| .. -*- coding: utf-8; mode: rst -*- | |
| .. _codec: | |
| *************** | |
| Codec Interface | |
| *************** | |
| 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 | |
| ``V4L2_CAP_VIDEO_M2M`` or ``V4L2_CAP_VIDEO_M2M_MPLANE`` capability set). | |
| 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 :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` | |
| for both capture and output to start the codec. | |
| Video compression codecs use the MPEG controls to setup their codec | |
| parameters | |
| .. note:: | |
| The MPEG controls actually support many more codecs than | |
| just MPEG. See :ref:`mpeg-controls`. | |
| Memory-to-memory devices function 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). |