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The Matroska demuxer splits every sequence of h Matroska Blocks into h * w / cfs packets of size cfs; here h (sub_packet_h), w (frame_size) and cfs (coded_framesize) are parameters from the track's CodecPrivate. It does this by splitting the Block's data in h/2 pieces of size cfs each and putting them into a buffer at offset m * 2 * w + n * cfs where m (range 0..(h/2 - 1)) indicates the index of the current piece in the current Block and n (range 0..(h - 1)) is the index of the current Block in the current sequence of Blocks. The data in this buffer is then used for the output packets. The problem is that there is currently no check to actually guarantee that no uninitialized data will be output. One instance where this is trivially so is if h == 1; another is if cfs * h is so small that the input pieces do not cover everything that is output. In order to preclude this, rmdec.c checks for h * cfs == 2 * w and h >= 2. The former requirement certainly makes much sense, as it means that for every given m the input pieces (corresponding to the h different values of n) form a nonoverlapping partition of the two adjacent frames of size w corresponding to m. But precluding h == 1 is not enough, other odd values can cause problems, too. That is because the assumption behind the code is that h frames of size w contain data to be output, although the real number is h/2 * 2. E.g. for h = 3, cfs = 2 and w = 3 the current code would output four (== h * w / cfs) packets. although only data for three (== h/2 * h) packets has been read. (Notice that if h * cfs == 2 * w, h being even is equivalent to cfs dividing w; the latter condition also seems very reasonable: It means that the subframes are a partition of the frames.) 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.