2024-09-09 08:52:07 +00:00
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<title>Codec Interface</title>
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2024-09-09 08:57:42 +00:00
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<para>A V4L2 codec can compress, decompress, transform, or otherwise
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convert 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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<constant>V4L2_CAP_VIDEO_M2M</constant> or <constant>V4L2_CAP_VIDEO_M2M_MPLANE</constant>
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capability set).
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</para>
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2024-09-09 08:52:07 +00:00
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2024-09-09 08:57:42 +00:00
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<para>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) and
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capture (receiving the processed frames from the codec hardware into memory)
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stream I/O. An application will have to setup the stream
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I/O for both sides and finally call &VIDIOC-STREAMON; for both capture and output
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to start the codec.</para>
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2024-09-09 08:52:07 +00:00
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2024-09-09 08:57:42 +00:00
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<para>Video compression codecs use the MPEG controls to setup their codec parameters
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(note that the MPEG controls actually support many more codecs than just MPEG).
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See <xref linkend="mpeg-controls"></xref>.</para>
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2024-09-09 08:52:07 +00:00
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2024-09-09 08:57:42 +00:00
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<para>Memory-to-memory devices can often be used as a shared resource: you can
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open the video node multiple times, each application setting up their own codec properties
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that are local to the file handle, and each can use it independently from the others.
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The driver will arbitrate access to the codec and reprogram it whenever another file
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handler gets access. This is different from the usual video node behavior where the video properties
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are global to the device (i.e. changing something through one file handle is visible
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through another file handle).</para>
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