37 lines
1.5 KiB
ReStructuredText
37 lines
1.5 KiB
ReStructuredText
|
.. -*- 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 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).
|