1 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2 .. c:namespace:: V4L 3 4 .. _capture: 5 6 *********************** 7 Video Capture Interface 8 *********************** 9 10 Video capture devices sample an analog video signal and store the 11 digitized images in memory. Today nearly all devices can capture at full 12 25 or 30 frames/second. With this interface applications can control the 13 capture process and move images from the driver into user space. 14 15 Conventionally V4L2 video capture devices are accessed through character 16 device 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 19 device. 20 21 .. note:: The same device file names are used for video output devices. 22 23 Querying Capabilities 24 ===================== 25 26 Devices supporting the video capture interface set the 27 ``V4L2_CAP_VIDEO_CAPTURE`` or ``V4L2_CAP_VIDEO_CAPTURE_MPLANE`` flag in 28 the ``capabilities`` field of struct 29 :c:type:`v4l2_capability` returned by the 30 :ref:`VIDIOC_QUERYCAP` ioctl. As secondary device 31 functions 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 34 streaming I/O methods must be supported. Tuners and audio inputs are 35 optional. 36 37 Supplemental Functions 38 ====================== 39 40 Video 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 45 capture devices. 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 :c: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 :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 69 structure. Drivers fill the struct 70 :c:type:`v4l2_pix_format` ``pix`` or the struct 71 :c:type:`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 :c:type:`v4l2_format` as above and initialize all 76 fields of the struct :c:type:`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 :c:type:`v4l2_pix_format` and 88 struct :c:type:`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 Reading Images 97 ============== 98 99 A video capture device may support the :ref:`read() function <func-read>` 100 and/or streaming (:ref:`memory mapping <func-mmap>` or 101 :ref:`user pointer <userp>`) I/O. See :ref:`io` for details.
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