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diff --git a/Documentation/userspace-api/media/v4l/dev-osd.rst b/Documentation/userspace-api/media/v4l/dev-osd.rst new file mode 100644 index 0000000000..8e4be9129e --- /dev/null +++ b/Documentation/userspace-api/media/v4l/dev-osd.rst @@ -0,0 +1,150 @@ +.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later + +.. _osd: + +****************************** +Video Output Overlay Interface +****************************** + +**Also known as On-Screen Display (OSD)** + +Some video output devices can overlay a framebuffer image onto the +outgoing video signal. Applications can set up such an overlay using +this interface, which borrows structures and ioctls of the +:ref:`Video Overlay <overlay>` interface. + +The OSD function is accessible through the same character special file +as the :ref:`Video Output <capture>` function. + +.. note:: + + The default function of such a ``/dev/video`` device is video + capturing or output. The OSD function is only available after calling + the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl. + + +Querying Capabilities +===================== + +Devices supporting the *Video Output Overlay* interface set the +``V4L2_CAP_VIDEO_OUTPUT_OVERLAY`` flag in the ``capabilities`` field of +struct :c:type:`v4l2_capability` returned by the +:ref:`VIDIOC_QUERYCAP` ioctl. + + +Framebuffer +=========== + +Contrary to the *Video Overlay* interface the framebuffer is normally +implemented on the TV card and not the graphics card. On Linux it is +accessible as a framebuffer device (``/dev/fbN``). Given a V4L2 device, +applications can find the corresponding framebuffer device by calling +the :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>` ioctl. It returns, amongst +other information, the physical address of the framebuffer in the +``base`` field of struct :c:type:`v4l2_framebuffer`. +The framebuffer device ioctl ``FBIOGET_FSCREENINFO`` returns the same +address in the ``smem_start`` field of struct +:c:type:`fb_fix_screeninfo`. The ``FBIOGET_FSCREENINFO`` +ioctl and struct :c:type:`fb_fix_screeninfo` are defined in +the ``linux/fb.h`` header file. + +The width and height of the framebuffer depends on the current video +standard. A V4L2 driver may reject attempts to change the video standard +(or any other ioctl which would imply a framebuffer size change) with an +``EBUSY`` error code until all applications closed the framebuffer device. + +Example: Finding a framebuffer device for OSD +--------------------------------------------- + +.. code-block:: c + + #include <linux/fb.h> + + struct v4l2_framebuffer fbuf; + unsigned int i; + int fb_fd; + + if (-1 == ioctl(fd, VIDIOC_G_FBUF, &fbuf)) { + perror("VIDIOC_G_FBUF"); + exit(EXIT_FAILURE); + } + + for (i = 0; i < 30; i++) { + char dev_name[16]; + struct fb_fix_screeninfo si; + + snprintf(dev_name, sizeof(dev_name), "/dev/fb%u", i); + + fb_fd = open(dev_name, O_RDWR); + if (-1 == fb_fd) { + switch (errno) { + case ENOENT: /* no such file */ + case ENXIO: /* no driver */ + continue; + + default: + perror("open"); + exit(EXIT_FAILURE); + } + } + + if (0 == ioctl(fb_fd, FBIOGET_FSCREENINFO, &si)) { + if (si.smem_start == (unsigned long)fbuf.base) + break; + } else { + /* Apparently not a framebuffer device. */ + } + + close(fb_fd); + fb_fd = -1; + } + + /* fb_fd is the file descriptor of the framebuffer device + for the video output overlay, or -1 if no device was found. */ + + +Overlay Window and Scaling +========================== + +The overlay is controlled by source and target rectangles. The source +rectangle selects a subsection of the framebuffer image to be overlaid, +the target rectangle an area in the outgoing video signal where the +image will appear. Drivers may or may not support scaling, and arbitrary +sizes and positions of these rectangles. Further drivers may support any +(or none) of the clipping/blending methods defined for the +:ref:`Video Overlay <overlay>` interface. + +A struct :c:type:`v4l2_window` defines the size of the +source rectangle, its position in the framebuffer and the +clipping/blending method to be used for the overlay. To get the current +parameters applications set the ``type`` field of a struct +:c:type:`v4l2_format` to +``V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY`` and call the +:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl. The driver fills the +struct :c:type:`v4l2_window` substructure named ``win``. It is not +possible to retrieve a previously programmed clipping list or bitmap. + +To program the source rectangle applications set the ``type`` field of a +struct :c:type:`v4l2_format` to +``V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY``, initialize the ``win`` +substructure and call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl. +The driver adjusts the parameters against hardware limits and returns +the actual parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does. Like :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`, +the :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl can be used to learn +about driver capabilities without actually changing driver state. Unlike +:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` this also works after the overlay has been enabled. + +A struct :c:type:`v4l2_crop` defines the size and position +of the target rectangle. The scaling factor of the overlay is implied by +the width and height given in struct :c:type:`v4l2_window` +and struct :c:type:`v4l2_crop`. The cropping API applies to +*Video Output* and *Video Output Overlay* devices in the same way as to +*Video Capture* and *Video Overlay* devices, merely reversing the +direction of the data flow. For more information see :ref:`crop`. + + +Enabling Overlay +================ + +There is no V4L2 ioctl to enable or disable the overlay, however the +framebuffer interface of the driver may support the ``FBIOBLANK`` ioctl. |