| .. -*- 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). |
