Documentation/userspace-api/media/v4l/dev-osd.rst

Source file repositories/reference/linux-study-clean/Documentation/userspace-api/media/v4l/dev-osd.rst

File Facts

System
Linux kernel
Corpus path
Documentation/userspace-api/media/v4l/dev-osd.rst
Extension
.rst
Size
5427 bytes
Lines
152
Domain
Support Tooling And Documentation
Bucket
Documentation
Inferred role
Support Tooling And Documentation: documentation
Status
atlas-only

Why This File Exists

Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

Dependency Surface

Detected Declarations

Annotated Snippet

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.

Annotation

Implementation Notes