Documentation/linux_tv/media/v4l/dev-capture.rst

   1 .. -*- coding: utf-8; mode: rst -*-
   2
   3 .. _capture:
   4
   5 ***********************
   6 Video Capture Interface
   7 ***********************
   8
   9 Video capture devices sample an analog video signal and store the
  10 digitized images in memory. Today nearly all devices can capture at full
  11 25 or 30 frames/second. With this interface applications can control the
  12 capture process and move images from the driver into user space.
  13
  14 Conventionally V4L2 video capture devices are accessed through character
  15 device special files named ``/dev/video`` and ``/dev/video0`` to
  16 ``/dev/video63`` with major number 81 and minor numbers 0 to 63.
  17 ``/dev/video`` is typically a symbolic link to the preferred video
  18 device. Note the same device files are used for video output devices.
  19
  20
  21 Querying Capabilities
  22 =====================
  23
  24 Devices supporting the video capture interface set the
  25 ``V4L2_CAP_VIDEO_CAPTURE`` or ``V4L2_CAP_VIDEO_CAPTURE_MPLANE`` flag in
  26 the ``capabilities`` field of struct
  27 :ref:`v4l2_capability <v4l2-capability>` returned by the
  28 :ref:`VIDIOC_QUERYCAP` ioctl. As secondary device
  29 functions they may also support the :ref:`video overlay <overlay>`
  30 (``V4L2_CAP_VIDEO_OVERLAY``) and the :ref:`raw VBI capture <raw-vbi>`
  31 (``V4L2_CAP_VBI_CAPTURE``) interface. At least one of the read/write or
  32 streaming I/O methods must be supported. Tuners and audio inputs are
  33 optional.
  34
  35
  36 Supplemental Functions
  37 ======================
  38
  39 Video capture devices shall support :ref:`audio input <audio>`,
  40 :ref:`tuner`, :ref:`controls <control>`,
  41 :ref:`cropping and scaling <crop>` and
  42 :ref:`streaming parameter <streaming-par>` ioctls as needed. The
  43 :ref:`video input <video>` and :ref:`video standard <standard>`
  44 ioctls must be supported by all video capture devices.
  45
  46
  47 Image Format Negotiation
  48 ========================
  49
  50 The result of a capture operation is determined by cropping and image
  51 format parameters. The former select an area of the video picture to
  52 capture, the latter how images are stored in memory, i. e. in RGB or YUV
  53 format, the number of bits per pixel or width and height. Together they
  54 also define how images are scaled in the process.
  55
  56 As usual these parameters are *not* reset at :ref:`open() <func-open>`
  57 time to permit Unix tool chains, programming a device and then reading
  58 from it as if it was a plain file. Well written V4L2 applications ensure
  59 they really get what they want, including cropping and scaling.
  60
  61 Cropping initialization at minimum requires to reset the parameters to
  62 defaults. An example is given in :ref:`crop`.
  63
  64 To query the current image format applications set the ``type`` field of
  65 a struct :ref:`v4l2_format <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
  69 structure. Drivers fill the struct
  70 :ref:`v4l2_pix_format <v4l2-pix-format>` ``pix`` or the struct
  71 :ref:`v4l2_pix_format_mplane <v4l2-pix-format-mplane>` ``pix_mp``
  72 member of the ``fmt`` union.
  73
  74 To request different parameters applications set the ``type`` field of a
  75 struct :ref:`v4l2_format <v4l2-format>` as above and initialize all
  76 fields of the struct :ref:`v4l2_pix_format <v4l2-pix-format>`
  77 ``vbi`` member of the ``fmt`` union, or better just modify the results
  78 of :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, and call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
  79 ioctl with a pointer to this structure. Drivers may adjust the
  80 parameters and finally return the actual parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`
  81 does.
  82
  83 Like :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` the :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl
  84 can be used to learn about hardware limitations without disabling I/O or
  85 possibly time consuming hardware preparations.
  86
  87 The contents of struct :ref:`v4l2_pix_format <v4l2-pix-format>` and
  88 struct :ref:`v4l2_pix_format_mplane <v4l2-pix-format-mplane>` are
  89 discussed 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
  91 details. Video capture devices must implement both the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`
  92 and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, even if :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ignores all
  93 requests 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
  96
  97 Reading Images
  98 ==============
  99
 100 A video capture device may support the :ref:`read() function <rw>`
 101 and/or streaming (:ref:`memory mapping <mmap>` or
 102 :ref:`user pointer <userp>`) I/O. See :ref:`io` for details.
 103
 104
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