41 lines
1.7 KiB
ReStructuredText
41 lines
1.7 KiB
ReStructuredText
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.. Permission is granted to copy, distribute and/or modify this
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.. document under the terms of the GNU Free Documentation License,
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.. Version 1.1 or any later version published by the Free Software
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.. Foundation, with no Invariant Sections, no Front-Cover Texts
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.. and no Back-Cover Texts. A copy of the license is included at
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.. Documentation/media/uapi/fdl-appendix.rst.
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..
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.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
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.. _streaming-par:
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********************
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Streaming Parameters
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********************
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Streaming parameters are intended to optimize the video capture process
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as well as I/O. Presently applications can request a high quality
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capture mode with the :ref:`VIDIOC_S_PARM <VIDIOC_G_PARM>` ioctl.
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The current video standard determines a nominal number of frames per
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second. If less than this number of frames is to be captured or output,
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applications can request frame skipping or duplicating on the driver
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side. This is especially useful when using the
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:ref:`read() <func-read>` or :ref:`write() <func-write>`, which are
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not augmented by timestamps or sequence counters, and to avoid
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unnecessary data copying.
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Finally these ioctls can be used to determine the number of buffers used
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internally by a driver in read/write mode. For implications see the
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section discussing the :ref:`read() <func-read>` function.
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To get and set the streaming parameters applications call the
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:ref:`VIDIOC_G_PARM <VIDIOC_G_PARM>` and
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:ref:`VIDIOC_S_PARM <VIDIOC_G_PARM>` ioctl, respectively. They take
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a pointer to a struct :c:type:`v4l2_streamparm`, which
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contains a union holding separate parameters for input and output
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devices.
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These ioctls are optional, drivers need not implement them. If so, they
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return the ``EINVAL`` error code.
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