181 lines
6.5 KiB
ReStructuredText
181 lines
6.5 KiB
ReStructuredText
.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
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.. c:namespace:: V4L
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.. _libv4l-introduction:
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************
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Introduction
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************
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libv4l is a collection of libraries which adds a thin abstraction layer
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on top of video4linux2 devices. The purpose of this (thin) layer is to
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make it easy for application writers to support a wide variety of
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devices without having to write separate code for different devices in
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the same class.
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An example of using libv4l is provided by
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:ref:`v4l2grab <v4l2grab-example>`.
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libv4l consists of 3 different libraries:
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libv4lconvert
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=============
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libv4lconvert is a library that converts several different pixelformats
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found in V4L2 drivers into a few common RGB and YUY formats.
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It currently accepts the following V4L2 driver formats:
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:ref:`V4L2_PIX_FMT_BGR24 <V4L2-PIX-FMT-BGR24>`,
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:ref:`V4L2_PIX_FMT_NV12_16L16 <V4L2-PIX-FMT-NV12-16L16>`,
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:ref:`V4L2_PIX_FMT_JPEG <V4L2-PIX-FMT-JPEG>`,
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:ref:`V4L2_PIX_FMT_MJPEG <V4L2-PIX-FMT-MJPEG>`,
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:ref:`V4L2_PIX_FMT_MR97310A <V4L2-PIX-FMT-MR97310A>`,
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:ref:`V4L2_PIX_FMT_OV511 <V4L2-PIX-FMT-OV511>`,
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:ref:`V4L2_PIX_FMT_OV518 <V4L2-PIX-FMT-OV518>`,
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:ref:`V4L2_PIX_FMT_PAC207 <V4L2-PIX-FMT-PAC207>`,
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:ref:`V4L2_PIX_FMT_PJPG <V4L2-PIX-FMT-PJPG>`,
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:ref:`V4L2_PIX_FMT_RGB24 <V4L2-PIX-FMT-RGB24>`,
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:ref:`V4L2_PIX_FMT_SBGGR8 <V4L2-PIX-FMT-SBGGR8>`,
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:ref:`V4L2_PIX_FMT_SGBRG8 <V4L2-PIX-FMT-SGBRG8>`,
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:ref:`V4L2_PIX_FMT_SGRBG8 <V4L2-PIX-FMT-SGRBG8>`,
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:ref:`V4L2_PIX_FMT_SN9C10X <V4L2-PIX-FMT-SN9C10X>`,
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:ref:`V4L2_PIX_FMT_SN9C20X_I420 <V4L2-PIX-FMT-SN9C20X-I420>`,
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:ref:`V4L2_PIX_FMT_SPCA501 <V4L2-PIX-FMT-SPCA501>`,
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:ref:`V4L2_PIX_FMT_SPCA505 <V4L2-PIX-FMT-SPCA505>`,
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:ref:`V4L2_PIX_FMT_SPCA508 <V4L2-PIX-FMT-SPCA508>`,
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:ref:`V4L2_PIX_FMT_SPCA561 <V4L2-PIX-FMT-SPCA561>`,
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:ref:`V4L2_PIX_FMT_SQ905C <V4L2-PIX-FMT-SQ905C>`,
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:ref:`V4L2_PIX_FMT_SRGGB8 <V4L2-PIX-FMT-SRGGB8>`,
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:ref:`V4L2_PIX_FMT_UYVY <V4L2-PIX-FMT-UYVY>`,
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:ref:`V4L2_PIX_FMT_YUV420 <V4L2-PIX-FMT-YUV420>`,
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:ref:`V4L2_PIX_FMT_YUYV <V4L2-PIX-FMT-YUYV>`,
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:ref:`V4L2_PIX_FMT_YVU420 <V4L2-PIX-FMT-YVU420>`, and
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:ref:`V4L2_PIX_FMT_YVYU <V4L2-PIX-FMT-YVYU>`.
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Later on libv4lconvert was expanded to also be able to do various video
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processing functions to improve webcam video quality. The video
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processing is split in to 2 parts: libv4lconvert/control and
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libv4lconvert/processing.
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The control part is used to offer video controls which can be used to
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control the video processing functions made available by
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libv4lconvert/processing. These controls are stored application wide
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(until reboot) by using a persistent shared memory object.
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libv4lconvert/processing offers the actual video processing
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functionality.
