dev-capture.rst

  1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
  2.. c:namespace:: V4L
  3
  4.. _capture:
  5
  6***********************
  7Video Capture Interface
  8***********************
  9
 10Video capture devices sample an analog video signal and store the
 11digitized images in memory. Today nearly all devices can capture at full
 1225 or 30 frames/second. With this interface applications can control the
 13capture process and move images from the driver into user space.
 14
 15Conventionally V4L2 video capture devices are accessed through character
 16device special files named ``/dev/video`` and ``/dev/video0`` to
 17``/dev/video63`` with major number 81 and minor numbers 0 to 63.
 18``/dev/video`` is typically a symbolic link to the preferred video
 19device.
 20
 21.. note:: The same device file names are used for video output devices.
 22
 23Querying Capabilities
 24=====================
 25
 26Devices supporting the video capture interface set the
 27``V4L2_CAP_VIDEO_CAPTURE`` or ``V4L2_CAP_VIDEO_CAPTURE_MPLANE`` flag in
 28the ``capabilities`` field of struct
 29:c:type:`v4l2_capability` returned by the
 30:ref:`VIDIOC_QUERYCAP` ioctl. As secondary device
 31functions they may also support the :ref:`video overlay <overlay>`
 32(``V4L2_CAP_VIDEO_OVERLAY``) and the :ref:`raw VBI capture <raw-vbi>`
 33(``V4L2_CAP_VBI_CAPTURE``) interface. At least one of the read/write or
 34streaming I/O methods must be supported. Tuners and audio inputs are
 35optional.
 36
 37Supplemental Functions
 38======================
 39
 40Video capture devices shall support :ref:`audio input <audio>`,
 41:ref:`tuner`, :ref:`controls <control>`,
 42:ref:`cropping and scaling <crop>` and
 43:ref:`streaming parameter <streaming-par>` ioctls as needed. The
 44:ref:`video input <video>` ioctls must be supported by all video
 45capture devices.
 46
 47Image Format Negotiation
 48========================
 49
 50The result of a capture operation is determined by cropping and image
 51format parameters. The former select an area of the video picture to
 52capture, the latter how images are stored in memory, i. e. in RGB or YUV
 53format, the number of bits per pixel or width and height. Together they
 54also define how images are scaled in the process.
 55
 56As usual these parameters are *not* reset at :c:func:`open()`
 57time to permit Unix tool chains, programming a device and then reading
 58from it as if it was a plain file. Well written V4L2 applications ensure
 59they really get what they want, including cropping and scaling.
 60
 61Cropping initialization at minimum requires to reset the parameters to
 62defaults. An example is given in :ref:`crop`.
 63
 64To query the current image format applications set the ``type`` field of
 65a struct :c:type:`v4l2_format` to
 66``V4L2_BUF_TYPE_VIDEO_CAPTURE`` or
 67``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE`` and call the
 68:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl with a pointer to this
 69structure. Drivers fill the struct
 70:c:type:`v4l2_pix_format` ``pix`` or the struct
 71:c:type:`v4l2_pix_format_mplane` ``pix_mp``
 72member of the ``fmt`` union.
 73
 74To request different parameters applications set the ``type`` field of a
 75struct :c:type:`v4l2_format` as above and initialize all
 76fields of the struct :c:type:`v4l2_pix_format`
 77``vbi`` member of the ``fmt`` union, or better just modify the results
 78of :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, and call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
 79ioctl with a pointer to this structure. Drivers may adjust the
 80parameters and finally return the actual parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`
 81does.
 82
 83Like :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` the :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl
 84can be used to learn about hardware limitations without disabling I/O or
 85possibly time consuming hardware preparations.
 86
 87The contents of struct :c:type:`v4l2_pix_format` and
 88struct :c:type:`v4l2_pix_format_mplane` are
 89discussed in :ref:`pixfmt`. See also the specification of the
 90:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctls for
 91details. Video capture devices must implement both the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`
 92and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, even if :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ignores all
 93requests and always returns default parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does.
 94:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` is optional.
 95
 96Reading Images
 97==============
 98
 99A video capture device may support the :ref:`read() function <func-read>`
100and/or streaming (:ref:`memory mapping <func-mmap>` or
101:ref:`user pointer <userp>`) I/O. See :ref:`io` for details.