VIDIOC_G_SELECTION·VIDIOC_S_SELECTION ioctl

목적, 호출 형식과 인자

`VIDIOC_G_SELECTION`과 `VIDIOC_S_SELECTION`은 cropping·composing 등 selection rectangle을 조회하거나 설정합니다. 두 명령은 `struct v4l2_selection *argp`를 사용합니다.

`fd`는 `open()`이 반환한 파일 디스크립터이며 구조체는 buffer type, selection target, 조정 flag와 `v4l2_rect`를 담습니다.

.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
.. c:namespace:: V4L

.. _VIDIOC_G_SELECTION:

********************************************
ioctl VIDIOC_G_SELECTION, VIDIOC_S_SELECTION
********************************************

Name
====

VIDIOC_G_SELECTION - VIDIOC_S_SELECTION - Get or set one of the selection rectangles

Synopsis
========

.. c:macro:: VIDIOC_G_SELECTION

``int ioctl(int fd, VIDIOC_G_SELECTION, struct v4l2_selection *argp)``

.. c:macro:: VIDIOC_S_SELECTION

``int ioctl(int fd, VIDIOC_S_SELECTION, struct v4l2_selection *argp)``

Arguments
=========

``fd``
    File descriptor returned by :c:func:`open()`.

``argp``
    Pointer to struct :c:type:`v4l2_selection`.

selection rectangle 조회

crop 또는 compose rectangle을 조회하려면 `type`을 해당 buffer type, `target`을 `V4L2_SEL_TGT_CROP` 또는 `V4L2_SEL_TGT_COMPOSE`로 설정합니다. 추가 target은 공통 selection 표와 Selection API 문서에 정의됩니다.

조회에서는 `flags`와 `reserved`가 무시되지만 반드시 0으로 채워야 합니다. 잘못된 buffer type 또는 target이면 `EINVAL`입니다.

cropping 또는 composing을 지원하지 않으면 active rectangle은 변경할 수 없고 항상 bounds rectangle과 같습니다. 성공하면 현재 좌표가 `r`에 돌아옵니다.

좌표 단위는 드라이버에 따라 다릅니다. 예외적으로 raw format 영상의 rectangle 좌표는 항상 pixel 단위입니다.

Description
===========

The ioctls are used to query and configure selection rectangles.

To query the cropping (composing) rectangle set struct
:c:type:`v4l2_selection` ``type`` field to the
respective buffer type. The next step is setting the
value of struct :c:type:`v4l2_selection` ``target``
field to ``V4L2_SEL_TGT_CROP`` (``V4L2_SEL_TGT_COMPOSE``). Please refer
to table :ref:`v4l2-selections-common` or :ref:`selection-api` for
additional targets. The ``flags`` and ``reserved`` fields of struct
:c:type:`v4l2_selection` are ignored and they must be
filled with zeros. The driver fills the rest of the structure or returns
EINVAL error code if incorrect buffer type or target was used. If
cropping (composing) is not supported then the active rectangle is not
mutable and it is always equal to the bounds rectangle. Finally, the
struct :c:type:`v4l2_rect` ``r`` rectangle is filled with
the current cropping (composing) coordinates. The coordinates are
expressed in driver-dependent units. The only exception are rectangles
for images in raw formats, whose coordinates are always expressed in
pixels.

rectangle 설정, 제약과 조정 순서

설정할 때도 `type`과 `target`을 지정하고 원하는 active area를 `r`에 넣으며 `reserved`를 0으로 만듭니다. 드라이버는 하드웨어와 pipeline 제약에 맞춰 요청 좌표를 조정할 수 있습니다.

조정은 하드웨어 한계뿐 아니라 capture·output window나 TV display가 정하는 bounds 등 pipeline의 다른 요소도 고려합니다.

성공하면 조정된 rectangle이 `r`에 반환됩니다. 적합하지 않으면 crop·compose 또는 영상 매개변수를 수정해 만족할 때까지 협상을 반복할 수 있습니다.

constraint flag를 지키는 rectangle이 하나도 없으면 `ERANGE`입니다. 단순히 요청값이 조정됐다는 뜻이 아니라 모든 제약을 동시에 만족하는 rectangle이 존재하지 않는다는 뜻입니다.

To change the cropping (composing) rectangle set the struct
:c:type:`v4l2_selection` ``type`` field to the
respective buffer type. The next step is setting the
value of struct :c:type:`v4l2_selection` ``target`` to
``V4L2_SEL_TGT_CROP`` (``V4L2_SEL_TGT_COMPOSE``). Please refer to table
:ref:`v4l2-selections-common` or :ref:`selection-api` for additional
targets. The struct :c:type:`v4l2_rect` ``r`` rectangle need
to be set to the desired active area. Field struct
:c:type:`v4l2_selection` ``reserved`` is ignored and
must be filled with zeros. The driver may adjust coordinates of the
requested rectangle. An application may introduce constraints to control
rounding behaviour. The struct :c:type:`v4l2_selection`
``flags`` field must be set to one of the following:

-  ``0`` - The driver can adjust the rectangle size freely and shall
   choose a crop/compose rectangle as close as possible to the requested
   one.

