35 lines
1.3 KiB
ReStructuredText
35 lines
1.3 KiB
ReStructuredText
.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
|
|
.. c:namespace:: V4L
|
|
|
|
.. _streaming-par:
|
|
|
|
********************
|
|
Streaming Parameters
|
|
********************
|
|
|
|
Streaming parameters are intended to optimize the video capture process
|
|
as well as I/O. Presently applications can request a high quality
|
|
capture mode with the :ref:`VIDIOC_S_PARM <VIDIOC_G_PARM>` ioctl.
|
|
|
|
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
|
|
:c:func:`read()` or :c:func:`write()`, which are
|
|
not augmented by timestamps or sequence counters, and to avoid
|
|
unnecessary data copying.
|
|
|
|
Finally 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 :c:func:`read()` function.
|
|
|
|
To get and set the streaming parameters applications call the
|
|
:ref:`VIDIOC_G_PARM <VIDIOC_G_PARM>` and
|
|
:ref:`VIDIOC_S_PARM <VIDIOC_G_PARM>` ioctl, respectively. They take
|
|
a pointer to a struct :c:type:`v4l2_streamparm`, which
|
|
contains a union holding separate parameters for input and output
|
|
devices.
|
|
|
|
These ioctls are optional, drivers need not implement them. If so, they
|
|
return the ``EINVAL`` error code.
|