1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2 3.. _capture: 4 5*********************** 6Video Capture Interface 7*********************** 8 9Video capture devices sample an analog video signal and store the 10digitized images in memory. Today nearly all devices can capture at full 1125 or 30 frames/second. With this interface applications can control the 12capture process and move images from the driver into user space. 13 14Conventionally V4L2 video capture devices are accessed through character 15device 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 18device. 19 20.. note:: The same device file names are used for video output devices. 21 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 37 38Supplemental Functions 39====================== 40 41Video capture devices shall support :ref:`audio input <audio>`, 42:ref:`tuner`, :ref:`controls <control>`, 43:ref:`cropping and scaling <crop>` and 44:ref:`streaming parameter <streaming-par>` ioctls as needed. The 45:ref:`video input <video>` ioctls must be supported by all video 46capture devices. 47 48 49Image Format Negotiation 50======================== 51 52The result of a capture operation is determined by cropping and image 53format parameters. The former select an area of the video picture to 54capture, the latter how images are stored in memory, i. e. in RGB or YUV 55format, the number of bits per pixel or width and height. Together they 56also define how images are scaled in the process. 57 58As usual these parameters are *not* reset at :ref:`open() <func-open>` 59time to permit Unix tool chains, programming a device and then reading 60from it as if it was a plain file. Well written V4L2 applications ensure 61they really get what they want, including cropping and scaling. 62 63Cropping initialization at minimum requires to reset the parameters to 64defaults. An example is given in :ref:`crop`. 65 66To query the current image format applications set the ``type`` field of 67a struct :c:type:`v4l2_format` to 68``V4L2_BUF_TYPE_VIDEO_CAPTURE`` or 69``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE`` and call the 70:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl with a pointer to this 71structure. Drivers fill the struct 72:c:type:`v4l2_pix_format` ``pix`` or the struct 73:c:type:`v4l2_pix_format_mplane` ``pix_mp`` 74member of the ``fmt`` union. 75 76To request different parameters applications set the ``type`` field of a 77struct :c:type:`v4l2_format` as above and initialize all 78fields of the struct :c:type:`v4l2_pix_format` 79``vbi`` member of the ``fmt`` union, or better just modify the results 80of :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, and call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` 81ioctl with a pointer to this structure. Drivers may adjust the 82parameters and finally return the actual parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` 83does. 84 85Like :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` the :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl 86can be used to learn about hardware limitations without disabling I/O or 87possibly time consuming hardware preparations. 88 89The contents of struct :c:type:`v4l2_pix_format` and 90struct :c:type:`v4l2_pix_format_mplane` are 91discussed in :ref:`pixfmt`. See also the specification of the 92:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctls for 93details. Video capture devices must implement both the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` 94and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, even if :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ignores all 95requests and always returns default parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does. 96:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` is optional. 97 98 99Reading Images 100============== 101 102A video capture device may support the :ref:`read() function <func-read>` 103and/or streaming (:ref:`memory mapping <func-mmap>` or 104:ref:`user pointer <userp>`) I/O. See :ref:`io` for details. 105