x264

x264 is a free software library and encoder for the H.264/MPEG-4 AVC video compression standard. It was developed as part of the open-source x264 project and released under the GNU General Public License (GPL) version 2 or later. The project is maintained by VideoLAN and a community of contributors, and is designed to serve as an encoding backend for a range of multimedia tools.

The encoder provides a C library and command-line utilities that convert raw video frames into H.264 bitstreams. It implements the core features of H.264, including intra and inter prediction, motion estimation, multiple reference frames, transform coding, and entropy coding with CABAC or CAVLC. It also includes a deblocking filter, rate control, and options for multi-pass encoding. Users can tune the quality and speed with a wide set of options, including a hierarchy of presets from ultrafast to veryslow and tuning profiles such as film or animation.

x264 is widely used as the backend for other software, notably FFmpeg and HandBrake, through the libx264 library. It supports various encoding modes, such as constant rate factor (CRF) quality-based encoding and bitrate-based modes, and scales with multi-core processors to improve speed on modern hardware. The combination of high quality, flexible tuning, and broad adoption has made x264 one of the de facto standards for software-based H.264 encoding.