151 lines · plain
1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later2 3.. _osd:4 5******************************6Video Output Overlay Interface7******************************8 9**Also known as On-Screen Display (OSD)**10 11Some video output devices can overlay a framebuffer image onto the12outgoing video signal. Applications can set up such an overlay using13this interface, which borrows structures and ioctls of the14:ref:`Video Overlay <overlay>` interface.15 16The OSD function is accessible through the same character special file17as the :ref:`Video Output <capture>` function.18 19.. note::20 21 The default function of such a ``/dev/video`` device is video22 capturing or output. The OSD function is only available after calling23 the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl.24 25 26Querying Capabilities27=====================28 29Devices supporting the *Video Output Overlay* interface set the30``V4L2_CAP_VIDEO_OUTPUT_OVERLAY`` flag in the ``capabilities`` field of31struct :c:type:`v4l2_capability` returned by the32:ref:`VIDIOC_QUERYCAP` ioctl.33 34 35Framebuffer36===========37 38Contrary to the *Video Overlay* interface the framebuffer is normally39implemented on the TV card and not the graphics card. On Linux it is40accessible as a framebuffer device (``/dev/fbN``). Given a V4L2 device,41applications can find the corresponding framebuffer device by calling42the :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>` ioctl. It returns, amongst43other information, the physical address of the framebuffer in the44``base`` field of struct :c:type:`v4l2_framebuffer`.45The framebuffer device ioctl ``FBIOGET_FSCREENINFO`` returns the same46address in the ``smem_start`` field of struct47:c:type:`fb_fix_screeninfo`. The ``FBIOGET_FSCREENINFO``48ioctl and struct :c:type:`fb_fix_screeninfo` are defined in49the ``linux/fb.h`` header file.50 51The width and height of the framebuffer depends on the current video52standard. A V4L2 driver may reject attempts to change the video standard53(or any other ioctl which would imply a framebuffer size change) with an54``EBUSY`` error code until all applications closed the framebuffer device.55 56Example: Finding a framebuffer device for OSD57---------------------------------------------58 59.. code-block:: c60 61 #include <linux/fb.h>62 63 struct v4l2_framebuffer fbuf;64 unsigned int i;65 int fb_fd;66 67 if (-1 == ioctl(fd, VIDIOC_G_FBUF, &fbuf)) {68 perror("VIDIOC_G_FBUF");69 exit(EXIT_FAILURE);70 }71 72 for (i = 0; i < 30; i++) {73 char dev_name[16];74 struct fb_fix_screeninfo si;75 76 snprintf(dev_name, sizeof(dev_name), "/dev/fb%u", i);77 78 fb_fd = open(dev_name, O_RDWR);79 if (-1 == fb_fd) {80 switch (errno) {81 case ENOENT: /* no such file */82 case ENXIO: /* no driver */83 continue;84 85 default:86 perror("open");87 exit(EXIT_FAILURE);88 }89 }90 91 if (0 == ioctl(fb_fd, FBIOGET_FSCREENINFO, &si)) {92 if (si.smem_start == (unsigned long)fbuf.base)93 break;94 } else {95 /* Apparently not a framebuffer device. */96 }97 98 close(fb_fd);99 fb_fd = -1;100 }101 102 /* fb_fd is the file descriptor of the framebuffer device103 for the video output overlay, or -1 if no device was found. */104 105 106Overlay Window and Scaling107==========================108 109The overlay is controlled by source and target rectangles. The source110rectangle selects a subsection of the framebuffer image to be overlaid,111the target rectangle an area in the outgoing video signal where the112image will appear. Drivers may or may not support scaling, and arbitrary113sizes and positions of these rectangles. Further drivers may support any114(or none) of the clipping/blending methods defined for the115:ref:`Video Overlay <overlay>` interface.116 117A struct :c:type:`v4l2_window` defines the size of the118source rectangle, its position in the framebuffer and the119clipping/blending method to be used for the overlay. To get the current120parameters applications set the ``type`` field of a struct121:c:type:`v4l2_format` to122``V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY`` and call the123:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl. The driver fills the124struct :c:type:`v4l2_window` substructure named ``win``. It is not125possible to retrieve a previously programmed clipping list or bitmap.126 127To program the source rectangle applications set the ``type`` field of a128struct :c:type:`v4l2_format` to129``V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY``, initialize the ``win``130substructure and call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl.131The driver adjusts the parameters against hardware limits and returns132the actual parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does. Like :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`,133the :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl can be used to learn134about driver capabilities without actually changing driver state. Unlike135:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` this also works after the overlay has been enabled.136 137A struct :c:type:`v4l2_crop` defines the size and position138of the target rectangle. The scaling factor of the overlay is implied by139the width and height given in struct :c:type:`v4l2_window`140and struct :c:type:`v4l2_crop`. The cropping API applies to141*Video Output* and *Video Output Overlay* devices in the same way as to142*Video Capture* and *Video Overlay* devices, merely reversing the143direction of the data flow. For more information see :ref:`crop`.144 145 146Enabling Overlay147================148 149There is no V4L2 ioctl to enable or disable the overlay, however the150framebuffer interface of the driver may support the ``FBIOBLANK`` ioctl.151