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+.. -*- coding: utf-8; mode: rst -*-
+
+.. _VIDIOC_G_PARM:
+
+**********************************
+ioctl VIDIOC_G_PARM, VIDIOC_S_PARM
+**********************************
+
+Name
+====
+
+VIDIOC_G_PARM - VIDIOC_S_PARM - Get or set streaming parameters
+
+
+Synopsis
+========
+
+.. cpp:function:: int ioctl( int fd, int request, v4l2_streamparm *argp )
+
+
+Arguments
+=========
+
+``fd``
+ File descriptor returned by :ref:`open() <func-open>`.
+
+``request``
+ VIDIOC_G_PARM, VIDIOC_S_PARM
+
+``argp``
+
+
+Description
+===========
+
+The current video standard determines a nominal number of frames per
+second. If less than this number of frames is to be captured or output,
+applications can request frame skipping or duplicating on the driver
+side. This is especially useful when using the :ref:`read() <func-read>` or
+:ref:`write() <func-write>`, which are not augmented by timestamps or sequence
+counters, and to avoid unnecessary data copying.
+
+Further these ioctls can be used to determine the number of buffers used
+internally by a driver in read/write mode. For implications see the
+section discussing the :ref:`read() <func-read>` function.
+
+To get and set the streaming parameters applications call the
+:ref:`VIDIOC_G_PARM <VIDIOC_G_PARM>` and :ref:`VIDIOC_S_PARM <VIDIOC_G_PARM>` ioctl, respectively. They take a
+pointer to a struct :ref:`struct v4l2_streamparm <v4l2-streamparm>` which contains a
+union holding separate parameters for input and output devices.
+
+
+.. _v4l2-streamparm:
+
+.. flat-table:: struct v4l2_streamparm
+ :header-rows: 0
+ :stub-columns: 0
+ :widths: 1 1 1 2
+
+
+ - .. row 1
+
+ - __u32
+
+ - ``type``
+
+ -
+ - The buffer (stream) type, same as struct
+ :ref:`v4l2_format <v4l2-format>` ``type``, set by the
+ application. See :ref:`v4l2-buf-type`
+
+ - .. row 2
+
+ - union
+
+ - ``parm``
+
+ -
+ -
+
+ - .. row 3
+
+ -
+ - struct :ref:`v4l2_captureparm <v4l2-captureparm>`
+
+ - ``capture``
+
+ - Parameters for capture devices, used when ``type`` is
+ ``V4L2_BUF_TYPE_VIDEO_CAPTURE``.
+
+ - .. row 4
+
+ -
+ - struct :ref:`v4l2_outputparm <v4l2-outputparm>`
+
+ - ``output``
+
+ - Parameters for output devices, used when ``type`` is
+ ``V4L2_BUF_TYPE_VIDEO_OUTPUT``.
+
+ - .. row 5
+
+ -
+ - __u8
+
+ - ``raw_data``\ [200]
+
+ - A place holder for future extensions.
+
+
+
+.. _v4l2-captureparm:
+
+.. flat-table:: struct v4l2_captureparm
+ :header-rows: 0
+ :stub-columns: 0
+ :widths: 1 1 2
+
+
+ - .. row 1
+
+ - __u32
+
+ - ``capability``
+
+ - See :ref:`parm-caps`.
+
+ - .. row 2
+
+ - __u32
+
+ - ``capturemode``
+
+ - Set by drivers and applications, see :ref:`parm-flags`.
+
+ - .. row 3
+
+ - struct :ref:`v4l2_fract <v4l2-fract>`
+
+ - ``timeperframe``
+
+ - This is the desired period between successive frames captured by
+ the driver, in seconds. The field is intended to skip frames on
+ the driver side, saving I/O bandwidth.
+
+ Applications store here the desired frame period, drivers return
+ the actual frame period, which must be greater or equal to the
+ nominal frame period determined by the current video standard
+ (struct :ref:`v4l2_standard <v4l2-standard>` ``frameperiod``
+ field). Changing the video standard (also implicitly by switching
+ the video input) may reset this parameter to the nominal frame
+ period. To reset manually applications can just set this field to
+ zero.
+
+ Drivers support this function only when they set the
+ ``V4L2_CAP_TIMEPERFRAME`` flag in the ``capability`` field.
+
+ - .. row 4
+
+ - __u32
+
+ - ``extendedmode``
+
+ - Custom (driver specific) streaming parameters. When unused,
+ applications and drivers must set this field to zero. Applications
+ using this field should check the driver name and version, see
+ :ref:`querycap`.
+
+ - .. row 5
+
+ - __u32
+
+ - ``readbuffers``
+
+ - Applications set this field to the desired number of buffers used
+ internally by the driver in :ref:`read() <func-read>` mode.
