fdFile descriptor returned by open().
requestVIDIOC_ENUM_FRAMEINTERVALS
argpPointer to a struct v4l2_frmivalenum structure that contains a pixel format and size and receives a frame interval.
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 VIDIOC_ENUM_FMT and 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. The
v4l2_frmivalenum.type field is set to
V4L2_FRMIVAL_TYPE_STEPWISE by the driver. Of the
union only the stepwise 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. The
v4l2_frmivalenum.type field is set to
V4L2_FRMIVAL_TYPE_CONTINUOUS by the driver. Of
the union only the stepwise member is valid
and the step 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 that 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.
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
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.
Table 1. struct v4l2_frmival_stepwise
| 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]. |
Table 2. struct v4l2_frmivalenum
| __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. |