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- <title>Video Output Overlay Interface</title>
- <subtitle>Also known as On-Screen Display (OSD)</subtitle>
-
- <note>
- <title>Experimental</title>
-
- <para>This is an <link linkend="experimental">experimental</link>
-interface and may change in the future.</para>
- </note>
-
- <para>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 <link
-linkend="overlay">Video Overlay</link> interface.</para>
-
- <para>The OSD function is accessible through the same character
-special file as the <link linkend="capture">Video Output</link> function.
-Note the default function of such a <filename>/dev/video</filename> device
-is video capturing or output. The OSD function is only available after
-calling the &VIDIOC-S-FMT; ioctl.</para>
-
- <section>
- <title>Querying Capabilities</title>
-
- <para>Devices supporting the <wordasword>Video Output
-Overlay</wordasword> interface set the
-<constant>V4L2_CAP_VIDEO_OUTPUT_OVERLAY</constant> flag in the
-<structfield>capabilities</structfield> field of &v4l2-capability;
-returned by the &VIDIOC-QUERYCAP; ioctl.</para>
- </section>
-
- <section>
- <title>Framebuffer</title>
-
- <para>Contrary to the <wordasword>Video Overlay</wordasword>
-interface the framebuffer is normally implemented on the TV card and
-not the graphics card. On Linux it is accessible as a framebuffer
-device (<filename>/dev/fbN</filename>). Given a V4L2 device,
-applications can find the corresponding framebuffer device by calling
-the &VIDIOC-G-FBUF; ioctl. It returns, amongst other information, the
-physical address of the framebuffer in the
-<structfield>base</structfield> field of &v4l2-framebuffer;. The
-framebuffer device ioctl <constant>FBIOGET_FSCREENINFO</constant>
-returns the same address in the <structfield>smem_start</structfield>
-field of struct <structname>fb_fix_screeninfo</structname>. The
-<constant>FBIOGET_FSCREENINFO</constant> ioctl and struct
-<structname>fb_fix_screeninfo</structname> are defined in the
-<filename>linux/fb.h</filename> header file.</para>
-
- <para>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; until all applications closed the
-framebuffer device.</para>
-
- <example>
- <title>Finding a framebuffer device for OSD</title>
-
- <programlisting>
-#include &lt;linux/fb.h&gt;
-
-&v4l2-framebuffer; fbuf;
-unsigned int i;
-int fb_fd;
-
-if (-1 == ioctl (fd, VIDIOC_G_FBUF, &amp;fbuf)) {
- perror ("VIDIOC_G_FBUF");
- exit (EXIT_FAILURE);
-}
-
-for (i = 0; i &gt; 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, &amp;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. */
-</programlisting>
- </example>
- </section>
-
- <section>
- <title>Overlay Window and Scaling</title>
-
- <para>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 <link linkend="overlay">Video Overlay</link> interface.</para>
-
- <para>A &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 <structfield>type</structfield> field of a &v4l2-format; to
-<constant>V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY</constant> and call the
-&VIDIOC-G-FMT; ioctl. The driver fills the
-<structname>v4l2_window</structname> substructure named
-<structfield>win</structfield>. It is not possible to retrieve a
-previously programmed clipping list or bitmap.</para>
-
- <para>To program the source rectangle applications set the
-<structfield>type</structfield> field of a &v4l2-format; to
-<constant>V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY</constant>, initialize
-the <structfield>win</structfield> substructure and call the
-&VIDIOC-S-FMT; ioctl. The driver adjusts the parameters against
-hardware limits and returns the actual parameters as
-<constant>VIDIOC_G_FMT</constant> does. Like
-<constant>VIDIOC_S_FMT</constant>, the &VIDIOC-TRY-FMT; ioctl can be
-used to learn about driver capabilities without actually changing
-driver state. Unlike <constant>VIDIOC_S_FMT</constant> this also works
-after the overlay has been enabled.</para>
-
- <para>A &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 &v4l2-window; and &v4l2-crop;. The cropping API
-applies to <wordasword>Video Output</wordasword> and <wordasword>Video
-Output Overlay</wordasword> devices in the same way as to
-<wordasword>Video Capture</wordasword> and <wordasword>Video
-Overlay</wordasword> devices, merely reversing the direction of the
-data flow. For more information see <xref linkend="crop" />.</para>
- </section>
-
- <section>
- <title>Enabling Overlay</title>
-
- <para>There is no V4L2 ioctl to enable or disable the overlay,
-however the framebuffer interface of the driver may support the
-<constant>FBIOBLANK</constant> ioctl.</para>
- </section>