+ Drivers return the actual number of buffers. When an application
+ requests zero buffers, drivers should just return the current
+ setting rather than the minimum or an error code. For details see
+ :ref:`rw`.
+
+ - .. row 6
+
+ - __u32
+
+ - ``reserved``\ [4]
+
+ - Reserved for future extensions. Drivers and applications must set
+ the array to zero.
+
+
+
+.. _v4l2-outputparm:
+
+.. flat-table:: struct v4l2_outputparm
+ :header-rows: 0
+ :stub-columns: 0
+ :widths: 1 1 2
+
+
+ - .. row 1
+
+ - __u32
+
+ - ``capability``
+
+ - See :ref:`parm-caps`.
+
+ - .. row 2
+
+ - __u32
+
+ - ``outputmode``
+
+ - Set by drivers and applications, see :ref:`parm-flags`.
+
+ - .. row 3
+
+ - struct :ref:`v4l2_fract <v4l2-fract>`
+
+ - ``timeperframe``
+
+ - This is the desired period between successive frames output by the
+ driver, in seconds.
+
+ - .. row 4
+
+ - :cspan:`2`
+
+ The field is intended to repeat frames on the driver side in
+ :ref:`write() <func-write>` mode (in streaming mode timestamps
+ can be used to throttle the output), saving I/O bandwidth.
+
+ Applications store here the desired frame period, drivers return
+ the actual frame period, which must be greater or equal to the
+ nominal frame period determined by the current video standard
+ (struct :ref:`v4l2_standard <v4l2-standard>` ``frameperiod``
+ field). Changing the video standard (also implicitly by switching
+ the video output) may reset this parameter to the nominal frame
+ period. To reset manually applications can just set this field to
+ zero.
+
+ Drivers support this function only when they set the
+ ``V4L2_CAP_TIMEPERFRAME`` flag in the ``capability`` field.
+
+ - .. row 5
+
+ - __u32
+
+ - ``extendedmode``
+
+ - Custom (driver specific) streaming parameters. When unused,
+ applications and drivers must set this field to zero. Applications
+ using this field should check the driver name and version, see
+ :ref:`querycap`.
+
+ - .. row 6
+
+ - __u32
+
+ - ``writebuffers``
+
+ - Applications set this field to the desired number of buffers used
+ internally by the driver in :ref:`write() <func-write>` mode. Drivers
+ return the actual number of buffers. When an application requests
+ zero buffers, drivers should just return the current setting
+ rather than the minimum or an error code. For details see
+ :ref:`rw`.
+
+ - .. row 7
+
+ - __u32
+
+ - ``reserved``\ [4]
+
+ - Reserved for future extensions. Drivers and applications must set
+ the array to zero.
+
+
+
+.. _parm-caps:
+
+.. flat-table:: Streaming Parameters Capabilites
+ :header-rows: 0
+ :stub-columns: 0
+ :widths: 3 1 4
+
+
+ - .. row 1
+
+ - ``V4L2_CAP_TIMEPERFRAME``
+
+ - 0x1000
+
+ - The frame skipping/repeating controlled by the ``timeperframe``
+ field is supported.
+
+
+
+.. _parm-flags:
+
+.. flat-table:: Capture Parameters Flags
+ :header-rows: 0
+ :stub-columns: 0
+ :widths: 3 1 4
+
+
+ - .. row 1
+
+ - ``V4L2_MODE_HIGHQUALITY``
+
+ - 0x0001
+
+ - High quality imaging mode. High quality mode is intended for still
+ imaging applications. The idea is to get the best possible image
+ quality that the hardware can deliver. It is not defined how the
+ driver writer may achieve that; it will depend on the hardware and
+ the ingenuity of the driver writer. High quality mode is a
+ different mode from the regular motion video capture modes. In
+ high quality mode:
+
+ - The driver may be able to capture higher resolutions than for
+ motion capture.
+
+ - The driver may support fewer pixel formats than motion capture
+ (eg; true color).
+
+ - The driver may capture and arithmetically combine multiple
+ successive fields or frames to remove color edge artifacts and
+ reduce the noise in the video data.
+
+ - The driver may capture images in slices like a scanner in order
+ to handle larger format images than would otherwise be
+ possible.
+
+ - An image capture operation may be significantly slower than
+ motion capture.
+
+ - Moving objects in the image might have excessive motion blur.
+
+ - Capture might only work through the :ref:`read() <func-read>` call.
+
+
+Return Value
+============
+
+On success 0 is returned, on error -1 and the ``errno`` variable is set
+appropriately. The generic error codes are described at the
+:ref:`Generic Error Codes <gen-errors>` chapter.