Video for Linux Two API Specification: Revision 0.24
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# ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF

## Name

VIDIOC_G_FBUF, VIDIOC_S_FBUF -- Get or set frame buffer overlay parameters

## Synopsis

``int ioctl`(int fd, int request, struct v4l2_framebuffer *argp);`

``int ioctl`(int fd, int request, const struct v4l2_framebuffer *argp);`

## Arguments

`fd`

  File descriptor returned by `open()`.

`request`

  VIDIOC_G_FBUF, VIDIOC_S_FBUF

`argp`

## Description

Applications can use the `VIDIOC_G_FBUF` and `VIDIOC_S_FBUF` ioctl to get and set the framebuffer parameters for a Video Overlay or Video Output Overlay (OSD). The type of overlay is implied by the device type (capture or output device) and can be determined with the `VIDIOC_QUERYCAP` ioctl. One `/dev/videoN` device must not support both kinds of overlay.

The V4L2 API distinguishes destructive and non-destructive overlays. A destructive overlay copies captured video images into the video memory of a graphics card. A non-destructive overlay blends video images into a VGA signal or graphics into a video signal. Video Output Overlays are always non-destructive.

To get the current parameters applications call the `VIDIOC_G_FBUF` ioctl with a pointer to a `v4l2_framebuffer` structure. The driver fills all fields of the structure or returns an EINVAL error code when overlays are not supported.

To set the parameters for a Video Output Overlay, applications must initialize the `flags` field of a struct `v4l2_framebuffer`. Since the framebuffer is implemented on the TV card all other parameters are determined by the driver. When an application calls `VIDIOC_S_FBUF` with a pointer to this structure, the driver prepares for the overlay and returns the framebuffer parameters as `VIDIOC_G_FBUF` does, or it returns an error code.

To set the parameters for a non-destructive Video Overlay, applications must initialize the `flags` field, the `fmt` substructure, and call `VIDIOC_S_FBUF`. Again the driver prepares for the overlay and returns the framebuffer parameters as `VIDIOC_G_FBUF` does, or it returns an error code.

For a destructive Video Overlay applications must additionally provide a `base` address. Setting up a DMA to a random memory location can jeopardize the system security, its stability or even damage the hardware, therefore only the superuser can set the parameters for a destructive video overlay.

Table 1. struct `v4l2_framebuffer`

__u32 | `capability` |  | Overlay capability flags set by the driver, see Table 2.
__u32 | `flags` |  | Overlay control flags set by application and driver, see Table 3
void * | `base` |  | Physical base address of the framebuffer, that is the address of the pixel in the top left corner of the framebuffer.a This field is irrelevant to non-destructive Video Overlays. For destructive Video Overlays applications must provide a base address. The driver may accept only base addresses which are a multiple of two, four or eight bytes. For Video Output Overlays the driver must return a valid base address, so applications can find the corresponding Linux framebuffer device (see Section 4.4).
struct v4l2_pix_format | `fmt` |  | Layout of the frame buffer. The `v4l2_pix_format` structure is defined in Chapter 2, for clarification the fields and acceptable values are listed below:
| __u32 | `width` | Width of the frame buffer in pixels.
| __u32 | `height` | Height of the frame buffer in pixels.
| __u32 | `pixelformat` | The pixel format of the framebuffer. For non-destructive Video Overlays this field only defines a format for the struct v4l2_window `chromakey` field. For destructive Video Overlays applications must initialize this field. For Video Output Overlays the driver must return a valid format.

Usually this is an RGB format (for example `V4L2_PIX_FMT_RGB565`) but YUV formats (only packed YUV formats when chroma keying is used, not including `V4L2_PIX_FMT_YUYV` and `V4L2_PIX_FMT_UYVY`) and the `V4L2_PIX_FMT_PAL8` format are also permitted. The behavior of the driver when an application requests a compressed format is undefined. See Chapter 2 for information on pixel formats.

| enum v4l2_field | `field` | Drivers and applications shall ignore this field. If applicable, the field order is selected with the `VIDIOC_S_FMT` ioctl, using the `field` field of struct v4l2_window.
| __u32 | `bytesperline` | Distance in bytes between the leftmost pixels in two adjacent lines.
This field is irrelevant to non-destructive Video Overlays. For destructive Video Overlays both applications and drivers can set this field to request padding bytes at the end of each line. Drivers however may ignore the requested value, returning `width` times bytes-per-pixel or a larger value required by the hardware. That implies applications can just set this field to zero to get a reasonable default.

For Video Output Overlays the driver must return a valid value.

Video hardware may access padding bytes, therefore they must reside in accessible memory. Consider for example the case where padding bytes after the last line of an image cross a system page boundary. Capture devices may write padding bytes, the value is undefined. Output devices ignore the contents of padding bytes.

