202 lines
5.4 KiB
ReStructuredText
202 lines
5.4 KiB
ReStructuredText
.. -*- coding: utf-8; mode: rst -*-
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.. _VIDIOC_ENUM_FRAMEINTERVALS:
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********************************
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ioctl VIDIOC_ENUM_FRAMEINTERVALS
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********************************
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Name
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====
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VIDIOC_ENUM_FRAMEINTERVALS - Enumerate frame intervals
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Synopsis
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========
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.. c:function:: int ioctl( int fd, VIDIOC_ENUM_FRAMEINTERVALS, struct v4l2_frmivalenum *argp )
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:name: VIDIOC_ENUM_FRAMEINTERVALS
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Arguments
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=========
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``fd``
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File descriptor returned by :ref:`open() <func-open>`.
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``argp``
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Pointer to struct :c:type:`v4l2_frmivalenum`
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that contains a pixel format and size and receives a frame interval.
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Description
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===========
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This ioctl allows applications to enumerate all frame intervals that the
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device supports for the given pixel format and frame size.
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The supported pixel formats and frame sizes can be obtained by using the
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:ref:`VIDIOC_ENUM_FMT` and
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:ref:`VIDIOC_ENUM_FRAMESIZES` functions.
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The return value and the content of the ``v4l2_frmivalenum.type`` field
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depend on the type of frame intervals the device supports. Here are the
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semantics of the function for the different cases:
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- **Discrete:** The function returns success if the given index value
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(zero-based) is valid. The application should increase the index by
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one for each call until ``EINVAL`` is returned. The
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`v4l2_frmivalenum.type` field is set to
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`V4L2_FRMIVAL_TYPE_DISCRETE` by the driver. Of the union only
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the `discrete` member is valid.
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- **Step-wise:** The function returns success if the given index value
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is zero and ``EINVAL`` for any other index value. The
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``v4l2_frmivalenum.type`` field is set to
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``V4L2_FRMIVAL_TYPE_STEPWISE`` by the driver. Of the union only the
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``stepwise`` member is valid.
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- **Continuous:** This is a special case of the step-wise type above.
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The function returns success if the given index value is zero and
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``EINVAL`` for any other index value. The ``v4l2_frmivalenum.type``
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field is set to ``V4L2_FRMIVAL_TYPE_CONTINUOUS`` by the driver. Of
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the union only the ``stepwise`` member is valid and the ``step``
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value is set to 1.
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When the application calls the function with index zero, it must check
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the ``type`` field to determine the type of frame interval enumeration
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the device supports. Only for the ``V4L2_FRMIVAL_TYPE_DISCRETE`` type
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does it make sense to increase the index value to receive more frame
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intervals.
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.. note::
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The order in which the frame intervals are returned has no
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special meaning. In particular does it not say anything about potential
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default frame intervals.
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Applications can assume that the enumeration data does not change
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without any interaction from the application itself. This means that the
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enumeration data is consistent if the application does not perform any
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other ioctl calls while it runs the frame interval enumeration.
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.. note::
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**Frame intervals and frame rates:** The V4L2 API uses frame
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intervals instead of frame rates. Given the frame interval the frame
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rate can be computed as follows:
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::
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frame_rate = 1 / frame_interval
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Structs
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=======
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In the structs below, *IN* denotes a value that has to be filled in by
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the application, *OUT* denotes values that the driver fills in. The
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application should zero out all members except for the *IN* fields.
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.. c:type:: v4l2_frmival_stepwise
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.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
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.. flat-table:: struct v4l2_frmival_stepwise
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:header-rows: 0
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:stub-columns: 0
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:widths: 1 1 2
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* - struct :c:type:`v4l2_fract`
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- ``min``
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- Minimum frame interval [s].
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* - struct :c:type:`v4l2_fract`
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- ``max``
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- Maximum frame interval [s].
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* - struct :c:type:`v4l2_fract`
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- ``step``
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- Frame interval step size [s].
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.. c:type:: v4l2_frmivalenum
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.. tabularcolumns:: |p{1.8cm}|p{4.4cm}|p{2.4cm}|p{8.9cm}|
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.. flat-table:: struct v4l2_frmivalenum
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:header-rows: 0
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:stub-columns: 0
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* - __u32
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- ``index``
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-
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- IN: Index of the given frame interval in the enumeration.
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* - __u32
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- ``pixel_format``
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-
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- IN: Pixel format for which the frame intervals are enumerated.
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* - __u32
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- ``width``
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-
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- IN: Frame width for which the frame intervals are enumerated.
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* - __u32
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- ``height``
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-
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- IN: Frame height for which the frame intervals are enumerated.
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* - __u32
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- ``type``
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-
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- OUT: Frame interval type the device supports.
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* - union
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-
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-
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- OUT: Frame interval with the given index.
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* -
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- struct :c:type:`v4l2_fract`
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- ``discrete``
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- Frame interval [s].
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* -
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- struct :c:type:`v4l2_frmival_stepwise`
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- ``stepwise``
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-
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* - __u32
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- ``reserved[2]``
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-
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- Reserved space for future use. Must be zeroed by drivers and
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applications.
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Enums
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=====
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.. c:type:: v4l2_frmivaltypes
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.. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
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.. flat-table:: enum v4l2_frmivaltypes
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:header-rows: 0
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:stub-columns: 0
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:widths: 3 1 4
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* - ``V4L2_FRMIVAL_TYPE_DISCRETE``
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- 1
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- Discrete frame interval.
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* - ``V4L2_FRMIVAL_TYPE_CONTINUOUS``
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- 2
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- Continuous frame interval.
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* - ``V4L2_FRMIVAL_TYPE_STEPWISE``
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- 3
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- Step-wise defined frame interval.
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Return Value
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============
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On success 0 is returned, on error -1 and the ``errno`` variable is set
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appropriately. The generic error codes are described at the
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:ref:`Generic Error Codes <gen-errors>` chapter.
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