Video for Linux Two API Specification: Revision 0.24
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# ioctl VIDIOC_G_PARM, VIDIOC_S_PARM

## Name

VIDIOC_G_PARM, VIDIOC_S_PARM -- Get or set streaming parameters

## Synopsis

``int ioctl`(int fd, int request, v4l2_streamparm *argp);`

## Arguments

`fd`

  File descriptor returned by `open()`.

`request`

  VIDIOC_G_PARM, VIDIOC_S_PARM

`argp`

## Description

The current video standard determines a nominal number of frames per second. If less than this number of frames is to be captured or output, applications can request frame skipping or duplicating on the driver side. This is especially useful when using the `read()` or `write()`, which are not augmented by timestamps or sequence counters, and to avoid unneccessary data copying.

Further these ioctls can be used to determine the number of buffers used internally by a driver in read/write mode. For implications see the section discussing the `read()` function.

To get and set the streaming parameters applications call the `VIDIOC_G_PARM` and `VIDIOC_S_PARM` ioctl, respectively. They take a pointer to a struct `v4l2_streamparm` which contains a union holding separate parameters for input and output devices.

Table 1. struct `v4l2_streamparm`

enum v4l2_buf_type | `type` |  | The buffer (stream) type, same as struct v4l2_format `type`, set by the application.
union | `parm` |  |
| struct v4l2_captureparm | `capture` | Parameters for capture devices, used when `type` is `V4L2_BUF_TYPE_VIDEO_CAPTURE`.
| struct v4l2_outputparm | `output` | Parameters for output devices, used when `type` is `V4L2_BUF_TYPE_VIDEO_OUTPUT`.
| __u8 | `raw_data`[200] | A place holder for future extensions and custom (driver defined) buffer types `V4L2_BUF_TYPE_PRIVATE` and higher.

Table 2. struct `v4l2_captureparm`

__u32 | `capability` | See Table 4.
__u32 | `capturemode` | Set by drivers and applications, see Table 5.
struct v4l2_fract | `timeperframe` | This is is the desired period between successive frames captured by the driver, in seconds. The field is intended to skip frames on the driver side, saving I/O bandwidth. Applications store here the desired frame period, drivers return the actual frame period, which must be greater or equal to the nominal frame period determined by the current video standard (struct v4l2_standard `frameperiod` field). Changing the video standard (also implicitly by switching the video input) may reset this parameter to the nominal frame period. To reset manually applications can just set this field to zero.

Drivers support this function only when they set the `V4L2_CAP_TIMEPERFRAME` flag in the `capability` field.

__u32 | `extendedmode` | Custom (driver specific) streaming parameters. When unused, applications and drivers must set this field to zero. Applications using this field should check the driver name and version, see Section 1.2.
__u32 | `readbuffers` | Applications set this field to the desired number of buffers used internally by the driver in `read()` mode. Drivers return the actual number of buffers. When an application requests zero buffers, drivers should just return the current setting rather than the minimum or an error code. For details see Section 3.1.
__u32 | `reserved`[4] | Reserved for future extensions. Drivers and applications must set the array to zero.

Table 3. struct `v4l2_outputparm`

__u32 | `capability` | See Table 4.
__u32 | `outputmode` | Set by drivers and applications, see Table 5.
struct v4l2_fract | `timeperframe` | This is is the desired period between successive frames output by the driver, in seconds.
The field is intended to repeat frames on the driver side in `write()` mode (in streaming mode timestamps can be used to throttle the output), saving I/O bandwidth. Applications store here the desired frame period, drivers return the actual frame period, which must be greater or equal to the nominal frame period determined by the current video standard (struct v4l2_standard `frameperiod` field). Changing the video standard (also implicitly by switching the video output) may reset this parameter to the nominal frame period. To reset manually applications can just set this field to zero.

Drivers support this function only when they set the `V4L2_CAP_TIMEPERFRAME` flag in the `capability` field.

__u32 | `extendedmode` | Custom (driver specific) streaming parameters. When unused, applications and drivers must set this field to zero. Applications using this field should check the driver name and version, see Section 1.2.
__u32 | `writebuffers` | Applications set this field to the desired number of buffers used internally by the driver in `write()` mode. Drivers return the actual number of buffers. When an application requests zero buffers, drivers should just return the current setting rather than the minimum or an error code. For details see Section 3.1.
__u32 | `reserved`[4] | Reserved for future extensions. Drivers and applications must set the array to zero.

Table 4. Streaming Parameters Capabilites

`V4L2_CAP_TIMEPERFRAME` | 0x1000 | The frame skipping/repeating controlled by the `timeperframe` field is supported.

Table 5. Capture Parameters Flags

`V4L2_MODE_HIGHQUALITY` | 0x0001 | High quality imaging mode. High quality mode is intended for still imaging applications. The idea is to get the best possible image quality that the hardware can deliver. It is not defined how the driver writer may achieve that; it will depend on the hardware and the ingenuity of the driver writer. High quality mode is a different mode from the the regular motion video capture modes. In high quality mode:

- The driver may be able to capture higher resolutions than for motion capture.

- The driver may support fewer pixel formats than motion capture (eg; true color).

- The driver may capture and arithmetically combine multiple successive fields or frames to remove color edge artifacts and reduce the noise in the video data.

- The driver may capture images in slices like a scanner in order to handle larger format images than would otherwise be possible.

- An image capture operation may be significantly slower than motion capture.

- Moving objects in the image might have excessive motion blur.

- Capture might only work through the `read()` call.

## Return Value

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

EINVAL

  This ioctl is not supported.

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