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diff --git a/Documentation/userspace-api/media/v4l/dev-capture.rst b/Documentation/userspace-api/media/v4l/dev-capture.rst new file mode 100644 index 000000000..fe58fd450 --- /dev/null +++ b/Documentation/userspace-api/media/v4l/dev-capture.rst @@ -0,0 +1,101 @@ +.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later +.. c:namespace:: V4L + +.. _capture: + +*********************** +Video Capture Interface +*********************** + +Video capture devices sample an analog video signal and store the +digitized images in memory. Today nearly all devices can capture at full +25 or 30 frames/second. With this interface applications can control the +capture process and move images from the driver into user space. + +Conventionally V4L2 video capture devices are accessed through character +device special files named ``/dev/video`` and ``/dev/video0`` to +``/dev/video63`` with major number 81 and minor numbers 0 to 63. +``/dev/video`` is typically a symbolic link to the preferred video +device. + +.. note:: The same device file names are used for video output devices. + +Querying Capabilities +===================== + +Devices supporting the video capture interface set the +``V4L2_CAP_VIDEO_CAPTURE`` or ``V4L2_CAP_VIDEO_CAPTURE_MPLANE`` flag in +the ``capabilities`` field of struct +:c:type:`v4l2_capability` returned by the +:ref:`VIDIOC_QUERYCAP` ioctl. As secondary device +functions they may also support the :ref:`video overlay <overlay>` +(``V4L2_CAP_VIDEO_OVERLAY``) and the :ref:`raw VBI capture <raw-vbi>` +(``V4L2_CAP_VBI_CAPTURE``) interface. At least one of the read/write or +streaming I/O methods must be supported. Tuners and audio inputs are +optional. + +Supplemental Functions +====================== + +Video capture devices shall support :ref:`audio input <audio>`, +:ref:`tuner`, :ref:`controls <control>`, +:ref:`cropping and scaling <crop>` and +:ref:`streaming parameter <streaming-par>` ioctls as needed. The +:ref:`video input <video>` ioctls must be supported by all video +capture devices. + +Image Format Negotiation +======================== + +The result of a capture operation is determined by cropping and image +format parameters. The former select an area of the video picture to +capture, the latter how images are stored in memory, i. e. in RGB or YUV +format, the number of bits per pixel or width and height. Together they +also define how images are scaled in the process. + +As usual these parameters are *not* reset at :c:func:`open()` +time to permit Unix tool chains, programming a device and then reading +from it as if it was a plain file. Well written V4L2 applications ensure +they really get what they want, including cropping and scaling. + +Cropping initialization at minimum requires to reset the parameters to +defaults. An example is given in :ref:`crop`. + +To query the current image format applications set the ``type`` field of +a struct :c:type:`v4l2_format` to +``V4L2_BUF_TYPE_VIDEO_CAPTURE`` or +``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE`` and call the +:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl with a pointer to this +structure. Drivers fill the struct +:c:type:`v4l2_pix_format` ``pix`` or the struct +:c:type:`v4l2_pix_format_mplane` ``pix_mp`` +member of the ``fmt`` union. + +To request different parameters applications set the ``type`` field of a +struct :c:type:`v4l2_format` as above and initialize all +fields of the struct :c:type:`v4l2_pix_format` +``vbi`` member of the ``fmt`` union, or better just modify the results +of :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, and call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` +ioctl with a pointer to this structure. Drivers may adjust the +parameters and finally return the actual parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` +does. + +Like :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` the :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl +can be used to learn about hardware limitations without disabling I/O or +possibly time consuming hardware preparations. + +The contents of struct :c:type:`v4l2_pix_format` and +struct :c:type:`v4l2_pix_format_mplane` are +discussed in :ref:`pixfmt`. See also the specification of the +:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctls for +details. Video capture devices must implement both the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` +and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, even if :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ignores all +requests and always returns default parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does. +:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` is optional. + +Reading Images +============== + +A video capture device may support the :ref:`read() function <func-read>` +and/or streaming (:ref:`memory mapping <func-mmap>` or +:ref:`user pointer <userp>`) I/O. See :ref:`io` for details. |