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+.. Permission is granted to copy, distribute and/or modify this
+.. document under the terms of the GNU Free Documentation License,
+.. Version 1.1 or any later version published by the Free Software
+.. Foundation, with no Invariant Sections, no Front-Cover Texts
+.. and no Back-Cover Texts. A copy of the license is included at
+.. Documentation/userspace-api/media/fdl-appendix.rst.
+..
+.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
+
+.. _libv4l-introduction:
+
+************
+Introduction
+************
+
+libv4l is a collection of libraries which adds a thin abstraction layer
+on top of video4linux2 devices. The purpose of this (thin) layer is to
+make it easy for application writers to support a wide variety of
+devices without having to write separate code for different devices in
+the same class.
+
+An example of using libv4l is provided by
+:ref:`v4l2grab <v4l2grab-example>`.
+
+libv4l consists of 3 different libraries:
+
+
+libv4lconvert
+=============
+
+libv4lconvert is a library that converts several different pixelformats
+found in V4L2 drivers into a few common RGB and YUY formats.
+
+It currently accepts the following V4L2 driver formats:
+:ref:`V4L2_PIX_FMT_BGR24 <V4L2-PIX-FMT-BGR24>`,
+:ref:`V4L2_PIX_FMT_HM12 <V4L2-PIX-FMT-HM12>`,
+:ref:`V4L2_PIX_FMT_JPEG <V4L2-PIX-FMT-JPEG>`,
+:ref:`V4L2_PIX_FMT_MJPEG <V4L2-PIX-FMT-MJPEG>`,
+:ref:`V4L2_PIX_FMT_MR97310A <V4L2-PIX-FMT-MR97310A>`,
+:ref:`V4L2_PIX_FMT_OV511 <V4L2-PIX-FMT-OV511>`,
+:ref:`V4L2_PIX_FMT_OV518 <V4L2-PIX-FMT-OV518>`,
+:ref:`V4L2_PIX_FMT_PAC207 <V4L2-PIX-FMT-PAC207>`,
+:ref:`V4L2_PIX_FMT_PJPG <V4L2-PIX-FMT-PJPG>`,
+:ref:`V4L2_PIX_FMT_RGB24 <V4L2-PIX-FMT-RGB24>`,
+:ref:`V4L2_PIX_FMT_SBGGR8 <V4L2-PIX-FMT-SBGGR8>`,
+:ref:`V4L2_PIX_FMT_SGBRG8 <V4L2-PIX-FMT-SGBRG8>`,
+:ref:`V4L2_PIX_FMT_SGRBG8 <V4L2-PIX-FMT-SGRBG8>`,
+:ref:`V4L2_PIX_FMT_SN9C10X <V4L2-PIX-FMT-SN9C10X>`,
+:ref:`V4L2_PIX_FMT_SN9C20X_I420 <V4L2-PIX-FMT-SN9C20X-I420>`,
+:ref:`V4L2_PIX_FMT_SPCA501 <V4L2-PIX-FMT-SPCA501>`,
+:ref:`V4L2_PIX_FMT_SPCA505 <V4L2-PIX-FMT-SPCA505>`,
+:ref:`V4L2_PIX_FMT_SPCA508 <V4L2-PIX-FMT-SPCA508>`,
+:ref:`V4L2_PIX_FMT_SPCA561 <V4L2-PIX-FMT-SPCA561>`,
+:ref:`V4L2_PIX_FMT_SQ905C <V4L2-PIX-FMT-SQ905C>`,
+:ref:`V4L2_PIX_FMT_SRGGB8 <V4L2-PIX-FMT-SRGGB8>`,
+:ref:`V4L2_PIX_FMT_UYVY <V4L2-PIX-FMT-UYVY>`,
+:ref:`V4L2_PIX_FMT_YUV420 <V4L2-PIX-FMT-YUV420>`,
+:ref:`V4L2_PIX_FMT_YUYV <V4L2-PIX-FMT-YUYV>`,
+:ref:`V4L2_PIX_FMT_YVU420 <V4L2-PIX-FMT-YVU420>`, and
+:ref:`V4L2_PIX_FMT_YVYU <V4L2-PIX-FMT-YVYU>`.
+
+Later on libv4lconvert was expanded to also be able to do various video
+processing functions to improve webcam video quality. The video
+processing is split in to 2 parts: libv4lconvert/control and
+libv4lconvert/processing.
+
+The control part is used to offer video controls which can be used to
+control the video processing functions made available by
+libv4lconvert/processing. These controls are stored application wide
+(until reboot) by using a persistent shared memory object.
+
+libv4lconvert/processing offers the actual video processing
+functionality.
+
+
+libv4l1
+=======
+
+This library offers functions that can be used to quickly make v4l1
+applications work with v4l2 devices. These functions work exactly like
+the normal open/close/etc, except that libv4l1 does full emulation of
+the v4l1 api on top of v4l2 drivers, in case of v4l1 drivers it will
+just pass calls through.
