD
Websitetheora.org

The libtheora video codec is the reference implementation of the Theora video compression format being developed by the Xiph.Org Foundation.[7][8]

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

Theora is named after Theora Jones, Edison Carter's Controller on the Max Headroom television program.[9]

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. Pixels are grouped into various structures, namely blocks, super blocks, and macroblocks. Theora 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 support interlacing, or bit-depths larger than 8 bits per component.[2]

Theora video streams can be stored in any suitable container format, but they are most commonly found in the Ogg container with Vorbis or FLAC audio streams. This combination provides a completely open, royalty-free multimedia format. It can also be used with the Matroska container.[10]

The Theora video-compression format is essentially compatible with the VP3 video-compression format, consisting of a backward-compatible superset.[11][12] Theora is a superset of VP3, and VP3 streams (with some minor syntactic modifications) can be converted into Theora streams without recompression (but not vice versa).[12] VP3 video compression can be decoded using Theora implementations, but Theora video compression usually cannot be decoded using old VP3 implementations.

History
See also: VP3

Theora's predecessor On2 TrueMotion VP3 was originally a proprietary and patent-encumbered video codec developed by On2 Technologies. VP3.1 was introduced in May 2000[13] and followed three months later by the VP3.2 release,[14] which is the basis for Theora.

Move to free software

In August 2001, On2 Technologies announced that they would be releasing an open source version of their VP3.2 video compression algorithm.[15][16] In September 2001, On2 Technologies published the source code of the VP3.2 codec under the VP3.2 Public License 0.1,[17] a custom open-source license.[18][19] The license only granted the right to modify the source code if the resulting larger work continued to support playback of VP3.2 data.[17][20]

In March 2002, On2 responded to the public's reception by relicensing the VP3 codec under the GNU Lesser General Public License.[21] In June 2002, On2 donated VP3 to the Xiph.Org Foundation and offered it under the Ogg Vorbis BSD-style license.[22][23][24][25] 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.[11][26] 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.[27] On2 declared Theora to be VP3's successor. On 3 October 2002, On2 and Xiph announced the completion and availability of the initial alpha code release of libtheora, Theora's reference implementation.[28]

There is no formal specification for VP3's bitstream format beyond the VP3 source code published by On2 Technologies. In 2003, Mike Melanson created an incomplete 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.[2][29]

Project Daala is working on the successor to Theora.

Theora I specification

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][30] 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.[31] Any later changes in the specification are minor updates.

The Theora reference implementation libtheora spent several years in alpha and beta status.[30] The first alpha version was released on 25 September 2002[32] and the first beta version was released on 22 September 2007. The first stable release of libtheora was made in November 2008.[33][34] Work then focused on improving the codec's performance in the "Thusnelda" branch, which was released as version 1.1 in September 2009 as the second stable libtheora release.[30][35] This release brought some technical improvements and new features, such as the new rate control module and the two-pass rate control.

The codename for the next version of libtheora is Ptalarbvorm.[36]

Theora is well established as a video format in open-source applications, and is the format used for Wikipedia's video content. However, the proposed adoption of Theora as part of the baseline video support in HTML5 resulted in controversy.[37]

Performance

Encoding performance

Evaluations of the VP3[38] and early Theora encoders[39][40][41] found that their subjective visual quality was inferior to that of 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 raised in 2009 about Theora's inferior performance by Chris DiBona, a Google employee.[42] 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.[43][44]

The performance characteristics of the Theora 1.0 reference implementation are dominated mostly by implementation problems inherited from the original VP3 code base.[45] Work leading up to the 1.1 stable release was focused on improving on or eliminating these. 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.[46] 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. In any case, the differences in quality, bitrate and file size between a YouTube H.264 video and a transcoded Ogg video file are negligible.[47] 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.[48] It began as a 2006 Google Summer of Code project, and it has been developed on both the Nios II and LEON processors.[49] However, there are currently no Theora decoder chips in production, and portable media players, smartphones and similar devices with limited computing power rely on such chips to provide efficient playback. But since decoding Theora is less CPU intensive than decoding H.264, the need for hardware-accelerated Theora decoding may be somewhat less.

