doc-rst: linux_tv DocBook to reST migration (docs-next)
This is the restructuredText (reST) migration of the ``media``
DocBook-XML set from the linux_tv project.
Signed-off-by: Markus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
diff --git a/Documentation/linux_tv/media/v4l/crop.rst b/Documentation/linux_tv/media/v4l/crop.rst
new file mode 100644
index 0000000..b9014d0
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+++ b/Documentation/linux_tv/media/v4l/crop.rst
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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. *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
+``VIDIOC_G/S_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 ``VIDIOC_G/S_CROP`` ioctls. Their size (and
+position where applicable) will be fixed in this case. *All capture and
+output device must support the ``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 ``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
+(:ref:`vbi-525`, :ref:`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
+``VIDIOC_G_CROP`` and ``VIDIOC_S_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 ``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.
+
+(A video capture device is assumed; change
+``V4L2_BUF_TYPE_VIDEO_CAPTURE`` for other devices.)
+
+
+.. 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);
+ }
+
+(A video capture device is assumed.)
+
+
+.. 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. */
+
+
+.. 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);
+ }
+
+(A video capture device is assumed.)
+
+
+.. 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;
+
+
+
+
+.. ------------------------------------------------------------------------------
+.. This file was automatically converted from DocBook-XML with the dbxml
+.. library (https://github.com/return42/sphkerneldoc). The origin XML comes
+.. from the linux kernel, refer to:
+..
+.. * https://github.com/torvalds/linux/tree/master/Documentation/DocBook
+.. ------------------------------------------------------------------------------