Based on kernel version 3.0.4. Page generated on 2011-10-03 22:07 EST.
1 <title>Video Output Overlay Interface</title> 2 <subtitle>Also known as On-Screen Display (OSD)</subtitle> 3 4 <note> 5 <title>Experimental</title> 6 7 <para>This is an <link linkend="experimental">experimental</link> 8 interface and may change in the future.</para> 9 </note> 10 11 <para>Some video output devices can overlay a framebuffer image onto 12 the outgoing video signal. Applications can set up such an overlay 13 using this interface, which borrows structures and ioctls of the <link 14 linkend="overlay">Video Overlay</link> interface.</para> 15 16 <para>The OSD function is accessible through the same character 17 special file as the <link linkend="capture">Video Output</link> function. 18 Note the default function of such a <filename>/dev/video</filename> device 19 is video capturing or output. The OSD function is only available after 20 calling the &VIDIOC-S-FMT; ioctl.</para> 21 22 <section> 23 <title>Querying Capabilities</title> 24 25 <para>Devices supporting the <wordasword>Video Output 26 Overlay</wordasword> interface set the 27 <constant>V4L2_CAP_VIDEO_OUTPUT_OVERLAY</constant> flag in the 28 <structfield>capabilities</structfield> field of &v4l2-capability; 29 returned by the &VIDIOC-QUERYCAP; ioctl.</para> 30 </section> 31 32 <section> 33 <title>Framebuffer</title> 34 35 <para>Contrary to the <wordasword>Video Overlay</wordasword> 36 interface the framebuffer is normally implemented on the TV card and 37 not the graphics card. On Linux it is accessible as a framebuffer 38 device (<filename>/dev/fbN</filename>). Given a V4L2 device, 39 applications can find the corresponding framebuffer device by calling 40 the &VIDIOC-G-FBUF; ioctl. It returns, amongst other information, the 41 physical address of the framebuffer in the 42 <structfield>base</structfield> field of &v4l2-framebuffer;. The 43 framebuffer device ioctl <constant>FBIOGET_FSCREENINFO</constant> 44 returns the same address in the <structfield>smem_start</structfield> 45 field of struct <structname>fb_fix_screeninfo</structname>. The 46 <constant>FBIOGET_FSCREENINFO</constant> ioctl and struct 47 <structname>fb_fix_screeninfo</structname> are defined in the 48 <filename>linux/fb.h</filename> header file.</para> 49 50 <para>The width and height of the framebuffer depends on the 51 current video standard. A V4L2 driver may reject attempts to change 52 the video standard (or any other ioctl which would imply a framebuffer 53 size change) with an &EBUSY; until all applications closed the 54 framebuffer device.</para> 55 56 <example> 57 <title>Finding a framebuffer device for OSD</title> 58 59 <programlisting> 60 #include <linux/fb.h> 61 62 &v4l2-framebuffer; fbuf; 63 unsigned int i; 64 int fb_fd; 65 66 if (-1 == ioctl (fd, VIDIOC_G_FBUF, &fbuf)) { 67 perror ("VIDIOC_G_FBUF"); 68 exit (EXIT_FAILURE); 69 } 70 71 for (i = 0; i > 30; ++i) { 72 char dev_name[16]; 73 struct fb_fix_screeninfo si; 74 75 snprintf (dev_name, sizeof (dev_name), "/dev/fb%u", i); 76 77 fb_fd = open (dev_name, O_RDWR); 78 if (-1 == fb_fd) { 79 switch (errno) { 80 case ENOENT: /* no such file */ 81 case ENXIO: /* no driver */ 82 continue; 83 84 default: 85 perror ("open"); 86 exit (EXIT_FAILURE); 87 } 88 } 89 90 if (0 == ioctl (fb_fd, FBIOGET_FSCREENINFO, &si)) { 91 if (si.smem_start == (unsigned long) fbuf.base) 92 break; 93 } else { 94 /* Apparently not a framebuffer device. */ 95 } 96 97 close (fb_fd); 98 fb_fd = -1; 99 } 100 101 /* fb_fd is the file descriptor of the framebuffer device 102 for the video output overlay, or -1 if no device was found. */ 103 </programlisting> 104 </example> 105 </section> 106 107 <section> 108 <title>Overlay Window and Scaling</title> 109 110 <para>The overlay is controlled by source and target rectangles. 111 The source rectangle selects a subsection of the framebuffer image to 112 be overlaid, the target rectangle an area in the outgoing video signal 113 where the image will appear. Drivers may or may not support scaling, 114 and arbitrary sizes and positions of these rectangles. Further drivers 115 may support any (or none) of the clipping/blending methods defined for 116 the <link linkend="overlay">Video Overlay</link> interface.</para> 117 118 <para>A &v4l2-window; defines the size of the source rectangle, 119 its position in the framebuffer and the clipping/blending method to be 120 used for the overlay. To get the current parameters applications set 121 the <structfield>type</structfield> field of a &v4l2-format; to 122 <constant>V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY</constant> and call the 123 &VIDIOC-G-FMT; ioctl. The driver fills the 124 <structname>v4l2_window</structname> substructure named 125 <structfield>win</structfield>. It is not possible to retrieve a 126 previously programmed clipping list or bitmap.</para> 127 128 <para>To program the source rectangle applications set the 129 <structfield>type</structfield> field of a &v4l2-format; to 130 <constant>V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY</constant>, initialize 131 the <structfield>win</structfield> substructure and call the 132 &VIDIOC-S-FMT; ioctl. The driver adjusts the parameters against 133 hardware limits and returns the actual parameters as 134 <constant>VIDIOC_G_FMT</constant> does. Like 135 <constant>VIDIOC_S_FMT</constant>, the &VIDIOC-TRY-FMT; ioctl can be 136 used to learn about driver capabilities without actually changing 137 driver state. Unlike <constant>VIDIOC_S_FMT</constant> this also works 138 after the overlay has been enabled.</para> 139 140 <para>A &v4l2-crop; defines the size and position of the target 141 rectangle. The scaling factor of the overlay is implied by the width 142 and height given in &v4l2-window; and &v4l2-crop;. The cropping API 143 applies to <wordasword>Video Output</wordasword> and <wordasword>Video 144 Output Overlay</wordasword> devices in the same way as to 145 <wordasword>Video Capture</wordasword> and <wordasword>Video 146 Overlay</wordasword> devices, merely reversing the direction of the 147 data flow. For more information see <xref linkend="crop" />.</para> 148 </section> 149 150 <section> 151 <title>Enabling Overlay</title> 152 153 <para>There is no V4L2 ioctl to enable or disable the overlay, 154 however the framebuffer interface of the driver may support the 155 <constant>FBIOBLANK</constant> ioctl.</para> 156 </section> 157 158 <!-- 159 Local Variables: 160 mode: sgml 161 sgml-parent-document: "v4l2.sgml" 162 indent-tabs-mode: nil 163 End: 164 -->
