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1 <title>Codec Interface</title> 2 3 <para>A V4L2 codec can compress, decompress, transform, or otherwise 4 convert video data from one format into another format, in memory. Typically 5 such devices are memory-to-memory devices (i.e. devices with the 6 <constant>V4L2_CAP_VIDEO_M2M</constant> or <constant>V4L2_CAP_VIDEO_M2M_MPLANE</constant> 7 capability set). 8 </para> 9 10 <para>A memory-to-memory video node acts just like a normal video node, but it 11 supports both output (sending frames from memory to the codec hardware) and 12 capture (receiving the processed frames from the codec hardware into memory) 13 stream I/O. An application will have to setup the stream 14 I/O for both sides and finally call &VIDIOC-STREAMON; for both capture and output 15 to start the codec.</para> 16 17 <para>Video compression codecs use the MPEG controls to setup their codec parameters 18 (note that the MPEG controls actually support many more codecs than just MPEG). 19 See <xref linkend="mpeg-controls"></xref>.</para> 20 21 <para>Memory-to-memory devices can often be used as a shared resource: you can 22 open the video node multiple times, each application setting up their own codec properties 23 that are local to the file handle, and each can use it independently from the others. 24 The driver will arbitrate access to the codec and reprogram it whenever another file 25 handler gets access. This is different from the usual video node behavior where the video properties 26 are global to the device (i.e. changing something through one file handle is visible 27 through another file handle).</para>
