Documentation/linux_tv/media/v4l/dev-osd.rst

   1 .. -*- coding: utf-8; mode: rst -*-
   2
   3 .. _osd:
   4
   5 ******************************
   6 Video Output Overlay Interface
   7 ******************************
   8
   9
  10 **Also known as On-Screen Display (OSD)**
  11 Some video output devices can overlay a framebuffer image onto the
  12 outgoing video signal. Applications can set up such an overlay using
  13 this interface, which borrows structures and ioctls of the
  14 :ref:`Video Overlay <overlay>` interface.
  15
  16 The OSD function is accessible through the same character special file
  17 as the :ref:`Video Output <capture>` function. Note the default
  18 function of such a ``/dev/video`` device is video capturing or output.
  19 The OSD function is only available after calling the
  20 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl.
  21
  22
  23 Querying Capabilities
  24 =====================
  25
  26 Devices supporting the *Video Output Overlay* interface set the
  27 ``V4L2_CAP_VIDEO_OUTPUT_OVERLAY`` flag in the ``capabilities`` field of
  28 struct :ref:`v4l2_capability <v4l2-capability>` returned by the
  29 :ref:`VIDIOC_QUERYCAP` ioctl.
  30
  31
  32 Framebuffer
  33 ===========
  34
  35 Contrary to the *Video Overlay* interface the framebuffer is normally
  36 implemented on the TV card and not the graphics card. On Linux it is
  37 accessible as a framebuffer device (``/dev/fbN``). Given a V4L2 device,
  38 applications can find the corresponding framebuffer device by calling
  39 the :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>` ioctl. It returns, amongst
  40 other information, the physical address of the framebuffer in the
  41 ``base`` field of struct :ref:`v4l2_framebuffer <v4l2-framebuffer>`.
  42 The framebuffer device ioctl ``FBIOGET_FSCREENINFO`` returns the same
  43 address in the ``smem_start`` field of struct
  44 :c:type:`struct fb_fix_screeninfo`. The ``FBIOGET_FSCREENINFO``
  45 ioctl and struct :c:type:`struct fb_fix_screeninfo` are defined in
  46 the ``linux/fb.h`` header file.
  47
  48 The width and height of the framebuffer depends on the current video
  49 standard. A V4L2 driver may reject attempts to change the video standard
  50 (or any other ioctl which would imply a framebuffer size change) with an
  51 EBUSY error code until all applications closed the framebuffer device.
  52
  53
  54 .. code-block:: c
  55
  56     #include <linux/fb.h>
  57
  58     struct v4l2_framebuffer fbuf;
  59     unsigned int i;
  60     int fb_fd;
  61
  62     if (-1 == ioctl(fd, VIDIOC_G_FBUF, &fbuf)) {
  63         perror("VIDIOC_G_FBUF");
  64         exit(EXIT_FAILURE);
  65     }
  66
  67     for (i = 0; i < 30; i++) {
  68         char dev_name[16];
  69         struct fb_fix_screeninfo si;
  70
  71         snprintf(dev_name, sizeof(dev_name), "/dev/fb%u", i);
  72
  73         fb_fd = open(dev_name, O_RDWR);
  74         if (-1 == fb_fd) {
  75             switch (errno) {
  76             case ENOENT: /* no such file */
  77             case ENXIO:  /* no driver */
  78                 continue;
  79
  80             default:
  81                 perror("open");
  82                 exit(EXIT_FAILURE);
  83             }
  84         }
  85
  86         if (0 == ioctl(fb_fd, FBIOGET_FSCREENINFO, &si)) {
  87             if (si.smem_start == (unsigned long)fbuf.base)
  88                 break;
  89         } else {
  90             /* Apparently not a framebuffer device. */
  91         }
  92
  93         close(fb_fd);
  94         fb_fd = -1;
  95     }
  96
  97     /* fb_fd is the file descriptor of the framebuffer device
  98        for the video output overlay, or -1 if no device was found. */
  99
 100
 101 Overlay Window and Scaling
 102 ==========================
 103
 104 The overlay is controlled by source and target rectangles. The source
 105 rectangle selects a subsection of the framebuffer image to be overlaid,
 106 the target rectangle an area in the outgoing video signal where the
 107 image will appear. Drivers may or may not support scaling, and arbitrary
 108 sizes and positions of these rectangles. Further drivers may support any
 109 (or none) of the clipping/blending methods defined for the
 110 :ref:`Video Overlay <overlay>` interface.
 111
 112 A struct :ref:`v4l2_window <v4l2-window>` defines the size of the
 113 source rectangle, its position in the framebuffer and the
 114 clipping/blending method to be used for the overlay. To get the current
 115 parameters applications set the ``type`` field of a struct
 116 :ref:`v4l2_format <v4l2-format>` to
 117 ``V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY`` and call the
 118 :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl. The driver fills the
 119 :ref:`struct v4l2_window <v4l2-window>` substructure named ``win``. It is not
 120 possible to retrieve a previously programmed clipping list or bitmap.
 121
 122 To program the source rectangle applications set the ``type`` field of a
 123 struct :ref:`v4l2_format <v4l2-format>` to
 124 ``V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY``, initialize the ``win``
 125 substructure and call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl.
 126 The driver adjusts the parameters against hardware limits and returns
 127 the actual parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does. Like :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`,
 128 the :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl can be used to learn
 129 about driver capabilities without actually changing driver state. Unlike
 130 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` this also works after the overlay has been enabled.
 131
 132 A struct :ref:`v4l2_crop <v4l2-crop>` defines the size and position
 133 of the target rectangle. The scaling factor of the overlay is implied by
 134 the width and height given in struct :ref:`v4l2_window <v4l2-window>`
 135 and struct :ref:`v4l2_crop <v4l2-crop>`. The cropping API applies to
 136 *Video Output* and *Video Output Overlay* devices in the same way as to
 137 *Video Capture* and *Video Overlay* devices, merely reversing the
 138 direction of the data flow. For more information see :ref:`crop`.
 139
 140
 141 Enabling Overlay
 142 ================
 143
 144 There is no V4L2 ioctl to enable or disable the overlay, however the
 145 framebuffer interface of the driver may support the ``FBIOBLANK`` ioctl.