1.. Permission is granted to copy, distribute and/or modify this 2.. document under the terms of the GNU Free Documentation License, 3.. Version 1.1 or any later version published by the Free Software 4.. Foundation, with no Invariant Sections, no Front-Cover Texts 5.. and no Back-Cover Texts. A copy of the license is included at 6.. Documentation/userspace-api/media/fdl-appendix.rst. 7.. 8.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections 9 10.. _capture: 11 12*********************** 13Video Capture Interface 14*********************** 15 16Video capture devices sample an analog video signal and store the 17digitized images in memory. Today nearly all devices can capture at full 1825 or 30 frames/second. With this interface applications can control the 19capture process and move images from the driver into user space. 20 21Conventionally V4L2 video capture devices are accessed through character 22device special files named ``/dev/video`` and ``/dev/video0`` to 23``/dev/video63`` with major number 81 and minor numbers 0 to 63. 24``/dev/video`` is typically a symbolic link to the preferred video 25device. 26 27.. note:: The same device file names are used for video output devices. 28 29 30Querying Capabilities 31===================== 32 33Devices supporting the video capture interface set the 34``V4L2_CAP_VIDEO_CAPTURE`` or ``V4L2_CAP_VIDEO_CAPTURE_MPLANE`` flag in 35the ``capabilities`` field of struct 36:c:type:`v4l2_capability` returned by the 37:ref:`VIDIOC_QUERYCAP` ioctl. As secondary device 38functions they may also support the :ref:`video overlay <overlay>` 39(``V4L2_CAP_VIDEO_OVERLAY``) and the :ref:`raw VBI capture <raw-vbi>` 40(``V4L2_CAP_VBI_CAPTURE``) interface. At least one of the read/write or 41streaming I/O methods must be supported. Tuners and audio inputs are 42optional. 43 44 45Supplemental Functions 46====================== 47 48Video capture devices shall support :ref:`audio input <audio>`, 49:ref:`tuner`, :ref:`controls <control>`, 50:ref:`cropping and scaling <crop>` and 51:ref:`streaming parameter <streaming-par>` ioctls as needed. The 52:ref:`video input <video>` ioctls must be supported by all video 53capture devices. 54 55 56Image Format Negotiation 57======================== 58 59The result of a capture operation is determined by cropping and image 60format parameters. The former select an area of the video picture to 61capture, the latter how images are stored in memory, i. e. in RGB or YUV 62format, the number of bits per pixel or width and height. Together they 63also define how images are scaled in the process. 64 65As usual these parameters are *not* reset at :ref:`open() <func-open>` 66time to permit Unix tool chains, programming a device and then reading 67from it as if it was a plain file. Well written V4L2 applications ensure 68they really get what they want, including cropping and scaling. 69 70Cropping initialization at minimum requires to reset the parameters to 71defaults. An example is given in :ref:`crop`. 72 73To query the current image format applications set the ``type`` field of 74a struct :c:type:`v4l2_format` to 75``V4L2_BUF_TYPE_VIDEO_CAPTURE`` or 76``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE`` and call the 77:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl with a pointer to this 78structure. Drivers fill the struct 79:c:type:`v4l2_pix_format` ``pix`` or the struct 80:c:type:`v4l2_pix_format_mplane` ``pix_mp`` 81member of the ``fmt`` union. 82 83To request different parameters applications set the ``type`` field of a 84struct :c:type:`v4l2_format` as above and initialize all 85fields of the struct :c:type:`v4l2_pix_format` 86``vbi`` member of the ``fmt`` union, or better just modify the results 87of :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, and call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` 88ioctl with a pointer to this structure. Drivers may adjust the 89parameters and finally return the actual parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` 90does. 91 92Like :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` the :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl 93can be used to learn about hardware limitations without disabling I/O or 94possibly time consuming hardware preparations. 95 96The contents of struct :c:type:`v4l2_pix_format` and 97struct :c:type:`v4l2_pix_format_mplane` are 98discussed in :ref:`pixfmt`. See also the specification of the 99:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctls for 100details. Video capture devices must implement both the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` 101and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, even if :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ignores all 102requests and always returns default parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does. 103:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` is optional. 104 105 106Reading Images 107============== 108 109A video capture device may support the :ref:`read() function <func-read>` 110and/or streaming (:ref:`memory mapping <func-mmap>` or 111:ref:`user pointer <userp>`) I/O. See :ref:`io` for details. 112