+
+Since those functions are emulations of the old V4L1 API, it shouldn't
+be used for new applications.
+
+
+libv4l2
+=======
+
+This library should be used for all modern V4L2 applications.
+
+It provides handles to call V4L2 open/ioctl/close/poll methods. Instead
+of just providing the raw output of the device, it enhances the calls in
+the sense that it will use libv4lconvert to provide more video formats
+and to enhance the image quality.
+
+In most cases, libv4l2 just passes the calls directly through to the
+v4l2 driver, intercepting the calls to
+:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>`,
+:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`,
+:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`,
+:ref:`VIDIOC_ENUM_FRAMESIZES <VIDIOC_ENUM_FRAMESIZES>` and
+:ref:`VIDIOC_ENUM_FRAMEINTERVALS <VIDIOC_ENUM_FRAMEINTERVALS>` in
+order to emulate the formats
+:ref:`V4L2_PIX_FMT_BGR24 <V4L2-PIX-FMT-BGR24>`,
+:ref:`V4L2_PIX_FMT_RGB24 <V4L2-PIX-FMT-RGB24>`,
+:ref:`V4L2_PIX_FMT_YUV420 <V4L2-PIX-FMT-YUV420>`, and
+:ref:`V4L2_PIX_FMT_YVU420 <V4L2-PIX-FMT-YVU420>`, if they aren't
+available in the driver. :ref:`VIDIOC_ENUM_FMT <VIDIOC_ENUM_FMT>`
+keeps enumerating the hardware supported formats, plus the emulated
+formats offered by libv4l at the end.
+
+
+.. _libv4l-ops:
+
+Libv4l device control functions
+-------------------------------
+
+The common file operation methods are provided by libv4l.
+
+Those functions operate just like the gcc function ``dup()`` and
+V4L2 functions
+:c:func:`open() <v4l2-open>`, :c:func:`close() <v4l2-close>`,
+:c:func:`ioctl() <v4l2-ioctl>`, :c:func:`read() <v4l2-read>`,
+:c:func:`mmap() <v4l2-mmap>` and :c:func:`munmap() <v4l2-munmap>`:
+
+.. c:function:: int v4l2_open(const char *file, int oflag, ...)
+
+   operates like the :c:func:`open() <v4l2-open>` function.
+
+.. c:function:: int v4l2_close(int fd)
+
+   operates like the :c:func:`close() <v4l2-close>` function.
+
+.. c:function:: int v4l2_dup(int fd)
+
+   operates like the libc ``dup()`` function, duplicating a file handler.
+
+.. c:function:: int v4l2_ioctl (int fd, unsigned long int request, ...)
+
+   operates like the :c:func:`ioctl() <v4l2-ioctl>` function.
+
+.. c:function:: int v4l2_read (int fd, void* buffer, size_t n)
+
+   operates like the :c:func:`read() <v4l2-read>` function.
+
+.. c:function:: void v4l2_mmap(void *start, size_t length, int prot, int flags, int fd, int64_t offset);
+
+   operates like the :c:func:`munmap() <v4l2-munmap>` function.
+
+.. c:function:: int v4l2_munmap(void *_start, size_t length);
+
+   operates like the :c:func:`munmap() <v4l2-munmap>` function.
+
+Those functions provide additional control:
+
+.. c:function:: int v4l2_fd_open(int fd, int v4l2_flags)
+
+   opens an already opened fd for further use through v4l2lib and possibly
+   modify libv4l2's default behavior through the ``v4l2_flags`` argument.
+   Currently, ``v4l2_flags`` can be ``V4L2_DISABLE_CONVERSION``, to disable
+   format conversion.
+
+.. c:function:: int v4l2_set_control(int fd, int cid, int value)
+
+   This function takes a value of 0 - 65535, and then scales that range to the
+   actual range of the given v4l control id, and then if the cid exists and is
+   not locked sets the cid to the scaled value.
+
+.. c:function:: int v4l2_get_control(int fd, int cid)
+
+   This function returns a value of 0 - 65535, scaled to from the actual range
+   of the given v4l control id. when the cid does not exist, could not be
+   accessed for some reason, or some error occurred 0 is returned.
+
+
+v4l1compat.so wrapper library
+=============================
+
+This library intercepts calls to
+:c:func:`open() <v4l2-open>`, :c:func:`close() <v4l2-close>`,
+:c:func:`ioctl() <v4l2-ioctl>`, :c:func:`mmap() <v4l2-mmap>` and
+:c:func:`munmap() <v4l2-munmap>`
+operations and redirects them to the libv4l counterparts, by using
+``LD_PRELOAD=/usr/lib/v4l1compat.so``. It also emulates V4L1 calls via V4L2
+API.
+
+It allows usage of binary legacy applications that still don't use
+libv4l.