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When the Matroska muxer writes an EBML ID, it calculates the length of said ID before; and it does this as if this were a number that needs to be encoded as EBML number: The formula used is (av_log2(id + 1) - 1) / 7 + 1. But the constants used already contain the VINT_MARKER (the leading bit indicating the length of the EBML number) and therefore the algorithm used makes no sense. Instead the position of the most significant byte set gives the desired length. The algorithm used until now worked because EBML numbers are subject to restrictions: If the EBML number takes up k bytes, then the bit 1 << (7 * k) is set and av_log2(id) is 7 * k. So the current algorithm produces the correct result unless the EBML ID is of the form 7 * k - 1 because of the "id + 1". But contrary to encoding lengths as EBML number (where the + 1 exists to avoid the encodings reserved for unknown length), such EBML numbers are simply forbidden as EBML IDs and as such none of them were ever written. 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.