avrecode: lossless re-compression of compressed video streams
avrecode reads an already-compressed video file and writes a more compressed file. Unlike transcoding, which loses fidelity, the compression algorithm used by avrecode is reversible. The decompressed bytes exactly match the original input file. However, avrecode's compressed format can only be read by avrecode -- an avrecode-compressed file cannot be played directly by standard software.
avrecode works by decoding the video stream into symbols using ffmpeg's libavcodec. It tries to predict each symbol as it arrives, and re-encodes the symbols to the compressed file using arithmetic coding. When avrecode's predictions are higher quality than the predictions specified by the H.264 standard, it achieves a better compression ratio.
Installing
avrecode consists of a compression/decompression program and a fork of the libavcodec library, part of the ffmpeg project. These live in separate github repositories:
The avrecode repository imports the libavcodec-hooks repository as a submodule,
so the git submodule command is used to keep them in sync.
Download the source:
git clone https://github.com/dropbox/avrecode
cd avrecode
git submodule update --init
Build and test:
brew install protobuf
cd ffmpeg
./configure
make
cd ..
make
./recode roundtrip data/GOPR4542.MP4
Warning
This is an experimental test bed for compression research: use on trusted inputs only This tool does not validate input.
License
avrecode is released under the BSD 3-clause license. See the LICENSE file for details. The required libavcodec-hooks patch to ffmpeg is licenced under the LGPL.
Contributing
avrecode was originally written by Chris Lesniewski during Dropbox Hack Week January 2016. It is a redesign of the first version written by Daniel Horn, Patrick Horn, Chris Lesniewski, and others during Dropbox Hack Week 2015. We welcome external contributions, but ask that contributors accept our Contributor License Agreement to grant us a license to distribute the code: