1  <title>Codec Interface</title>
2
3  <para>A V4L2 codec can compress, decompress, transform, or otherwise
4convert video data from one format into another format, in memory. Typically
5such 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>
7capability set).
8</para>
9
10  <para>A memory-to-memory video node acts just like a normal video node, but it
11supports both output (sending frames from memory to the codec hardware) and
12capture (receiving the processed frames from the codec hardware into memory)
13stream I/O. An application will have to setup the stream
14I/O for both sides and finally call &VIDIOC-STREAMON; for both capture and output
15to 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).
19See <xref linkend="mpeg-controls"></xref>.</para>
20
21  <para>Memory-to-memory devices can often be used as a shared resource: you can
22open the video node multiple times, each application setting up their own codec properties
23that are local to the file handle, and each can use it independently from the others.
24The driver will arbitrate access to the codec and reprogram it whenever another file
25handler gets access. This is different from the usual video node behavior where the video properties
26are global to the device (i.e. changing something through one file handle is visible
27through another file handle).</para>
28