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. 102