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