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1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later2 3.. _crop:4 5*****************************************************6Image Cropping, Insertion and Scaling -- the CROP API7*****************************************************8 9.. note::10 11 The CROP API is mostly superseded by the newer :ref:`SELECTION API12 <selection-api>`. The new API should be preferred in most cases,13 with the exception of pixel aspect ratio detection, which is14 implemented by :ref:`VIDIOC_CROPCAP <VIDIOC_CROPCAP>` and has no15 equivalent in the SELECTION API. See :ref:`selection-vs-crop` for a16 comparison of the two APIs.17 18Some video capture devices can sample a subsection of the picture and19shrink or enlarge it to an image of arbitrary size. We call these20abilities cropping and scaling. Some video output devices can scale an21image up or down and insert it at an arbitrary scan line and horizontal22offset into a video signal.23 24Applications can use the following API to select an area in the video25signal, query the default area and the hardware limits.26 27.. note::28 29 Despite their name, the :ref:`VIDIOC_CROPCAP <VIDIOC_CROPCAP>`,30 :ref:`VIDIOC_G_CROP <VIDIOC_G_CROP>` and :ref:`VIDIOC_S_CROP31 <VIDIOC_G_CROP>` ioctls apply to input as well as output devices.32 33Scaling requires a source and a target. On a video capture or overlay34device the source is the video signal, and the cropping ioctls determine35the area actually sampled. The target are images read by the application36or overlaid onto the graphics screen. Their size (and position for an37overlay) is negotiated with the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`38and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctls.39 40On a video output device the source are the images passed in by the41application, and their size is again negotiated with the42:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`43ioctls, or may be encoded in a compressed video stream. The target is44the video signal, and the cropping ioctls determine the area where the45images are inserted.46 47Source and target rectangles are defined even if the device does not48support scaling or the :ref:`VIDIOC_G_CROP <VIDIOC_G_CROP>` and49:ref:`VIDIOC_S_CROP <VIDIOC_G_CROP>` ioctls. Their size (and position50where applicable) will be fixed in this case.51 52.. note::53 54 All capture and output devices that support the CROP or SELECTION55 API will also support the :ref:`VIDIOC_CROPCAP <VIDIOC_CROPCAP>`56 ioctl.57 58Cropping Structures59===================60 61 62.. _crop-scale:63 64.. kernel-figure:: crop.svg65 :alt: crop.svg66 :align: center67 68 Image Cropping, Insertion and Scaling69 70 The cropping, insertion and scaling process71 72 73 74For capture devices the coordinates of the top left corner, width and75height of the area which can be sampled is given by the ``bounds``76substructure of the struct :c:type:`v4l2_cropcap` returned77by the :ref:`VIDIOC_CROPCAP <VIDIOC_CROPCAP>` ioctl. To support a wide78range of hardware this specification does not define an origin or units.79However by convention drivers should horizontally count unscaled samples80relative to 0H (the leading edge of the horizontal sync pulse, see81:ref:`vbi-hsync`). Vertically ITU-R line numbers of the first field82(see ITU R-525 line numbering for :ref:`525 lines <vbi-525>` and for83:ref:`625 lines <vbi-625>`), multiplied by two if the driver84can capture both fields.85 86The top left corner, width and height of the source rectangle, that is87the area actually sampled, is given by struct88:c:type:`v4l2_crop` using the same coordinate system as89struct :c:type:`v4l2_cropcap`. Applications can use the90:ref:`VIDIOC_G_CROP <VIDIOC_G_CROP>` and :ref:`VIDIOC_S_CROP <VIDIOC_G_CROP>`91ioctls to get and set this rectangle. It must lie completely within the92capture boundaries and the driver may further adjust the requested size93and/or position according to hardware limitations.94 95Each capture device has a default source rectangle, given by the96``defrect`` substructure of struct97:c:type:`v4l2_cropcap`. The center of this rectangle98shall align with the center of the active picture area of the video99signal, and cover what the driver writer considers the complete picture.100Drivers shall reset the source rectangle to the default when the driver101is first loaded, but not later.102 103For output devices these structures and ioctls are used accordingly,104defining the *target* rectangle where the images will be inserted into105the video signal.106 107 108Scaling Adjustments109===================110 111Video hardware can have various cropping, insertion and scaling112limitations. It may only scale up or down, support only discrete scaling113factors, or have different scaling abilities in horizontal and vertical114direction. Also it may not support scaling at all. At the same time the115struct :c:type:`v4l2_crop` rectangle may have to be aligned,116and both the source and target rectangles may have arbitrary upper and117lower size limits. In particular the maximum ``width`` and ``height`` in118struct :c:type:`v4l2_crop` may be smaller than the struct119:c:type:`v4l2_cropcap`. ``bounds`` area. Therefore, as120usual, drivers are expected to adjust the requested parameters and121return the actual values selected.122 123Applications can change the source or the target rectangle first, as124they may prefer a particular image size or a certain area in the video125signal. If the driver has to adjust both to satisfy hardware126limitations, the last requested rectangle shall take priority, and the127driver should preferably adjust the opposite one. The128:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl however shall not change129the driver state and therefore only adjust