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+.. -*- coding: utf-8; mode: rst -*-
+
+.. _crop:
+
+*************************************
+Image Cropping, Insertion and Scaling
+*************************************
+
+Some video capture devices can sample a subsection of the picture and
+shrink or enlarge it to an image of arbitrary size. We call these
+abilities cropping and scaling. Some video output devices can scale an
+image up or down and insert it at an arbitrary scan line and horizontal
+offset into a video signal.
+
+Applications can use the following API to select an area in the video
+signal, query the default area and the hardware limits.
+
+.. note:: Despite their name, the :ref:`VIDIOC_CROPCAP <VIDIOC_CROPCAP>`,
+   :ref:`VIDIOC_G_CROP <VIDIOC_G_CROP>` and :ref:`VIDIOC_S_CROP
+   <VIDIOC_G_CROP>` ioctls apply to input as well as output devices.
+
+Scaling requires a source and a target. On a video capture or overlay
+device the source is the video signal, and the cropping ioctls determine
+the area actually sampled. The target are images read by the application
+or overlaid onto the graphics screen. Their size (and position for an
+overlay) is negotiated with the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`
+and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctls.
+
+On a video output device the source are the images passed in by the
+application, and their size is again negotiated with the
+:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
+ioctls, or may be encoded in a compressed video stream. The target is
+the video signal, and the cropping ioctls determine the area where the
+images are inserted.
+
+Source and target rectangles are defined even if the device does not
+support scaling or the :ref:`VIDIOC_G_CROP <VIDIOC_G_CROP>` and
+:ref:`VIDIOC_S_CROP <VIDIOC_G_CROP>` ioctls. Their size (and position
+where applicable) will be fixed in this case.
+
+.. note:: All capture and output devices must support the
+   :ref:`VIDIOC_CROPCAP <VIDIOC_CROPCAP>` ioctl such that applications
+   can determine if scaling takes place.
+
+
+Cropping Structures
+===================
+
+
+.. _crop-scale:
+
+.. figure::  crop_files/crop.*
+    :alt:    crop.pdf / crop.gif
+    :align:  center
+
+    Image Cropping, Insertion and Scaling
+
+    The cropping, insertion and scaling process
+
+
+
+For capture devices the coordinates of the top left corner, width and
+height of the area which can be sampled is given by the ``bounds``
+substructure of the struct :ref:`v4l2_cropcap <v4l2-cropcap>` returned
+by the :ref:`VIDIOC_CROPCAP <VIDIOC_CROPCAP>` ioctl. To support a wide
+range of hardware this specification does not define an origin or units.
+However by convention drivers should horizontally count unscaled samples
+relative to 0H (the leading edge of the horizontal sync pulse, see
+:ref:`vbi-hsync`). Vertically ITU-R line numbers of the first field
+(see ITU R-525 line numbering for :ref:`525 lines <vbi-525>` and for
+:ref:`625 lines <vbi-625>`), multiplied by two if the driver
+can capture both fields.
+
+The top left corner, width and height of the source rectangle, that is
+the area actually sampled, is given by struct
+:ref:`v4l2_crop <v4l2-crop>` using the same coordinate system as
+struct :ref:`v4l2_cropcap <v4l2-cropcap>`. Applications can use the
+:ref:`VIDIOC_G_CROP <VIDIOC_G_CROP>` and :ref:`VIDIOC_S_CROP <VIDIOC_G_CROP>`
+ioctls to get and set this rectangle. It must lie completely within the
+capture boundaries and the driver may further adjust the requested size
+and/or position according to hardware limitations.
+
+Each capture device has a default source rectangle, given by the
+``defrect`` substructure of struct
+:ref:`v4l2_cropcap <v4l2-cropcap>`. The center of this rectangle
+shall align with the center of the active picture area of the video
+signal, and cover what the driver writer considers the complete picture.
+Drivers shall reset the source rectangle to the default when the driver
+is first loaded, but not later.