Playback

Native browser playback
Main article: Use of Ogg formats in HTML5

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

  • Mozilla Firefox 3.5 and later versions[50][51] including Firefox for mobile (Fennec).[52]
  • Google Chrome as of version 3.0.182.2[53][54] including Chromium as of 14 July 2009.[55]
  • Tizen browser
  • SeaMonkey as of version 2.0.[56]
  • Konqueror 4.4.2[57][58]
  • Opera as of version 10.50.[59][60] It was also supported in Opera 9.5 experimental video builds.[61][62]
  • Web uses WebKitGTK+ as its rendering engine. As WebKitGTK+ uses GStreamer to implement the HTML5 media player, and all the formats GStreamer supports (including Theora) are available in browser.
  • Midori is another example of a browser that supports Theora by using WebKitGTK+.

Browser plugins

Supporting media frameworks
  • DirectShow with use of OpenCodecs
  • GStreamer supported via Theora or FFmpeg module, supports GStreamer based applications e.g. Totem and Songbird
  • Phonon
  • QuickTime (including but not limited to Safari) with use of Xiph QuickTime Components
  • Silverlight Highgate media suite is going to bring an Open Source Theora/Vorbis implementation in Silverlight. It will enable installation-free support for HTML5 streaming video.[66]

Supporting applications
  • FFmpeg (own implementation)
  • Helix Player
  • Miro Media Player (formerly known as Democracy Player)
  • MPlayer and front-ends
  • Songbird, Totem, Moovida and all GStreamer-based players
  • VLC (native support)
  • xine and all libxine-based players like Kaffeine
  • Dragon player and all Phonon-based players

Encoding

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

Name Description Operating Systems Supported
Unix-like OS X Windows
Firefogg[67]
 A Firefox browser extension implementation of ffmpeg2theora Yes Yes Yes
ffmpeg2theora[68]
 A command-line program that transcodes video by decoding with FFmpeg and reencoding with libtheora to encode it Yes Yes Yes
VLC
 Can transcode to single-pass Theora 1.0 and optionally stream it Yes Yes Yes
OggConvert
 Transcodes supported media to Vorbis, Theora, or Dirac Yes ? Yes
FreeJ
 "Video DJing" software that can encode to and stream Theora Yes Yes ?
Kdenlive
 The video editor supplied with KDE Yes ? ?
Pitivi
 The video editor supplied with GNOME Yes ? ?
LiVES
 Video editing software for Linux. Can edit, encode and stream theora. Yes Yes ?
Thoggen
 A GTK+ and GStreamer based DVD backup utility Yes ? ?
HandBrake
 Can output to Theora only with the Matroska container Yes Yes Yes
RecordMyDesktop
 Records the screen to Ogg Theora with optional Vorbis audio Yes ? ?

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.[69][70][71] 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[72][73]

Editing
Name Description Operating Systems Supported
Unix-like OS X Windows
LiVES
 Video editing software for Linux. Can edit, encode and stream theora. Yes Yes ?
Kdenlive
 The KDE video editor. Yes ? ?
OpenShot
 Yes ? ?
Pitivi
 The GNOME video editor. Yes ? ?
Cinelerra
 CVS versions of the Cinelerra non-linear video editing system support Theora, as of August 2005. Yes Yes ?
oggz-tools by Xiph.org
 Command line programs to examine and edit Ogg files. Yes ? Yes
Ogg Video Tools by yornstreamnik
 Tools to resize, cut, split, join, and others[74] Yes Yes Yes
AVS Video Editor
 ? ? Yes

Streaming

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

Name Description Operating Systems Supported
Unix-like OS X Windows
VLC
 Yes Yes Yes
Icecast
 Yes ? Yes
FreeCast
 Peer-to-peer streaming. Written in Java Yes ? Yes
LiVES
 Can stream ogg/theora/vorbis in realtime to a file or fifo. Yes Yes ?
Flumotion
 Streaming media server. Yes ? ?

Makers

Elphel is the main maker of cameras that record in theora.

gollark: BAN it for it is UNSAFE.
gollark: But just don't use it™™.
gollark: YES.
gollark: WRONG.
gollark: WRONG.

See also

References
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