192 lines
7.0 KiB
ReStructuredText
192 lines
7.0 KiB
ReStructuredText
.. 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/media/uapi/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.
|