the requested rectangle.130 131Suppose scaling on a video capture device is restricted to a factor 1:1132or 2:1 in either direction and the target image size must be a multiple133of 16 × 16 pixels. The source cropping rectangle is set to defaults,134which are also the upper limit in this example, of 640 × 400 pixels at135offset 0, 0. An application requests an image size of 300 × 225 pixels,136assuming video will be scaled down from the "full picture" accordingly.137The driver sets the image size to the closest possible values 304 × 224,138then chooses the cropping rectangle closest to the requested size, that139is 608 × 224 (224 × 2:1 would exceed the limit 400). The offset 0, 0 is140still valid, thus unmodified. Given the default cropping rectangle141reported by :ref:`VIDIOC_CROPCAP <VIDIOC_CROPCAP>` the application can142easily propose another offset to center the cropping rectangle.143 144Now the application may insist on covering an area using a picture145aspect ratio closer to the original request, so it asks for a cropping146rectangle of 608 × 456 pixels. The present scaling factors limit147cropping to 640 × 384, so the driver returns the cropping size 608 × 384148and adjusts the image size to closest possible 304 × 192.149 150 151Examples152========153 154Source and target rectangles shall remain unchanged across closing and155reopening a device, such that piping data into or out of a device will156work without special preparations. More advanced applications should157ensure the parameters are suitable before starting I/O.158 159.. note::160 161 On the next two examples, a video capture device is assumed;162 change ``V4L2_BUF_TYPE_VIDEO_CAPTURE`` for other types of device.163 164Example: Resetting the cropping parameters165==========================================166 167.. code-block:: c168 169 struct v4l2_cropcap cropcap;170 struct v4l2_crop crop;171 172 memset (&cropcap, 0, sizeof (cropcap));173 cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;174 175 if (-1 == ioctl (fd, VIDIOC_CROPCAP, &cropcap)) {176 perror ("VIDIOC_CROPCAP");177 exit (EXIT_FAILURE);178 }179 180 memset (&crop, 0, sizeof (crop));181 crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;182 crop.c = cropcap.defrect;183 184 /* Ignore if cropping is not supported (EINVAL). */185 186 if (-1 == ioctl (fd, VIDIOC_S_CROP, &crop)187 && errno != EINVAL) {188 perror ("VIDIOC_S_CROP");189 exit (EXIT_FAILURE);190 }191 192 193Example: Simple downscaling194===========================195 196.. code-block:: c197 198 struct v4l2_cropcap cropcap;199 struct v4l2_format format;200 201 reset_cropping_parameters ();202 203 /* Scale down to 1/4 size of full picture. */204 205 memset (&format, 0, sizeof (format)); /* defaults */206 207 format.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;208 209 format.fmt.pix.width = cropcap.defrect.width >> 1;210 format.fmt.pix.height = cropcap.defrect.height >> 1;211 format.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;212 213 if (-1 == ioctl (fd, VIDIOC_S_FMT, &format)) {214 perror ("VIDIOC_S_FORMAT");215 exit (EXIT_FAILURE);216 }217 218 /* We could check the actual image size now, the actual scaling factor219 or if the driver can scale at all. */220 221Example: Selecting an output area222=================================223 224.. note:: This example assumes an output device.225 226.. code-block:: c227 228 struct v4l2_cropcap cropcap;229 struct v4l2_crop crop;230 231 memset (&cropcap, 0, sizeof (cropcap));232 cropcap.type = V4L2_BUF_TYPE_VIDEO_OUTPUT;233 234 if (-1 == ioctl (fd, VIDIOC_CROPCAP;, &cropcap)) {235 perror ("VIDIOC_CROPCAP");236 exit (EXIT_FAILURE);237 }238 239 memset (&crop, 0, sizeof (crop));240 241 crop.type = V4L2_BUF_TYPE_VIDEO_OUTPUT;242 crop.c = cropcap.defrect;243 244 /* Scale the width and height to 50 % of their original size245 and center the output. */246 247 crop.c.width /= 2;248 crop.c.height /= 2;249 crop.c.left += crop.c.width / 2;250 crop.c.top += crop.c.height / 2;251 252 /* Ignore if cropping is not supported (EINVAL). */253 254 if (-1 == ioctl (fd, VIDIOC_S_CROP, &crop)255 && errno != EINVAL) {256 perror ("VIDIOC_S_CROP");257 exit (EXIT_FAILURE);258 }259 260Example: Current scaling factor and pixel aspect261================================================262 263.. note:: This example assumes a video capture device.264 265.. code-block:: c266 267 struct v4l2_cropcap cropcap;268 struct v4l2_crop crop;269 struct v4l2_format format;270 double hscale, vscale;271 double aspect;272 int dwidth, dheight;273 274 memset (&cropcap, 0, sizeof (cropcap));275 cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;276 277 if (-1 == ioctl (fd, VIDIOC_CROPCAP, &cropcap)) {278 perror ("VIDIOC_CROPCAP");279 exit (EXIT_FAILURE);280 }281 282 memset (&crop, 0, sizeof (crop));283 crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;284 285 if (-1 == ioctl (fd, VIDIOC_G_CROP, &crop)) {286 if (errno != EINVAL) {287 perror ("VIDIOC_G_CROP");288 exit (EXIT_FAILURE);289 }290 291 /* Cropping not supported. */292 crop.c = cropcap.defrect;293 }294 295 memset (&format, 0, sizeof (format));296 format.fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;297 298 if (-1 == ioctl (fd, VIDIOC_G_FMT, &format)) {299 perror ("VIDIOC_G_FMT");300 exit (EXIT_FAILURE);301 }302 303 /* The scaling applied by the driver. */304 305 hscale = format.fmt.pix.width / (double) crop.c.width;306 vscale = format.fmt.pix.height / (double) crop.c.height;307 308 aspect = cropcap.pixelaspect.numerator /309 (double) cropcap.pixelaspect.denominator;310 aspect = aspect * hscale / vscale;311 312 /* Devices following ITU-R BT.601 do not capture313 square pixels. For playback on a computer monitor314 we should scale the images to this size. */315 316 dwidth = format.fmt.pix.width / aspect;317 dheight = format.fmt.pix.height;318