310 lines
10 KiB
ReStructuredText
310 lines
10 KiB
ReStructuredText
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.. -*- coding: utf-8; mode: rst -*-
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.. _crop:
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*************************************
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Image Cropping, Insertion and Scaling
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*************************************
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Some video capture devices can sample a subsection of the picture and
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shrink or enlarge it to an image of arbitrary size. We call these
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abilities cropping and scaling. Some video output devices can scale an
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image up or down and insert it at an arbitrary scan line and horizontal
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offset into a video signal.
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Applications can use the following API to select an area in the video
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signal, query the default area and the hardware limits.
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.. note::
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Despite their name, the :ref:`VIDIOC_CROPCAP <VIDIOC_CROPCAP>`,
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:ref:`VIDIOC_G_CROP <VIDIOC_G_CROP>` and :ref:`VIDIOC_S_CROP
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<VIDIOC_G_CROP>` ioctls apply to input as well as output devices.
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Scaling requires a source and a target. On a video capture or overlay
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device the source is the video signal, and the cropping ioctls determine
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the area actually sampled. The target are images read by the application
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or overlaid onto the graphics screen. Their size (and position for an
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overlay) is negotiated with the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`
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and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctls.
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On a video output device the source are the images passed in by the
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application, and their size is again negotiated with the
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:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
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ioctls, or may be encoded in a compressed video stream. The target is
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the video signal, and the cropping ioctls determine the area where the
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images are inserted.
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Source and target rectangles are defined even if the device does not
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support scaling or the :ref:`VIDIOC_G_CROP <VIDIOC_G_CROP>` and
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:ref:`VIDIOC_S_CROP <VIDIOC_G_CROP>` ioctls. Their size (and position
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where applicable) will be fixed in this case.
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.. note::
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All capture and output devices must support the
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:ref:`VIDIOC_CROPCAP <VIDIOC_CROPCAP>` ioctl such that applications
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can determine if scaling takes place.
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Cropping Structures
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===================
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.. _crop-scale:
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.. kernel-figure:: crop.svg
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:alt: crop.svg
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:align: center
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Image Cropping, Insertion and Scaling
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The cropping, insertion and scaling process
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For capture devices the coordinates of the top left corner, width and
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height of the area which can be sampled is given by the ``bounds``
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substructure of the struct :c:type:`v4l2_cropcap` returned
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by the :ref:`VIDIOC_CROPCAP <VIDIOC_CROPCAP>` ioctl. To support a wide
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range of hardware this specification does not define an origin or units.
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However by convention drivers should horizontally count unscaled samples
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relative to 0H (the leading edge of the horizontal sync pulse, see
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:ref:`vbi-hsync`). Vertically ITU-R line numbers of the first field
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(see ITU R-525 line numbering for :ref:`525 lines <vbi-525>` and for
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:ref:`625 lines <vbi-625>`), multiplied by two if the driver
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can capture both fields.
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The top left corner, width and height of the source rectangle, that is
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the area actually sampled, is given by struct
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:c:type:`v4l2_crop` using the same coordinate system as
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struct :c:type:`v4l2_cropcap`. Applications can use the
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:ref:`VIDIOC_G_CROP <VIDIOC_G_CROP>` and :ref:`VIDIOC_S_CROP <VIDIOC_G_CROP>`
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ioctls to get and set this rectangle. It must lie completely within the
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capture boundaries and the driver may further adjust the requested size
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and/or position according to hardware limitations.
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Each capture device has a default source rectangle, given by the
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``defrect`` substructure of struct
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:c:type:`v4l2_cropcap`. The center of this rectangle
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shall align with the center of the active picture area of the video
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signal, and cover what the driver writer considers the complete picture.
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Drivers shall reset the source rectangle to the default when the driver
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is first loaded, but not later.
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For output devices these structures and ioctls are used accordingly,
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defining the *target* rectangle where the images will be inserted into
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the video signal.
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Scaling Adjustments
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===================
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Video hardware can have various cropping, insertion and scaling
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limitations. It may only scale up or down, support only discrete scaling
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factors, or have different scaling abilities in horizontal and vertical
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direction. Also it may not support scaling at all. At the same time the
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struct :c:type:`v4l2_crop` rectangle may have to be aligned,
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and both the source and target rectangles may have arbitrary upper and
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lower size limits. In particular the maximum ``width`` and ``height`` in
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struct :c:type:`v4l2_crop` may be smaller than the struct
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:c:type:`v4l2_cropcap`. ``bounds`` area. Therefore, as
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usual, drivers are expected to adjust the requested parameters and
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return the actual values selected.
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Applications can change the source or the target rectangle first, as
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they may prefer a particular image size or a certain area in the video
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signal. If the driver has to adjust both to satisfy hardware
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limitations, the last requested rectangle shall take priority, and the
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driver should preferably adjust the opposite one. The
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:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl however shall not change
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the driver state and therefore only adjust the requested rectangle.
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Suppose scaling on a video capture device is restricted to a factor 1:1
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or 2:1 in either direction and the target image size must be a multiple
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of 16 × 16 pixels. The source cropping rectangle is set to defaults,
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which are also the upper limit in this example, of 640 × 400 pixels at
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offset 0, 0. An application requests an image size of 300 × 225 pixels,
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assuming video will be scaled down from the "full picture" accordingly.
