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MagicYUV transmits its Huffman trees by providing the length of the code corresponding to each symbol; then the decoder has to assemble the table in such a way that (i) longer codes are to the left of the tree and (ii) for codes of the same length the symbols are ascending from left to right. Up until now the decoder did this as follows: It counted the number of codes of each length and derived the first code of a given length via (ii). Then the array of lengths is traversed a second time to create the codes; there is one running counter for each length to do so. This process creates a default symbol table (that is omitted). This commit changes this as follows: Everything is indexed by the position in the tree (with codes to the left first); given (i), we can calculate the ranges occupied by the codes of each length; and with (ii) we can derive the actual symbols of each code; the running counters for each length are now used for the symbols and not for the codes. Doing so allows us to switch to ff_init_vlc_from_lengths(); this has the advantage that the codes table needs only be traversed once and that the codes need not be sorted any more (right now, the codes that are so long that they will be put into subtables need to be sorted so that codes that end up in the same subtable are contiguous). For a sample produced by our encoder (natural content, 4000 frames, YUV420p, ten iterations, GCC 9.3) this decreased the amount of decicycles for each call to build_huffman() from 1336049 to 1309401. Notice that our encoder restricts the code lengths to 12 and our decoder only uses subtables when the code is longer than 12 bits, so the sorting that can be avoided does not happen at the moment. If one reduces the decoder's tables to nine bits, the performance improvement becomes more apparent: The amount of decicycles for build_huffman() decreased from 1165210 to 654055. Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
FFmpeg README
FFmpeg is a collection of libraries and tools to process multimedia content such as audio, video, subtitles and related metadata.
Libraries
libavcodecprovides implementation of a wider range of codecs.libavformatimplements streaming protocols, container formats and basic I/O access.libavutilincludes hashers, decompressors and miscellaneous utility functions.libavfilterprovides a mean to alter decoded Audio and Video through chain of filters.libavdeviceprovides an abstraction to access capture and playback devices.libswresampleimplements audio mixing and resampling routines.libswscaleimplements color conversion and scaling routines.
Tools
- ffmpeg is a command line toolbox to manipulate, convert and stream multimedia content.
- ffplay is a minimalistic multimedia player.
- ffprobe is a simple analysis tool to inspect multimedia content.
- Additional small tools such as
aviocat,ismindexandqt-faststart.
Documentation
The offline documentation is available in the doc/ directory.
The online documentation is available in the main website and in the wiki.
Examples
Coding examples are available in the doc/examples directory.
License
FFmpeg codebase is mainly LGPL-licensed with optional components licensed under GPL. Please refer to the LICENSE file for detailed information.
Contributing
Patches should be submitted to the ffmpeg-devel mailing list using
git format-patch or git send-email. Github pull requests should be
avoided because they are not part of our review process and will be ignored.