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Theora

 
Theora
Filename Extension .ogv, .ogg
Internet Media Type video/ogg
Developed by Xiph.org
Initial release 1 June 2004
Latest release Theora I / 16 March 2011
Type of format Compressed video
Contained by Ogg, Matroska
Extended from VP3

Theora, 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, is a free lossy video compression format. Theora is derived from the 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.


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 super blocks, 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.

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

The Theora video-compression format is essentially compatible with the VP3 video-compression format, consisting of a backward-compatible superset. 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). VP3 video compression can be decoded using Theora implementations, but Theora video compression usually cannot be decoded using old VP3 implementations.


Performance

Encoding performance

Evaluations of the VP3 and early Theora encoders 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 raised in 2009 about Theora's inferior performance by Chris DiBona, a Google employee. 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.

The performance characteristics of the Theora 1.0 reference implementation are dominated mostly by implementation problems inherited from the original VP3 code base. 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. 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. 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. It began as a 2006 Google Summer of Code project, and it has been developed on both the Nios II and LEON processors. 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. However since decoding Theora is less CPU intensive than decoding H.264, the need for hardware acceleration may be somewhat obviated.


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