Theora

libtheora.ogvOgg Theoraogv
Theora is a free lossy video compression format.wikipedia
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Video coding format

video compression formatvideo compression standardvideo compression
Theora is a free lossy video compression format.
Examples of video coding formats include MPEG-2 Part 2, MPEG-4 Part 2, H.264 (MPEG-4 Part 10), HEVC, Theora, RealVideo RV40, VP9, and AV1.

Ogg

.oggOGMogg123
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. 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.
In the Ogg multimedia framework, Theora provides a lossy video layer.

Dirac (video compression format)

DiracDirac codecDirac video codec
It is comparable in open standards philosophy to the BBC's Dirac codec.
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, e.g. Theora, and WMV.

FLAC

.flacFree Lossless Audio CodecFLAC
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.
Xiph.org is behind other free compression formats such as Vorbis, Theora, Speex and Opus.

Daala

Project Daala is working on the successor to Theora.
Like Theora and Opus, Daala is available free of any royalties and its reference implementation is being developed as free and open-source software.

Use of Ogg formats in HTML5

resulted in controversyretracted
However, the proposed adoption of Theora as part of the baseline video support in HTML5 resulted in controversy.
The HTML5 draft specification adds and elements for embedding video and audio in HTML documents. The specification had formerly recommended support for playback of Theora video and Vorbis audio encapsulated in Ogg containers to provide for easier distribution of audio and video over the internet by using open standards, but the recommendation was soon after dropped.

Discrete cosine transform

DCTiDCTinverse discrete cosine transform
Theora is a variable-bitrate, DCT-based video compression scheme.
The DCT is used in JPEG image compression, MJPEG, MPEG, DV, Daala, and Theora video compression.

Lossy compression

lossylossy data compressioncompressed
Theora is a free lossy video compression format.
Ogg Theora (noted for its lack of patent restrictions)

VP3

4VP4
Theora is derived from the formerly proprietary VP3 codec, released into the public domain by On2 Technologies.
In June 2002 On2 donated VP3 to the Xiph.org Foundation under a BSD-like open source license to make VP3 the basis of a new, free (e.g. patent- and royalty-free) video codec, Theora.

Video codec

codecvideo decodingcodecs
The libtheora video codec is the reference implementation of the Theora video compression format being developed by the Xiph.Org Foundation.
libtheora: A reference implementation of the Theora video compression format developed by the Xiph.org Foundation, based upon On2 Technologies' VP3 codec, and christened by On2 as the successor in VP3's lineage. Theora is targeted at competing with MPEG-4 video and similar lower-bitrate video compression schemes.

Vorbis

Ogg VorbisOGGaoTuV
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. 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.
Opera 10.5 browser has support for Vorbis audio, WAVE PCM audio and Theora video.

Variable bitrate

VBRvariable bit ratevariable
Theora is a variable-bitrate, DCT-based video compression scheme.
Variable bit rate encoding is also commonly used on MPEG-2 video, MPEG-4 Part 2 video (Xvid, DivX, etc.), MPEG-4 Part 10/H.264 video, Theora, Dirac and other video compression formats.

Digital container format

container formatcontainercontainer formats
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.
Ogg (standard container for Xiph.org audio formats Vorbis and Opus and video format Theora)

Firefox 3.5

3.5
Mozilla Firefox 3.5 and later versions including Firefox for mobile (Fennec).
These include support for the and elements defined in the HTML 5 draft specification, including native support for Ogg Theora encoded video and Vorbis encoded audio.

Cortado (software)

Cortado
Cortado, a Java based applet
Cortado is a streaming Java applet for Ogg formats Vorbis, Theora and Kate, µ-law, MJPEG and Smoke (a custom MJPEG variant), released under the GPL.

WebM

.webm155-165 kbps Opus WebM
Wikipedia stopped preferring Ogg Theora and now prefers WebM.
Theora

OggConvert

OggConvert is a free and open-source transcoder for digital audio and video files of various types into the free Ogg Vorbis audio format, and the Theora, VP8 and Dirac video formats.

Annodex

Annodex Association
Annodex plugin via OggPlay
By late 2004, Andre Pang developed the Annodex Plug-in for Mozilla Firefox Browsers, allowing for the playback of Annodex media encoded with the Ogg Theora video codec and the Ogg Vorbis audio codec.

H.264/MPEG-4 AVC

H.264AVCH.264/AVC
Theora supports intra-coded frames and forward-predictive frames, but not bi-predictive frames which are found in H.264 and VC-1. A flaw in the version of FFmpeg used in the test initially led to incorrect reports of Theora PSNR surpassing that of H.264.
In 2009, the HTML5 working group was split between supporters of Ogg Theora, a free video format which is thought to be unencumbered by patents, and H.264, which contains patented technology.

LiVES

Some formats can be opened instantly using decoder plugins (e.g. flv, asf, dv, webm and ogg/theora).

Helix (multimedia project)

HelixHelix PlayerHelix DNA
Helix Player
Video formats: Theora, RealVideo, WMV, H.263, H.264, VC-1, H.261, MJPEG and container formats RealMedia file format, 3GP, 3G2, AVI, ASF

FFmpeg

SnowlibavformatNUT
A flaw in the version of FFmpeg used in the test initially led to incorrect reports of Theora PSNR surpassing that of H.264.

MPlayer

liba52mplayer2GNOME MPV
MPlayer and front-ends
Video formats: Cinepak, DV, H.263, H.264/MPEG-4 AVC, HuffYUV, Indeo, MJPEG, MPEG-1, MPEG-2, MPEG-4 Part 2, RealVideo, Sorenson, Theora, WMV, Bink

Miro (software)

Miro
Miro Media Player (formerly known as Democracy Player)
It is based on FFmpeg with profiles for the Theora (.ogv ), .mp4, and WebM video formats supported by various devices.

Xine

gxine
xine and all libxine-based players like Kaffeine
Video formats: Cinepak, DV, H.263, H.264/MPEG-4 AVC, HuffYUV, Indeo, MJPEG, MPEG-1, MPEG-2, MPEG-4 ASP, RealVideo, Sorenson, Theora, WMV (partial, including WMV1, WMV2 and WMV3; via FFmpeg)