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When using ff_init_vlc_sparse() to create a VLC, three input tables are used: A table for lengths, one for codes and one for symbols; the latter one can be omitted, then a default one will be used. These input tables will be traversed twice, once to get the long codes (which will be put into subtables) and once for the small codes. The long codes are then sorted so that entries that should be in the same subtable are contiguous. This commit adds an alternative to ff_init_vlc_sparse(): ff_init_vlc_from_lengths(). It is based upon the observation that if lengths, codes and symbols tables are permuted (in the same way) so that the codes are ordered from left to right in the corresponding tree and if said tree is complete (i.e. every non-leaf node has two children), the codes can be easily computed from the lengths and are therefore redundant. This means that if one initializes such a VLC with explicitly coded lengths, codes and symbols, the codes can be avoided; and even if one has no explicitly coded symbols, it might still be beneficial to remove the codes even when one has to add a new symbol table, because codes are typically longer than symbols so that the latter often fit into a smaller type, saving space. Furthermore, given that the codes here are by definition ordered from left to right, it is unnecessary to sort them again; for the same reason, one does not have to traverse the input twice. This function proved to be faster than ff_init_vlc_sparse() whenever it has been benchmarked. This function is usable for static tables (they can simply be permuted once) as well as in scenarios where the tables are naturally ordered from left to right in the tree; the latter e.g. happens with Smacker, Theora and several other formats. In order to make it also usable for (static) tables with incomplete trees, negative lengths are used to indicate that there is an open end of a certain length. Finally, ff_init_vlc_from_lengths() has one downside compared to ff_init_vlc_sparse(): The latter uses tables that can be reused by encoders. Of course, one could calculate the needed table at runtime if one so wishes, but it is nevertheless an obstacle. 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.