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Theora is a free lossy video compression format.〔.〕 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. The libtheora video codec is the reference implementation of the Theora video compression format being developed by the Xiph.Org Foundation. 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.〔(【引用サイトリンク】 Theora FAQ )〕 ==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.〔 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.〔(【引用サイトリンク】 Matroska Codec Specs )〕 The Theora video-compression format is essentially compatible with the VP3 video-compression format, consisting of a backward-compatible superset.〔〔Xiph.org (FAQ – Theora and VP3 ). Retrieved 2 September 2009〕 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Theora」の詳細全文を読む スポンサード リンク
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