194 lines
6.4 KiB
ReStructuredText
194 lines
6.4 KiB
ReStructuredText
.. -*- coding: utf-8; mode: rst -*-
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.. _VIDIOC_G_SELECTION:
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********************************************
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ioctl VIDIOC_G_SELECTION, VIDIOC_S_SELECTION
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********************************************
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Name
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====
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VIDIOC_G_SELECTION - VIDIOC_S_SELECTION - Get or set one of the selection rectangles
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Synopsis
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========
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.. c:function:: int ioctl( int fd, VIDIOC_G_SELECTION, struct v4l2_selection *argp )
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:name: VIDIOC_G_SELECTION
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.. c:function:: int ioctl( int fd, VIDIOC_S_SELECTION, struct v4l2_selection *argp )
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:name: VIDIOC_S_SELECTION
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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_selection`.
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Description
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===========
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The ioctls are used to query and configure selection rectangles.
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To query the cropping (composing) rectangle set struct
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:c:type:`v4l2_selection` ``type`` field to the
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respective buffer type. The next step is setting the
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value of struct :c:type:`v4l2_selection` ``target``
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field to ``V4L2_SEL_TGT_CROP`` (``V4L2_SEL_TGT_COMPOSE``). Please refer
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to table :ref:`v4l2-selections-common` or :ref:`selection-api` for
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additional targets. The ``flags`` and ``reserved`` fields of struct
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:c:type:`v4l2_selection` are ignored and they must be
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filled with zeros. The driver fills the rest of the structure or returns
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EINVAL error code if incorrect buffer type or target was used. If
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cropping (composing) is not supported then the active rectangle is not
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mutable and it is always equal to the bounds rectangle. Finally, the
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struct :c:type:`v4l2_rect` ``r`` rectangle is filled with
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the current cropping (composing) coordinates. The coordinates are
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expressed in driver-dependent units. The only exception are rectangles
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for images in raw formats, whose coordinates are always expressed in
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pixels.
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To change the cropping (composing) rectangle set the struct
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:c:type:`v4l2_selection` ``type`` field to the
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respective buffer type. The next step is setting the
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value of struct :c:type:`v4l2_selection` ``target`` to
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``V4L2_SEL_TGT_CROP`` (``V4L2_SEL_TGT_COMPOSE``). Please refer to table
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:ref:`v4l2-selections-common` or :ref:`selection-api` for additional
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targets. The struct :c:type:`v4l2_rect` ``r`` rectangle need
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to be set to the desired active area. Field struct
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:c:type:`v4l2_selection` ``reserved`` is ignored and
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must be filled with zeros. The driver may adjust coordinates of the
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requested rectangle. An application may introduce constraints to control
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rounding behaviour. The struct :c:type:`v4l2_selection`
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``flags`` field must be set to one of the following:
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- ``0`` - The driver can adjust the rectangle size freely and shall
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choose a crop/compose rectangle as close as possible to the requested
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one.
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- ``V4L2_SEL_FLAG_GE`` - The driver is not allowed to shrink the
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rectangle. The original rectangle must lay inside the adjusted one.
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- ``V4L2_SEL_FLAG_LE`` - The driver is not allowed to enlarge the
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rectangle. The adjusted rectangle must lay inside the original one.
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- ``V4L2_SEL_FLAG_GE | V4L2_SEL_FLAG_LE`` - The driver must choose the
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size exactly the same as in the requested rectangle.
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Please refer to :ref:`sel-const-adjust`.
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The driver may have to adjusts the requested dimensions against hardware
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limits and other parts as the pipeline, i.e. the bounds given by the
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capture/output window or TV display. The closest possible values of
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horizontal and vertical offset and sizes are chosen according to
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following priority:
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1. Satisfy constraints from struct
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:c:type:`v4l2_selection` ``flags``.
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2. Adjust width, height, left, and top to hardware limits and
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alignments.
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3. Keep center of adjusted rectangle as close as possible to the
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original one.
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4. Keep width and height as close as possible to original ones.
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5. Keep horizontal and vertical offset as close as possible to original
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ones.
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On success the struct :c:type:`v4l2_rect` ``r`` field
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contains the adjusted rectangle. When the parameters are unsuitable the
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application may modify the cropping (composing) or image parameters and
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repeat the cycle until satisfactory parameters have been negotiated. If
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constraints flags have to be violated at then ``ERANGE`` is returned. The
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error indicates that *there exist no rectangle* that satisfies the
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constraints.
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Selection targets and flags are documented in
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:ref:`v4l2-selections-common`.
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.. _sel-const-adjust:
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.. kernel-figure:: constraints.svg
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:alt: constraints.svg
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:align: center
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Size adjustments with constraint flags.
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Behaviour of rectangle adjustment for different constraint flags.
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.. c:type:: v4l2_selection
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.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
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.. flat-table:: struct v4l2_selection
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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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* - __u32
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- ``type``
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- Type of the buffer (from enum
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:c:type:`v4l2_buf_type`).
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* - __u32
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- ``target``
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- Used to select between
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:ref:`cropping and composing rectangles <v4l2-selections-common>`.
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* - __u32
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- ``flags``
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- Flags controlling the selection rectangle adjustments, refer to
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:ref:`selection flags <v4l2-selection-flags>`.
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* - struct :c:type:`v4l2_rect`
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- ``r``
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- The selection rectangle.
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* - __u32
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- ``reserved[9]``
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- Reserved fields for future use. Drivers and applications must zero
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this array.
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.. note::
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Unfortunately in the case of multiplanar buffer types
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(``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE`` and ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``)
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this API was messed up with regards to how the :c:type:`v4l2_selection` ``type`` field
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should be filled in. Some drivers only accepted the ``_MPLANE`` buffer type while
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other drivers only accepted a non-multiplanar buffer type (i.e. without the
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``_MPLANE`` at the end).
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Starting with kernel 4.13 both variations are allowed.
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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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EINVAL
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Given buffer type ``type`` or the selection target ``target`` is not
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supported, or the ``flags`` argument is not valid.
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ERANGE
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It is not possible to adjust struct :c:type:`v4l2_rect`
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``r`` rectangle to satisfy all constraints given in the ``flags``
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argument.
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ENODATA
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Selection is not supported for this input or output.
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EBUSY
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It is not possible to apply change of the selection rectangle at the
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moment. Usually because streaming is in progress.
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