1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2
3.. _osd:
4
5******************************
6Video Output Overlay Interface
7******************************
8
9**Also known as On-Screen Display (OSD)**
10
11Some video output devices can overlay a framebuffer image onto the
12outgoing video signal. Applications can set up such an overlay using
13this interface, which borrows structures and ioctls of the
14:ref:`Video Overlay <overlay>` interface.
15
16The OSD function is accessible through the same character special file
17as the :ref:`Video Output <capture>` function.
18
19.. note::
20
21   The default function of such a ``/dev/video`` device is video
22   capturing or output. The OSD function is only available after calling
23   the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl.
24
25
26Querying Capabilities
27=====================
28
29Devices supporting the *Video Output Overlay* interface set the
30``V4L2_CAP_VIDEO_OUTPUT_OVERLAY`` flag in the ``capabilities`` field of
31struct :c:type:`v4l2_capability` returned by the
32:ref:`VIDIOC_QUERYCAP` ioctl.
33
34
35Framebuffer
36===========
37
38Contrary to the *Video Overlay* interface the framebuffer is normally
39implemented on the TV card and not the graphics card. On Linux it is
40accessible as a framebuffer device (``/dev/fbN``). Given a V4L2 device,
41applications can find the corresponding framebuffer device by calling
42the :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>` ioctl. It returns, amongst
43other information, the physical address of the framebuffer in the
44``base`` field of struct :c:type:`v4l2_framebuffer`.
45The framebuffer device ioctl ``FBIOGET_FSCREENINFO`` returns the same
46address in the ``smem_start`` field of struct
47:c:type:`fb_fix_screeninfo`. The ``FBIOGET_FSCREENINFO``
48ioctl and struct :c:type:`fb_fix_screeninfo` are defined in
49the ``linux/fb.h`` header file.
50
51The width and height of the framebuffer depends on the current video
52standard. A V4L2 driver may reject attempts to change the video standard
53(or any other ioctl which would imply a framebuffer size change) with an
54``EBUSY`` error code until all applications closed the framebuffer device.
55
56Example: Finding a framebuffer device for OSD
57---------------------------------------------
58
59.. code-block:: c
60
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 device
103       for the video output overlay, or -1 if no device was found. */
104
105
106Overlay Window and Scaling
107==========================
108
109The overlay is controlled by source and target rectangles. The source
110rectangle selects a subsection of the framebuffer image to be overlaid,
111the target rectangle an area in the outgoing video signal where the
112image will appear. Drivers may or may not support scaling, and arbitrary
113sizes and positions of these rectangles. Further drivers may support any
114(or none) of the clipping/blending methods defined for the
115:ref:`Video Overlay <overlay>` interface.
116
117A struct :c:type:`v4l2_window` defines the size of the
118source rectangle, its position in the framebuffer and the
119clipping/blending method to be used for the overlay. To get the current
120parameters applications set the ``type`` field of a struct
121:c:type:`v4l2_format` to
122``V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY`` and call the
123:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl. The driver fills the
124struct :c:type:`v4l2_window` substructure named ``win``. It is not
125possible to retrieve a previously programmed clipping list or bitmap.
126
127To program the source rectangle applications set the ``type`` field of a
128struct :c:type:`v4l2_format` to
129``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 returns
132the 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 learn
134about driver capabilities without actually changing driver state. Unlike
135: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 position
138of the target rectangle. The scaling factor of the overlay is implied by
139the width and height given in struct :c:type:`v4l2_window`
140and struct :c:type:`v4l2_crop`. The cropping API applies to
141*Video Output* and *Video Output Overlay* devices in the same way as to
142*Video Capture* and *Video Overlay* devices, merely reversing the
143direction of the data flow. For more information see :ref:`crop`.
144
145
146Enabling Overlay
147================
148
149There is no V4L2 ioctl to enable or disable the overlay, however the
150framebuffer interface of the driver may support the ``FBIOBLANK`` ioctl.
151