8.16. ioctl VIDIOC_ENUM_FRAMEINTERVALS¶
8.16.1. Name¶
VIDIOC_ENUM_FRAMEINTERVALS - Enumerate frame intervals
8.16.3. Arguments¶
fd
- File descriptor returned by open().
request
- VIDIOC_ENUM_FRAMEINTERVALS
argp
- Pointer to a struct v4l2_frmivalenum structure that contains a pixel format and size and receives a frame interval.
8.16.4. Description¶
This ioctl allows applications to enumerate all frame intervals that the device supports for the given pixel format and frame size.
The supported pixel formats and frame sizes can be obtained by using the ioctl VIDIOC_ENUM_FMT and ioctl VIDIOC_ENUM_FRAMESIZES functions.
The return value and the content of the v4l2_frmivalenum.type
field
depend on the type of frame intervals the device supports. Here are the
semantics of the function for the different cases:
- Discrete: The function returns success if the given index value
(zero-based) is valid. The application should increase the index by
one for each call until
EINVAL
is returned. The v4l2_frmivalenum.type field is set to V4L2_FRMIVAL_TYPE_DISCRETE by the driver. Of the union only the discrete member is valid. - Step-wise: The function returns success if the given index value
is zero and
EINVAL
for any other index value. Thev4l2_frmivalenum.type
field is set toV4L2_FRMIVAL_TYPE_STEPWISE
by the driver. Of the union only thestepwise
member is valid. - Continuous: This is a special case of the step-wise type above.
The function returns success if the given index value is zero and
EINVAL
for any other index value. Thev4l2_frmivalenum.type
field is set toV4L2_FRMIVAL_TYPE_CONTINUOUS
by the driver. Of the union only thestepwise
member is valid and thestep
value is set to 1.
When the application calls the function with index zero, it must check
the type
field to determine the type of frame interval enumeration
the device supports. Only for the V4L2_FRMIVAL_TYPE_DISCRETE
type
does it make sense to increase the index value to receive more frame
intervals.
Note
The order in which the frame intervals are returned has no special meaning. In particular does it not say anything about potential default frame intervals.
Applications can assume that the enumeration data does not change without any interaction from the application itself. This means that the enumeration data is consistent if the application does not perform any other ioctl calls while it runs the frame interval enumeration.
Note
Frame intervals and frame rates: The V4L2 API uses frame intervals instead of frame rates. Given the frame interval the frame rate can be computed as follows:
frame_rate = 1 / frame_interval
8.16.5. Structs¶
In the structs below, IN denotes a value that has to be filled in by the application, OUT denotes values that the driver fills in. The application should zero out all members except for the IN fields.
struct v4l2_fract | min |
Minimum frame interval [s]. |
struct v4l2_fract | max |
Maximum frame interval [s]. |
struct v4l2_fract | step |
Frame interval step size [s]. |
__u32 | index |
IN: Index of the given frame interval in the enumeration. | |
__u32 | pixel_format |
IN: Pixel format for which the frame intervals are enumerated. | |
__u32 | width |
IN: Frame width for which the frame intervals are enumerated. | |
__u32 | height |
IN: Frame height for which the frame intervals are enumerated. | |
__u32 | type |
OUT: Frame interval type the device supports. | |
union | OUT: Frame interval with the given index. | ||
struct v4l2_fract | discrete |
Frame interval [s]. | |
struct v4l2_frmival_stepwise | stepwise |
||
__u32 | reserved[2] |
Reserved space for future use. Must be zeroed by drivers and applications. |
8.16.6. Enums¶
V4L2_FRMIVAL_TYPE_DISCRETE |
1 | Discrete frame interval. |
V4L2_FRMIVAL_TYPE_CONTINUOUS |
2 | Continuous frame interval. |
V4L2_FRMIVAL_TYPE_STEPWISE |
3 | Step-wise defined frame interval. |
8.16.7. Return Value¶
On success 0 is returned, on error -1 and the errno
variable is set
appropriately. The generic error codes are described at the
Generic Error Codes chapter.