4.4. 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 Video Overlay interface.

The OSD function is accessible through the same character special file as the Video Output 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 VIDIOC_S_FMT ioctl.

4.4.1. Querying Capabilities

Devices supporting the Video Output Overlay interface set the V4L2_CAP_VIDEO_OUTPUT_OVERLAY flag in the capabilities field of struct v4l2_capability returned by the ioctl VIDIOC_QUERYCAP ioctl.

4.4.2. 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 VIDIOC_G_FBUF ioctl. It returns, amongst other information, the physical address of the framebuffer in the base field of struct v4l2_framebuffer. The framebuffer device ioctl FBIOGET_FSCREENINFO returns the same address in the smem_start field of struct struct fb_fix_screeninfo. The FBIOGET_FSCREENINFO ioctl and struct 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.

4.4.2.1. Example: Finding a framebuffer device for OSD

#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. */

4.4.3. 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 Video Overlay interface.

A struct 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 v4l2_format to V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY and call the VIDIOC_G_FMT ioctl. The driver fills the struct 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 v4l2_format to V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY, initialize the win substructure and call the VIDIOC_S_FMT ioctl. The driver adjusts the parameters against hardware limits and returns the actual parameters as VIDIOC_G_FMT does. Like VIDIOC_S_FMT, the VIDIOC_TRY_FMT ioctl can be used to learn about driver capabilities without actually changing driver state. Unlike VIDIOC_S_FMT this also works after the overlay has been enabled.

A struct 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 v4l2_window and struct 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 Image Cropping, Insertion and Scaling – the CROP API.

4.4.4. 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.