+
+For output devices these structures and ioctls are used accordingly,
+defining the *target* rectangle where the images will be inserted into
+the video signal.
+
+
+Scaling Adjustments
+===================
+
+Video hardware can have various cropping, insertion and scaling
+limitations. It may only scale up or down, support only discrete scaling
+factors, or have different scaling abilities in horizontal and vertical
+direction. Also it may not support scaling at all. At the same time the
+struct :ref:`v4l2_crop <v4l2-crop>` rectangle may have to be aligned,
+and both the source and target rectangles may have arbitrary upper and
+lower size limits. In particular the maximum ``width`` and ``height`` in
+struct :ref:`v4l2_crop <v4l2-crop>` may be smaller than the struct
+:ref:`v4l2_cropcap <v4l2-cropcap>`. ``bounds`` area. Therefore, as
+usual, drivers are expected to adjust the requested parameters and
+return the actual values selected.
+
+Applications can change the source or the target rectangle first, as
+they may prefer a particular image size or a certain area in the video
+signal. If the driver has to adjust both to satisfy hardware
+limitations, the last requested rectangle shall take priority, and the
+driver should preferably adjust the opposite one. The
+:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl however shall not change
+the driver state and therefore only adjust the requested rectangle.
+
+Suppose scaling on a video capture device is restricted to a factor 1:1
+or 2:1 in either direction and the target image size must be a multiple
+of 16 × 16 pixels. The source cropping rectangle is set to defaults,
+which are also the upper limit in this example, of 640 × 400 pixels at
+offset 0, 0. An application requests an image size of 300 × 225 pixels,
+assuming video will be scaled down from the "full picture" accordingly.
+The driver sets the image size to the closest possible values 304 × 224,
+then chooses the cropping rectangle closest to the requested size, that
+is 608 × 224 (224 × 2:1 would exceed the limit 400). The offset 0, 0 is
+still valid, thus unmodified. Given the default cropping rectangle
+reported by :ref:`VIDIOC_CROPCAP <VIDIOC_CROPCAP>` the application can
+easily propose another offset to center the cropping rectangle.
+
+Now the application may insist on covering an area using a picture
+aspect ratio closer to the original request, so it asks for a cropping
+rectangle of 608 × 456 pixels. The present scaling factors limit
+cropping to 640 × 384, so the driver returns the cropping size 608 × 384
+and adjusts the image size to closest possible 304 × 192.
+
+
+Examples
+========
+
+Source and target rectangles shall remain unchanged across closing and
+reopening a device, such that piping data into or out of a device will
+work without special preparations. More advanced applications should
+ensure the parameters are suitable before starting I/O.
+
+.. note:: On the next two examples, a video capture device is assumed;
+   change ``V4L2_BUF_TYPE_VIDEO_CAPTURE`` for other types of device.
+
+Example: Resetting the cropping parameters
+==========================================
+
+.. code-block:: c
+
+    struct v4l2_cropcap cropcap;
+    struct v4l2_crop crop;
+
+    memset (&cropcap, 0, sizeof (cropcap));
+    cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+
+    if (-1 == ioctl (fd, VIDIOC_CROPCAP, &cropcap)) {
+	perror ("VIDIOC_CROPCAP");
+	exit (EXIT_FAILURE);
+    }
+
+    memset (&crop, 0, sizeof (crop));
+    crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+    crop.c = cropcap.defrect;
+
+    /* Ignore if cropping is not supported (EINVAL). */
+
+    if (-1 == ioctl (fd, VIDIOC_S_CROP, &crop)
+	&& errno != EINVAL) {
+	perror ("VIDIOC_S_CROP");
+	exit (EXIT_FAILURE);
+    }
+
+
+Example: Simple downscaling
+===========================
+
+.. code-block:: c
+
+    struct v4l2_cropcap cropcap;
+    struct v4l2_format format;
+
+    reset_cropping_parameters ();
+
+    /* Scale down to 1/4 size of full picture. */
+
+    memset (&format, 0, sizeof (format)); /* defaults */
+
+    format.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+
+    format.fmt.pix.width = cropcap.defrect.width >> 1;
+    format.fmt.pix.height = cropcap.defrect.height >> 1;
+    format.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
+
+    if (-1 == ioctl (fd, VIDIOC_S_FMT, &format)) {
+	perror ("VIDIOC_S_FORMAT");
+	exit (EXIT_FAILURE);
+    }
+
+    /* We could check the actual image size now, the actual scaling factor
+       or if the driver can scale at all. */
+
+Example: Selecting an output area
+=================================
+
+.. note:: This example assumes an output device.
