Documentation/media/uapi/v4l/selection-api-006.rst

   1 .. -*- coding: utf-8; mode: rst -*-
   2
   3 ********
   4 Examples
   5 ********
   6
   7 (A video capture device is assumed; change
   8 ``V4L2_BUF_TYPE_VIDEO_CAPTURE`` for other devices; change target to
   9 ``V4L2_SEL_TGT_COMPOSE_*`` family to configure composing area)
  10
  11 Example: Resetting the cropping parameters
  12 ==========================================
  13
  14 .. code-block:: c
  15
  16         struct v4l2_selection sel = {
  17             .type = V4L2_BUF_TYPE_VIDEO_CAPTURE,
  18             .target = V4L2_SEL_TGT_CROP_DEFAULT,
  19         };
  20         ret = ioctl(fd, VIDIOC_G_SELECTION, &sel);
  21         if (ret)
  22             exit(-1);
  23         sel.target = V4L2_SEL_TGT_CROP;
  24         ret = ioctl(fd, VIDIOC_S_SELECTION, &sel);
  25         if (ret)
  26             exit(-1);
  27
  28 Setting a composing area on output of size of *at most* half of limit
  29 placed at a center of a display.
  30
  31 Example: Simple downscaling
  32 ===========================
  33
  34 .. code-block:: c
  35
  36         struct v4l2_selection sel = {
  37             .type = V4L2_BUF_TYPE_VIDEO_OUTPUT,
  38             .target = V4L2_SEL_TGT_COMPOSE_BOUNDS,
  39         };
  40         struct v4l2_rect r;
  41
  42         ret = ioctl(fd, VIDIOC_G_SELECTION, &sel);
  43         if (ret)
  44             exit(-1);
  45         /* setting smaller compose rectangle */
  46         r.width = sel.r.width / 2;
  47         r.height = sel.r.height / 2;
  48         r.left = sel.r.width / 4;
  49         r.top = sel.r.height / 4;
  50         sel.r = r;
  51         sel.target = V4L2_SEL_TGT_COMPOSE;
  52         sel.flags = V4L2_SEL_FLAG_LE;
  53         ret = ioctl(fd, VIDIOC_S_SELECTION, &sel);
  54         if (ret)
  55             exit(-1);
  56
  57 A video output device is assumed; change ``V4L2_BUF_TYPE_VIDEO_OUTPUT``
  58 for other devices
  59
  60 Example: Querying for scaling factors
  61 =====================================
  62
  63 .. code-block:: c
  64
  65         struct v4l2_selection compose = {
  66             .type = V4L2_BUF_TYPE_VIDEO_OUTPUT,
  67             .target = V4L2_SEL_TGT_COMPOSE,
  68         };
  69         struct v4l2_selection crop = {
  70             .type = V4L2_BUF_TYPE_VIDEO_OUTPUT,
  71             .target = V4L2_SEL_TGT_CROP,
  72         };
  73         double hscale, vscale;
  74
  75         ret = ioctl(fd, VIDIOC_G_SELECTION, &compose);
  76         if (ret)
  77             exit(-1);
  78         ret = ioctl(fd, VIDIOC_G_SELECTION, &crop);
  79         if (ret)
  80             exit(-1);
  81
  82         /* computing scaling factors */
  83         hscale = (double)compose.r.width / crop.r.width;
  84         vscale = (double)compose.r.height / crop.r.height;