-  ``V4L2_SEL_FLAG_GE`` - The driver is not allowed to shrink the
   rectangle. The original rectangle must lay inside the adjusted one.

-  ``V4L2_SEL_FLAG_LE`` - The driver is not allowed to enlarge the
   rectangle. The adjusted rectangle must lay inside the original one.

-  ``V4L2_SEL_FLAG_GE | V4L2_SEL_FLAG_LE`` - The driver must choose the
   size exactly the same as in the requested rectangle.

Please refer to :ref:`sel-const-adjust`.

The driver may have to adjusts the requested dimensions against hardware
limits and other parts as the pipeline, i.e. the bounds given by the
capture/output window or TV display. The closest possible values of
horizontal and vertical offset and sizes are chosen according to
following priority:

1. Satisfy constraints from struct
   :c:type:`v4l2_selection` ``flags``.

2. Adjust width, height, left, and top to hardware limits and
   alignments.

3. Keep center of adjusted rectangle as close as possible to the
   original one.

4. Keep width and height as close as possible to original ones.

5. Keep horizontal and vertical offset as close as possible to original
   ones.

On success the struct :c:type:`v4l2_rect` ``r`` field
contains the adjusted rectangle. When the parameters are unsuitable the
application may modify the cropping (composing) or image parameters and
repeat the cycle until satisfactory parameters have been negotiated. If
constraints flags have to be violated at then ``ERANGE`` is returned. The
error indicates that *there exist no rectangle* that satisfies the
constraints.

Selection targets and flags are documented in
:ref:`v4l2-selections-common`.

제약 flag에 따른 크기 조정 도식

원문의 `constraints.svg` 도식은 `GE`, `LE`, 두 flag의 결합에 따라 요청 rectangle과 조정 rectangle의 포함 관계가 어떻게 달라지는지 보여 줍니다.

.. _sel-const-adjust:

.. kernel-figure::  constraints.svg
    :alt:    constraints.svg
    :align:  center

    Size adjustments with constraint flags.

    Behaviour of rectangle adjustment for different constraint flags.

struct v4l2_selection과 MPLANE 호환

multi-planar buffer type의 `type` 처리에는 역사적 불일치가 있습니다. 일부 드라이버는 `_MPLANE` type만, 다른 드라이버는 접미사가 없는 non-multiplanar type만 받았습니다.

Linux kernel 4.13부터 `V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE`·`V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE`과 대응 non-multiplanar 변형을 모두 허용합니다.

.. c:type:: v4l2_selection

.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm}|

.. flat-table:: struct v4l2_selection
    :header-rows:  0
    :stub-columns: 0
    :widths:       1 1 2

    * - __u32
      - ``type``
      - Type of the buffer (from enum
	:c:type:`v4l2_buf_type`).
    * - __u32
      - ``target``
      - Used to select between
	:ref:`cropping and composing rectangles <v4l2-selections-common>`.
    * - __u32
      - ``flags``
      - Flags controlling the selection rectangle adjustments, refer to
	:ref:`selection flags <v4l2-selection-flags>`.
    * - struct :c:type:`v4l2_rect`
      - ``r``
      - The selection rectangle.
    * - __u32
      - ``reserved[9]``
      - Reserved fields for future use. Drivers and applications must zero
	this array.

.. note::
   Unfortunately in the case of multiplanar buffer types
   (``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE`` and ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``)
   this API was messed up with regards to how the :c:type:`v4l2_selection` ``type`` field
   should be filled in. Some drivers only accepted the ``_MPLANE`` buffer type while
   other drivers only accepted a non-multiplanar buffer type (i.e. without the
   ``_MPLANE`` at the end).

   Starting with kernel 4.13 both variations are allowed.

반환값과 오류

성공하면 0을 반환합니다. 오류가 발생하면 -1을 반환하고 `errno`를 설정하며 Generic Error Codes 장의 공통 오류도 적용됩니다.

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.

EINVAL
    Given buffer type ``type`` or the selection target ``target`` is not
    supported, or the ``flags`` argument is not valid.

ERANGE
    It is not possible to adjust struct :c:type:`v4l2_rect`
    ``r`` rectangle to satisfy all constraints given in the ``flags``
    argument.

ENODATA
    Selection is not supported for this input or output.

EBUSY
    It is not possible to apply change of the selection rectangle at the
    moment. Usually because streaming is in progress.