+
+.. code-block:: c
+
+    struct v4l2_cropcap cropcap;
+    struct v4l2_crop crop;
+
+    memset (&cropcap, 0, sizeof (cropcap));
+    cropcap.type = V4L2_BUF_TYPE_VIDEO_OUTPUT;
+
+    if (-1 == ioctl (fd, VIDIOC_CROPCAP;, &cropcap)) {
+	perror ("VIDIOC_CROPCAP");
+	exit (EXIT_FAILURE);
+    }
+
+    memset (&crop, 0, sizeof (crop));
+
+    crop.type = V4L2_BUF_TYPE_VIDEO_OUTPUT;
+    crop.c = cropcap.defrect;
+
+    /* Scale the width and height to 50 % of their original size
+       and center the output. */
+
+    crop.c.width /= 2;
+    crop.c.height /= 2;
+    crop.c.left += crop.c.width / 2;
+    crop.c.top += crop.c.height / 2;
+
+    /* Ignore if cropping is not supported (EINVAL). */
+
+    if (-1 == ioctl (fd, VIDIOC_S_CROP, &crop)
+	&& errno != EINVAL) {
+	perror ("VIDIOC_S_CROP");
+	exit (EXIT_FAILURE);
+    }
+
+Example: Current scaling factor and pixel aspect
+================================================
+
+.. note:: This example assumes a video capture device.
+
+.. code-block:: c
+
+    struct v4l2_cropcap cropcap;
+    struct v4l2_crop crop;
+    struct v4l2_format format;
+    double hscale, vscale;
+    double aspect;
+    int dwidth, dheight;
+
+    memset (&cropcap, 0, sizeof (cropcap));
+    cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+
+    if (-1 == ioctl (fd, VIDIOC_CROPCAP, &cropcap)) {
+	perror ("VIDIOC_CROPCAP");
+	exit (EXIT_FAILURE);
+    }
+
+    memset (&crop, 0, sizeof (crop));
+    crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+
+    if (-1 == ioctl (fd, VIDIOC_G_CROP, &crop)) {
+	if (errno != EINVAL) {
+	    perror ("VIDIOC_G_CROP");
+	    exit (EXIT_FAILURE);
+	}
+
+	/* Cropping not supported. */
+	crop.c = cropcap.defrect;
+    }
+
+    memset (&format, 0, sizeof (format));
+    format.fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+
+    if (-1 == ioctl (fd, VIDIOC_G_FMT, &format)) {
+	perror ("VIDIOC_G_FMT");
+	exit (EXIT_FAILURE);
+    }
+
+    /* The scaling applied by the driver. */
+
+    hscale = format.fmt.pix.width / (double) crop.c.width;
+    vscale = format.fmt.pix.height / (double) crop.c.height;
+
+    aspect = cropcap.pixelaspect.numerator /
+	 (double) cropcap.pixelaspect.denominator;
+    aspect = aspect * hscale / vscale;
+
+    /* Devices following ITU-R BT.601 do not capture
+       square pixels. For playback on a computer monitor
+       we should scale the images to this size. */
+
+    dwidth = format.fmt.pix.width / aspect;
+    dheight = format.fmt.pix.height;