x264 is a free and open-source software library and a command-line utility developed by VideoLAN for encoding video streams into the H.264/MPEG-4 AVC video coding format. It is released under the terms of the GNU General Public License. History x264 was originally developed by Laurent Aimar, who stopped development in 2004 after being hired by ATEME. Loren Merritt then took over development. Later, in 2008, Fiona Glaser joined the project. They both stopped contributing in 2014. Today, x264 is primarily developed by Anton Mitrofanov and Henrik Gramner. Capabilities x264 provides a command line interface as well as an API. The former is used by many graphical user interfaces, such as Staxrip and MeGUI. The latter is used by many other interfaces, such as HandBrake and FFmpeg. x264 implements a large number of features compared to other H.264 encoders. x264 contains some psychovisual enhancements which aim to increase the subjective video quality of the encoded video. Adaptive quantisation in two modes using VAQ. The second mode, a later addition, adapts the strength per frame in an attempt to improve the quality. Psychovisual rate–distortion optimization which attempts to maintain a similar complexity. The complexity is measured using a combination of sum-of-squares optimization (SSD) and sum of absolute transformed differences (SATD). Macroblock-tree rate control, which controls the quality by tracking how often parts of the frame are used for predicting future frames. x264 has won awards in the following codec comparisons: Third Annual MSU MPEG-4 AVC/H.264 Video Codec Comparison, 2006 Fourth Annual MSU MPEG-4 AVC/H.264 Video Codec Comparison, 2007 Fifth Annual MSU MPEG-4 AVC/H.264 Video Codec Comparison, 2009 Sixth Annual MSU MPEG-4 AVC/H.264 Video Codec Comparison, 2010 Seventh Annual MSU MPEG-4 AVC/H.264 Video Codec Comparison, 2011 Eighth Annual MSU MPEG-4 AVC/H.264 Video Codec Comparison, 2012 x264 has SIMD assembly code acceleration on x86, PowerPC (using AltiVec), and ARMv7 (using NEON) platforms. x264 is able to use Periodic Intra Refresh instead of keyframes, which enables each frame to be capped to the same size enabling each slice to be immediately transmitted in a single UDP or TCP packet and on arrival immediately decoded. Periodic Intra Refresh can replace keyframes by using a column of intra blocks that move across the video from one side to the other, thereby "refreshing" the image. In effect, instead of a big keyframe, the keyframe is "spread" over many frames. The video is still seekable: a special header, called