xref: /kernel/linux/linux-5.10/Documentation/userspace-api/media/v4l/selection-api-examples.rst

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

********
Examples
********

(A video capture device is assumed; change
``V4L2_BUF_TYPE_VIDEO_CAPTURE`` for other devices; change target to
``V4L2_SEL_TGT_COMPOSE_*`` family to configure composing area)

Example: Resetting the cropping parameters
==========================================

.. code-block:: c

	struct v4l2_selection sel = {
	    .type = V4L2_BUF_TYPE_VIDEO_CAPTURE,
	    .target = V4L2_SEL_TGT_CROP_DEFAULT,
	};
	ret = ioctl(fd, VIDIOC_G_SELECTION, &sel);
	if (ret)
	    exit(-1);
	sel.target = V4L2_SEL_TGT_CROP;
	ret = ioctl(fd, VIDIOC_S_SELECTION, &sel);
	if (ret)
	    exit(-1);

Setting a composing area on output of size of *at most* half of limit
placed at a center of a display.

Example: Simple downscaling
===========================

.. code-block:: c

	struct v4l2_selection sel = {
	    .type = V4L2_BUF_TYPE_VIDEO_OUTPUT,
	    .target = V4L2_SEL_TGT_COMPOSE_BOUNDS,
	};
	struct v4l2_rect r;

	ret = ioctl(fd, VIDIOC_G_SELECTION, &sel);
	if (ret)
	    exit(-1);
	/* setting smaller compose rectangle */
	r.width = sel.r.width / 2;
	r.height = sel.r.height / 2;
	r.left = sel.r.width / 4;
	r.top = sel.r.height / 4;
	sel.r = r;
	sel.target = V4L2_SEL_TGT_COMPOSE;
	sel.flags = V4L2_SEL_FLAG_LE;
	ret = ioctl(fd, VIDIOC_S_SELECTION, &sel);
	if (ret)
	    exit(-1);

A video output device is assumed; change ``V4L2_BUF_TYPE_VIDEO_OUTPUT``
for other devices

Example: Querying for scaling factors
=====================================

.. code-block:: c

	struct v4l2_selection compose = {
	    .type = V4L2_BUF_TYPE_VIDEO_OUTPUT,
	    .target = V4L2_SEL_TGT_COMPOSE,
	};
	struct v4l2_selection crop = {
	    .type = V4L2_BUF_TYPE_VIDEO_OUTPUT,
	    .target = V4L2_SEL_TGT_CROP,
	};
	double hscale, vscale;

	ret = ioctl(fd, VIDIOC_G_SELECTION, &compose);
	if (ret)
	    exit(-1);
	ret = ioctl(fd, VIDIOC_G_SELECTION, &crop);
	if (ret)
	    exit(-1);

	/* computing scaling factors */
	hscale = (double)compose.r.width / crop.r.width;
	vscale = (double)compose.r.height / crop.r.height;