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libv4l1
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=======
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This library offers functions that can be used to quickly make v4l1
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applications work with v4l2 devices. These functions work exactly like
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the normal open/close/etc, except that libv4l1 does full emulation of
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the v4l1 api on top of v4l2 drivers, in case of v4l1 drivers it will
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just pass calls through.
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Since those functions are emulations of the old V4L1 API, it shouldn't
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be used for new applications.
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libv4l2
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=======
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This library should be used for all modern V4L2 applications.
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It provides handles to call V4L2 open/ioctl/close/poll methods. Instead
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of just providing the raw output of the device, it enhances the calls in
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the sense that it will use libv4lconvert to provide more video formats
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and to enhance the image quality.
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In most cases, libv4l2 just passes the calls directly through to the
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v4l2 driver, intercepting the calls to
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:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>`,
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:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`,
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:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`,
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:ref:`VIDIOC_ENUM_FRAMESIZES <VIDIOC_ENUM_FRAMESIZES>` and
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:ref:`VIDIOC_ENUM_FRAMEINTERVALS <VIDIOC_ENUM_FRAMEINTERVALS>` in
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order to emulate the formats
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:ref:`V4L2_PIX_FMT_BGR24 <V4L2-PIX-FMT-BGR24>`,
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:ref:`V4L2_PIX_FMT_RGB24 <V4L2-PIX-FMT-RGB24>`,
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:ref:`V4L2_PIX_FMT_YUV420 <V4L2-PIX-FMT-YUV420>`, and
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:ref:`V4L2_PIX_FMT_YVU420 <V4L2-PIX-FMT-YVU420>`, if they aren't
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available in the driver. :ref:`VIDIOC_ENUM_FMT <VIDIOC_ENUM_FMT>`
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keeps enumerating the hardware supported formats, plus the emulated
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formats offered by libv4l at the end.
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.. _libv4l-ops:
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Libv4l device control functions
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-------------------------------
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The common file operation methods are provided by libv4l.
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Those functions operate just like the gcc function ``dup()`` and
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V4L2 functions
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:c:func:`open()`, :c:func:`close()`,
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:c:func:`ioctl()`, :c:func:`read()`,
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:c:func:`mmap()` and :c:func:`munmap()`:
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.. c:function:: int v4l2_open(const char *file, int oflag, ...)
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operates like the :c:func:`open()` function.
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.. c:function:: int v4l2_close(int fd)
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operates like the :c:func:`close()` function.
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.. c:function:: int v4l2_dup(int fd)
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operates like the libc ``dup()`` function, duplicating a file handler.
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.. c:function:: int v4l2_ioctl (int fd, unsigned long int request, ...)
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operates like the :c:func:`ioctl()` function.
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.. c:function:: int v4l2_read (int fd, void* buffer, size_t n)
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operates like the :c:func:`read()` function.
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.. c:function:: void *v4l2_mmap(void *start, size_t length, int prot, int flags, int fd, int64_t offset);
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operates like the :c:func:`mmap()` function.
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.. c:function:: int v4l2_munmap(void *_start, size_t length);
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operates like the :c:func:`munmap()` function.
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Those functions provide additional control:
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.. c:function:: int v4l2_fd_open(int fd, int v4l2_flags)
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opens an already opened fd for further use through v4l2lib and possibly
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modify libv4l2's default behavior through the ``v4l2_flags`` argument.
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Currently, ``v4l2_flags`` can be ``V4L2_DISABLE_CONVERSION``, to disable
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format conversion.
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.. c:function:: int v4l2_set_control(int fd, int cid, int value)
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This function takes a value of 0 - 65535, and then scales that range to the
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actual range of the given v4l control id, and then if the cid exists and is
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not locked sets the cid to the scaled value.
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.. c:function:: int v4l2_get_control(int fd, int cid)
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This function returns a value of 0 - 65535, scaled to from the actual range
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of the given v4l control id. when the cid does not exist, could not be
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accessed for some reason, or some error occurred 0 is returned.
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v4l1compat.so wrapper library
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=============================
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This library intercepts calls to
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:c:func:`open()`, :c:func:`close()`,
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:c:func:`ioctl()`, :c:func:`mmap()` and
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:c:func:`munmap()`
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operations and redirects them to the libv4l counterparts, by using
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``LD_PRELOAD=/usr/lib/v4l1compat.so``. It also emulates V4L1 calls via V4L2
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API.
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It allows usage of binary legacy applications that still don't use
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libv4l.
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