105 lines
4.3 KiB
ReStructuredText
105 lines
4.3 KiB
ReStructuredText
.. -*- coding: utf-8; mode: rst -*-
|
|
|
|
.. _capture:
|
|
|
|
***********************
|
|
Video Capture Interface
|
|
***********************
|
|
|
|
Video capture devices sample an analog video signal and store the
|
|
digitized images in memory. Today nearly all devices can capture at full
|
|
25 or 30 frames/second. With this interface applications can control the
|
|
capture process and move images from the driver into user space.
|
|
|
|
Conventionally V4L2 video capture devices are accessed through character
|
|
device special files named ``/dev/video`` and ``/dev/video0`` to
|
|
``/dev/video63`` with major number 81 and minor numbers 0 to 63.
|
|
``/dev/video`` is typically a symbolic link to the preferred video
|
|
device.
|
|
|
|
.. note:: The same device file names are used for video output devices.
|
|
|
|
|
|
Querying Capabilities
|
|
=====================
|
|
|
|
Devices supporting the video capture interface set the
|
|
``V4L2_CAP_VIDEO_CAPTURE`` or ``V4L2_CAP_VIDEO_CAPTURE_MPLANE`` flag in
|
|
the ``capabilities`` field of struct
|
|
:c:type:`v4l2_capability` returned by the
|
|
:ref:`VIDIOC_QUERYCAP` ioctl. As secondary device
|
|
functions they may also support the :ref:`video overlay <overlay>`
|
|
(``V4L2_CAP_VIDEO_OVERLAY``) and the :ref:`raw VBI capture <raw-vbi>`
|
|
(``V4L2_CAP_VBI_CAPTURE``) interface. At least one of the read/write or
|
|
streaming I/O methods must be supported. Tuners and audio inputs are
|
|
optional.
|
|
|
|
|
|
Supplemental Functions
|
|
======================
|
|
|
|
Video capture devices shall support :ref:`audio input <audio>`,
|
|
:ref:`tuner`, :ref:`controls <control>`,
|
|
:ref:`cropping and scaling <crop>` and
|
|
:ref:`streaming parameter <streaming-par>` ioctls as needed. The
|
|
:ref:`video input <video>` ioctls must be supported by all video
|
|
capture devices.
|
|
|
|
|
|
Image Format Negotiation
|
|
========================
|
|
|
|
The result of a capture operation is determined by cropping and image
|
|
format parameters. The former select an area of the video picture to
|
|
capture, the latter how images are stored in memory, i. e. in RGB or YUV
|
|
format, the number of bits per pixel or width and height. Together they
|
|
also define how images are scaled in the process.
|
|
|
|
As usual these parameters are *not* reset at :ref:`open() <func-open>`
|
|
time to permit Unix tool chains, programming a device and then reading
|
|
from it as if it was a plain file. Well written V4L2 applications ensure
|
|
they really get what they want, including cropping and scaling.
|
|
|
|
Cropping initialization at minimum requires to reset the parameters to
|
|
defaults. An example is given in :ref:`crop`.
|
|
|
|
To query the current image format applications set the ``type`` field of
|
|
a struct :c:type:`v4l2_format` to
|
|
``V4L2_BUF_TYPE_VIDEO_CAPTURE`` or
|
|
``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE`` and call the
|
|
:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl with a pointer to this
|
|
structure. Drivers fill the struct
|
|
:c:type:`v4l2_pix_format` ``pix`` or the struct
|
|
:c:type:`v4l2_pix_format_mplane` ``pix_mp``
|
|
member of the ``fmt`` union.
|
|
|
|
To request different parameters applications set the ``type`` field of a
|
|
struct :c:type:`v4l2_format` as above and initialize all
|
|
fields of the struct :c:type:`v4l2_pix_format`
|
|
``vbi`` member of the ``fmt`` union, or better just modify the results
|
|
of :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, and call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
|
|
ioctl with a pointer to this structure. Drivers may adjust the
|
|
parameters and finally return the actual parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`
|
|
does.
|
|
|
|
Like :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` the :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl
|
|
can be used to learn about hardware limitations without disabling I/O or
|
|
possibly time consuming hardware preparations.
|
|
|
|
The contents of struct :c:type:`v4l2_pix_format` and
|
|
struct :c:type:`v4l2_pix_format_mplane` are
|
|
discussed in :ref:`pixfmt`. See also the specification of the
|
|
:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctls for
|
|
details. Video capture devices must implement both the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`
|
|
and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, even if :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ignores all
|
|
requests and always returns default parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does.
|
|
:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` is optional.
|
|
|
|
|
|
Reading Images
|
|
==============
|
|
|
|
A video capture device may support the ::ref:`read() function <func-read>`
|
|
and/or streaming (:ref:`memory mapping <func-mmap>` or
|
|
:ref:`user pointer <userp>`) I/O. See :ref:`io` for details.
|