Markus Heiser | 5377d91 | 2016-06-30 15:18:56 +0200 | [diff] [blame] | 1 | .. -*- coding: utf-8; mode: rst -*- |
| 2 | |
| 3 | .. _codec: |
| 4 | |
| 5 | *************** |
| 6 | Codec Interface |
| 7 | *************** |
| 8 | |
| 9 | A V4L2 codec can compress, decompress, transform, or otherwise convert |
| 10 | video data from one format into another format, in memory. Typically |
| 11 | such devices are memory-to-memory devices (i.e. devices with the |
| 12 | ``V4L2_CAP_VIDEO_M2M`` or ``V4L2_CAP_VIDEO_M2M_MPLANE`` capability set). |
| 13 | |
| 14 | A memory-to-memory video node acts just like a normal video node, but it |
| 15 | supports both output (sending frames from memory to the codec hardware) |
| 16 | and capture (receiving the processed frames from the codec hardware into |
| 17 | memory) stream I/O. An application will have to setup the stream I/O for |
Mauro Carvalho Chehab | 7347081 | 2016-07-01 13:58:44 -0300 | [diff] [blame] | 18 | both sides and finally call :ref:`VIDIOC_STREAMON` |
Markus Heiser | 5377d91 | 2016-06-30 15:18:56 +0200 | [diff] [blame] | 19 | for both capture and output to start the codec. |
| 20 | |
| 21 | Video compression codecs use the MPEG controls to setup their codec |
| 22 | parameters (note that the MPEG controls actually support many more |
| 23 | codecs than just MPEG). See :ref:`mpeg-controls`. |
| 24 | |
| 25 | Memory-to-memory devices can often be used as a shared resource: you can |
| 26 | open the video node multiple times, each application setting up their |
| 27 | own codec properties that are local to the file handle, and each can use |
| 28 | it independently from the others. The driver will arbitrate access to |
| 29 | the codec and reprogram it whenever another file handler gets access. |
| 30 | This is different from the usual video node behavior where the video |
| 31 | properties are global to the device (i.e. changing something through one |
| 32 | file handle is visible through another file handle). |
| 33 | |
| 34 | |
| 35 | .. ------------------------------------------------------------------------------ |
| 36 | .. This file was automatically converted from DocBook-XML with the dbxml |
| 37 | .. library (https://github.com/return42/sphkerneldoc). The origin XML comes |
| 38 | .. from the linux kernel, refer to: |
| 39 | .. |
| 40 | .. * https://github.com/torvalds/linux/tree/master/Documentation/DocBook |
| 41 | .. ------------------------------------------------------------------------------ |