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This uses Stein's binary GCD algorithm: https://en.wikipedia.org/wiki/Binary_GCD_algorithm to get a roughly 4x speedup over Euclidean GCD on standard architectures with a compiler intrinsic for ctzll, and a roughly 2x speedup otherwise. At the moment, the compiler intrinsic is used on GCC and Clang due to its easy availability. Quick note regarding overflow: yes, subtractions on int64_t can, but the llabs takes care of that. The llabs is also guaranteed to be safe, with no annoying INT64_MIN business since INT64_MIN being a power of 2, is shifted down before being sent to llabs. The binary GCD needs ff_ctzll, an extension of ff_ctz for long long (int64_t). On GCC, this is provided by a built-in. On Microsoft, there is a BitScanForward64 analog of BitScanForward that should work; but I can't confirm. Apparently it is not available on 32 bit builds; so this may or may not work correctly. On Intel, per the documentation there is only an intrinsic for _bit_scan_forward and people have posted on forums regarding _bit_scan_forward64, but often their documentation is woeful. Again, I don't have it, so I can't test. As such, to be safe, for now only the GCC/Clang intrinsic is added, the rest use a compiled version based on the De-Bruijn method of Leiserson et al: http://supertech.csail.mit.edu/papers/debruijn.pdf. Tested with FATE, sample benchmark (x86-64, GCC 5.2.0, Haswell) with a START_TIMER and STOP_TIMER in libavutil/rationsl.c, followed by a make fate. aac-am00_88.err: builtin: 714 decicycles in av_gcd, 4095 runs, 1 skips de-bruijn: 1440 decicycles in av_gcd, 4096 runs, 0 skips previous: 2889 decicycles in av_gcd, 4096 runs, 0 skips Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com> Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
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.
- ffserver is a multimedia streaming server for live broadcasts.
- 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. Few developers
follow pull requests so they will likely be ignored.