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The driver sets the image size to the closest possible values 304 × 224,
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then chooses the cropping rectangle closest to the requested size, that
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is 608 × 224 (224 × 2:1 would exceed the limit 400). The offset 0, 0 is
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still valid, thus unmodified. Given the default cropping rectangle
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reported by :ref:`VIDIOC_CROPCAP <VIDIOC_CROPCAP>` the application can
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easily propose another offset to center the cropping rectangle.
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Now the application may insist on covering an area using a picture
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aspect ratio closer to the original request, so it asks for a cropping
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rectangle of 608 × 456 pixels. The present scaling factors limit
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cropping to 640 × 384, so the driver returns the cropping size 608 × 384
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and adjusts the image size to closest possible 304 × 192.
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Examples
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========
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Source and target rectangles shall remain unchanged across closing and
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reopening a device, such that piping data into or out of a device will
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work without special preparations. More advanced applications should
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ensure the parameters are suitable before starting I/O.
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.. note::
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On the next two examples, a video capture device is assumed;
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change ``V4L2_BUF_TYPE_VIDEO_CAPTURE`` for other types of device.
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Example: Resetting the cropping parameters
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==========================================
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.. code-block:: c
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struct v4l2_cropcap cropcap;
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struct v4l2_crop crop;
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memset (&cropcap, 0, sizeof (cropcap));
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cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
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if (-1 == ioctl (fd, VIDIOC_CROPCAP, &cropcap)) {
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perror ("VIDIOC_CROPCAP");
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exit (EXIT_FAILURE);
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}
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memset (&crop, 0, sizeof (crop));
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crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
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crop.c = cropcap.defrect;
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/* Ignore if cropping is not supported (EINVAL). */
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if (-1 == ioctl (fd, VIDIOC_S_CROP, &crop)
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&& errno != EINVAL) {
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perror ("VIDIOC_S_CROP");
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exit (EXIT_FAILURE);
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}
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Example: Simple downscaling
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===========================
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.. code-block:: c
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struct v4l2_cropcap cropcap;
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struct v4l2_format format;
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reset_cropping_parameters ();
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/* Scale down to 1/4 size of full picture. */
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memset (&format, 0, sizeof (format)); /* defaults */
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format.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
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format.fmt.pix.width = cropcap.defrect.width >> 1;
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format.fmt.pix.height = cropcap.defrect.height >> 1;
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format.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
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if (-1 == ioctl (fd, VIDIOC_S_FMT, &format)) {
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perror ("VIDIOC_S_FORMAT");
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exit (EXIT_FAILURE);
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}
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/* We could check the actual image size now, the actual scaling factor
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or if the driver can scale at all. */
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Example: Selecting an output area
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=================================
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.. note:: This example assumes an output device.
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.. code-block:: c
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struct v4l2_cropcap cropcap;
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struct v4l2_crop crop;
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memset (&cropcap, 0, sizeof (cropcap));
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cropcap.type = V4L2_BUF_TYPE_VIDEO_OUTPUT;
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if (-1 == ioctl (fd, VIDIOC_CROPCAP;, &cropcap)) {
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perror ("VIDIOC_CROPCAP");
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exit (EXIT_FAILURE);
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}
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memset (&crop, 0, sizeof (crop));
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crop.type = V4L2_BUF_TYPE_VIDEO_OUTPUT;
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crop.c = cropcap.defrect;
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/* Scale the width and height to 50 % of their original size
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and center the output. */
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crop.c.width /= 2;
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crop.c.height /= 2;
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crop.c.left += crop.c.width / 2;
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crop.c.top += crop.c.height / 2;
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/* Ignore if cropping is not supported (EINVAL). */
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if (-1 == ioctl (fd, VIDIOC_S_CROP, &crop)
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&& errno != EINVAL) {
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perror ("VIDIOC_S_CROP");
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exit (EXIT_FAILURE);
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}
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Example: Current scaling factor and pixel aspect
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================================================
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.. note:: This example assumes a video capture device.
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.. code-block:: c
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struct v4l2_cropcap cropcap;
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struct v4l2_crop crop;
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struct v4l2_format format;
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double hscale, vscale;
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double aspect;
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int dwidth, dheight;
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memset (&cropcap, 0, sizeof (cropcap));
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cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
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if (-1 == ioctl (fd, VIDIOC_CROPCAP, &cropcap)) {
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perror ("VIDIOC_CROPCAP");
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exit (EXIT_FAILURE);
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}
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memset (&crop, 0, sizeof (crop));
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crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
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if (-1 == ioctl (fd, VIDIOC_G_CROP, &crop)) {
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if (errno != EINVAL) {
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perror ("VIDIOC_G_CROP");
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exit (EXIT_FAILURE);
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}
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/* Cropping not supported. */
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crop.c = cropcap.defrect;
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}
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memset (&format, 0, sizeof (format));
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format.fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
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if (-1 == ioctl (fd, VIDIOC_G_FMT, &format)) {
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perror ("VIDIOC_G_FMT");
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exit (EXIT_FAILURE);
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}
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/* The scaling applied by the driver. */
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hscale = format.fmt.pix.width / (double) crop.c.width;
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vscale = format.fmt.pix.height / (double) crop.c.height;
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aspect = cropcap.pixelaspect.numerator /
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(double) cropcap.pixelaspect.denominator;
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aspect = aspect * hscale / vscale;
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/* Devices following ITU-R BT.601 do not capture
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square pixels. For playback on a computer monitor
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we should scale the images to this size. */
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dwidth = format.fmt.pix.width / aspect;
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dheight = format.fmt.pix.height;
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