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+.. -*- 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).