2016-06-30 21:18:56 +08:00
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.. _codec:
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***************
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Codec Interface
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***************
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A V4L2 codec can compress, decompress, transform, or otherwise convert
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video data from one format into another format, in memory. Typically
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such devices are memory-to-memory devices (i.e. devices with the
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``V4L2_CAP_VIDEO_M2M`` or ``V4L2_CAP_VIDEO_M2M_MPLANE`` capability set).
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A memory-to-memory video node acts just like a normal video node, but it
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supports both output (sending frames from memory to the codec hardware)
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and capture (receiving the processed frames from the codec hardware into
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memory) stream I/O. An application will have to setup the stream I/O for
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2016-07-05 05:05:55 +08:00
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both sides and finally call :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>`
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2016-06-30 21:18:56 +08:00
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for both capture and output to start the codec.
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Video compression codecs use the MPEG controls to setup their codec
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2016-07-10 22:57:43 +08:00
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parameters
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2016-08-16 04:49:50 +08:00
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.. note::
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The MPEG controls actually support many more codecs than
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2016-07-10 22:57:43 +08:00
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just MPEG. See :ref:`mpeg-controls`.
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2016-06-30 21:18:56 +08:00
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2016-10-18 21:15:32 +08:00
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Memory-to-memory devices function as a shared resource: you can
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2016-06-30 21:18:56 +08:00
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open the video node multiple times, each application setting up their
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own codec properties that are local to the file handle, and each can use
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it independently from the others. The driver will arbitrate access to
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the codec and reprogram it whenever another file handler gets access.
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This is different from the usual video node behavior where the video
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properties are global to the device (i.e. changing something through one
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file handle is visible through another file handle).
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