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+.. 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.