When the image format is planar the `bytesperline` value applies to the largest plane and is divided by the same factor as the `width` field for any smaller planes. For example the Cb and Cr planes of a YUV 4:2:0 image have half as many padding bytes following each line as the Y plane. To avoid ambiguities drivers must return a `bytesperline` value rounded up to a multiple of the scale factor.

| __u32 | `sizeimage` | This field is irrelevant to non-destructive Video Overlays. For destructive Video Overlays applications must initialize this field. For Video Output Overlays the driver must return a valid format. Together with `base` it defines the framebuffer memory accessible by the driver.
| enum v4l2_colorspace | `colorspace` | This information supplements the `pixelformat` and must be set by the driver, see Section 2.2.
| __u32 | `priv` | Reserved for additional information about custom (driver defined) formats. When not used drivers and applications must set this field to zero.
Notes: a. A physical base address may not suit all platforms. GK notes in theory we should pass something like PCI device + memory region + offset instead. If you encounter problems please discuss on the Video4Linux mailing list: https://listman.redhat.com/mailman/listinfo/video4linux-list.

Table 2. Frame Buffer Capability Flags

`V4L2_FBUF_CAP_EXTERNOVERLAY` | 0x0001 | The device is capable of non-destructive overlays. When the driver clears this flag, only destructive overlays are supported. There are no drivers yet which support both destructive and non-destructive overlays.
`V4L2_FBUF_CAP_CHROMAKEY` | 0x0002 | The device supports clipping by chroma-keying the images. That is, image pixels replace pixels in the VGA or video signal only where the latter assume a certain color. Chroma-keying makes no sense for destructive overlays.
`V4L2_FBUF_CAP_LIST_CLIPPING` | 0x0004 | The device supports clipping using a list of clip rectangles.
`V4L2_FBUF_CAP_BITMAP_CLIPPING` | 0x0008 | The device supports clipping using a bit mask.
`V4L2_FBUF_CAP_LOCAL_ALPHA` | 0x0010 | The device supports clipping/blending using the alpha channel of the framebuffer or VGA signal. Alpha blending makes no sense for destructive overlays.
`V4L2_FBUF_CAP_GLOBAL_ALPHA` | 0x0020 | The device supports alpha blending using a global alpha value. Alpha blending makes no sense for destructive overlays.
`V4L2_FBUF_CAP_LOCAL_INV_ALPHA` | 0x0040 | The device supports clipping/blending using the inverted alpha channel of the framebuffer or VGA signal. Alpha blending makes no sense for destructive overlays.

Table 3. Frame Buffer Flags

`V4L2_FBUF_FLAG_PRIMARY` | 0x0001 | The framebuffer is the primary graphics surface. In other words, the overlay is destructive. [?]
`V4L2_FBUF_FLAG_OVERLAY` | 0x0002 | The frame buffer is an overlay surface the same size as the capture. [?]
The purpose of `V4L2_FBUF_FLAG_PRIMARY` and `V4L2_FBUF_FLAG_OVERLAY` was never quite clear. Most drivers seem to ignore these flags. For compatibility with the bttv driver applications should set the `V4L2_FBUF_FLAG_OVERLAY` flag.
`V4L2_FBUF_FLAG_CHROMAKEY` | 0x0004 | Use chroma-keying. The chroma-key color is determined by the `chromakey` field of struct v4l2_window and negotiated with the `VIDIOC_S_FMT` ioctl, see Section 4.2 and Section 4.4.
There are no flags to enable clipping using a list of clip rectangles or a bitmap. These methods are negotiated with the `VIDIOC_S_FMT` ioctl, see Section 4.2 and Section 4.4.
`V4L2_FBUF_FLAG_LOCAL_ALPHA` | 0x0008 | Use the alpha channel of the framebuffer to clip or blend framebuffer pixels with video images. The blend function is: output = framebuffer pixel * alpha + video pixel * (1 - alpha). The actual alpha depth depends on the framebuffer pixel format.
`V4L2_FBUF_FLAG_GLOBAL_ALPHA` | 0x0010 | Use a global alpha value to blend the framebuffer with video images. The blend function is: output = (framebuffer pixel * alpha + video pixel * (255 - alpha)) / 255. The alpha value is determined by the `global_alpha` field of struct v4l2_window and negotiated with the `VIDIOC_S_FMT` ioctl, see Section 4.2 and Section 4.4.
`V4L2_FBUF_FLAG_LOCAL_INV_ALPHA` | 0x0020 | Like `V4L2_FBUF_FLAG_LOCAL_ALPHA`, use the alpha channel of the framebuffer to clip or blend framebuffer pixels with video images, but with an inverted alpha value. The blend function is: output = framebuffer pixel * (1 - alpha) + video pixel * alpha. The actual alpha depth depends on the framebuffer pixel format.

## Return Value

On success 0 is returned, on error -1 and the `errno` variable is set appropriately:

EPERM

  `VIDIOC_S_FBUF` can only be called by a privileged user to negotiate the parameters for a destructive overlay.

EBUSY

  The framebuffer parameters cannot be changed at this time because overlay is already enabled, or capturing is enabled and the hardware cannot capture and overlay simultaneously.

EINVAL

  The ioctl is not supported or the `VIDIOC_S_FBUF` parameters are unsuitable.

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