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The Ut video format uses Huffman trees which are only implicitly coded in the bitstream: Only the lengths of the codes are coded, the rest has to be inferred by the decoder according to the rule that the longer codes are to the left of shorter codes in the tree and on each level the symbols are descending from left to right. Because longer codes are to the left of shorter codes, one needs to know how many non-leaf nodes there are on each level in order to know the code of the next left-most leaf (which belongs to the highest symbol on that level). The current code does this by sorting the entries to be ascending according to length and (for entries with the same length) ascending according to their symbols. This array is then traversed in reverse order, so that the lowest level is dealt with first, so that the number of non-leaf nodes of the next higher level is known when processing said level. But this can also be calculated without sorting: Simply count how many leaf nodes there are on each level. Then one can calculate the number of non-leaf nodes on each level iteratively from the lowest level upwards: It is just half the number of nodes of the level below. This improves performance: For the sample from ticket #4044 the amount of decicycles for one call to build_huff() decreased from 1055489 to 446310 for Clang 10 and from 1080306 to 535155 for GCC 9. Reviewed-by: Paul B Mahol <onemda@gmail.com> 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.