1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2.. c:namespace:: V4L 3 4.. _libv4l-introduction: 5 6************ 7Introduction 8************ 9 10libv4l is a collection of libraries which adds a thin abstraction layer 11on top of video4linux2 devices. The purpose of this (thin) layer is to 12make it easy for application writers to support a wide variety of 13devices without having to write separate code for different devices in 14the same class. 15 16An example of using libv4l is provided by 17:ref:`v4l2grab <v4l2grab-example>`. 18 19libv4l consists of 3 different libraries: 20 21libv4lconvert 22============= 23 24libv4lconvert is a library that converts several different pixelformats 25found in V4L2 drivers into a few common RGB and YUY formats. 26 27It currently accepts the following V4L2 driver formats: 28:ref:`V4L2_PIX_FMT_BGR24 <V4L2-PIX-FMT-BGR24>`, 29:ref:`V4L2_PIX_FMT_HM12 <V4L2-PIX-FMT-HM12>`, 30:ref:`V4L2_PIX_FMT_JPEG <V4L2-PIX-FMT-JPEG>`, 31:ref:`V4L2_PIX_FMT_MJPEG <V4L2-PIX-FMT-MJPEG>`, 32:ref:`V4L2_PIX_FMT_MR97310A <V4L2-PIX-FMT-MR97310A>`, 33:ref:`V4L2_PIX_FMT_OV511 <V4L2-PIX-FMT-OV511>`, 34:ref:`V4L2_PIX_FMT_OV518 <V4L2-PIX-FMT-OV518>`, 35:ref:`V4L2_PIX_FMT_PAC207 <V4L2-PIX-FMT-PAC207>`, 36:ref:`V4L2_PIX_FMT_PJPG <V4L2-PIX-FMT-PJPG>`, 37:ref:`V4L2_PIX_FMT_RGB24 <V4L2-PIX-FMT-RGB24>`, 38:ref:`V4L2_PIX_FMT_SBGGR8 <V4L2-PIX-FMT-SBGGR8>`, 39:ref:`V4L2_PIX_FMT_SGBRG8 <V4L2-PIX-FMT-SGBRG8>`, 40:ref:`V4L2_PIX_FMT_SGRBG8 <V4L2-PIX-FMT-SGRBG8>`, 41:ref:`V4L2_PIX_FMT_SN9C10X <V4L2-PIX-FMT-SN9C10X>`, 42:ref:`V4L2_PIX_FMT_SN9C20X_I420 <V4L2-PIX-FMT-SN9C20X-I420>`, 43:ref:`V4L2_PIX_FMT_SPCA501 <V4L2-PIX-FMT-SPCA501>`, 44:ref:`V4L2_PIX_FMT_SPCA505 <V4L2-PIX-FMT-SPCA505>`, 45:ref:`V4L2_PIX_FMT_SPCA508 <V4L2-PIX-FMT-SPCA508>`, 46:ref:`V4L2_PIX_FMT_SPCA561 <V4L2-PIX-FMT-SPCA561>`, 47:ref:`V4L2_PIX_FMT_SQ905C <V4L2-PIX-FMT-SQ905C>`, 48:ref:`V4L2_PIX_FMT_SRGGB8 <V4L2-PIX-FMT-SRGGB8>`, 49:ref:`V4L2_PIX_FMT_UYVY <V4L2-PIX-FMT-UYVY>`, 50:ref:`V4L2_PIX_FMT_YUV420 <V4L2-PIX-FMT-YUV420>`, 51:ref:`V4L2_PIX_FMT_YUYV <V4L2-PIX-FMT-YUYV>`, 52:ref:`V4L2_PIX_FMT_YVU420 <V4L2-PIX-FMT-YVU420>`, and 53:ref:`V4L2_PIX_FMT_YVYU <V4L2-PIX-FMT-YVYU>`. 54 55Later on libv4lconvert was expanded to also be able to do various video 56processing functions to improve webcam video quality. The video 57processing is split in to 2 parts: libv4lconvert/control and 58libv4lconvert/processing. 59 60The control part is used to offer video controls which can be used to 61control the video processing functions made available by 62libv4lconvert/processing. These controls are stored application wide 63(until reboot) by using a persistent shared memory object. 64 65libv4lconvert/processing offers the actual video processing 66functionality. 67 68libv4l1 69======= 70 71This library offers functions that can be used to quickly make v4l1 72applications work with v4l2 devices. These functions work exactly like 73the normal open/close/etc, except that libv4l1 does full emulation of 74the v4l1 api on top of v4l2 drivers, in case of v4l1 drivers it will 75just pass calls through. 76 77Since those functions are emulations of the old V4L1 API, it shouldn't 78be used for new applications. 79 80libv4l2 81======= 82 83This library should be used for all modern V4L2 applications. 84 85It provides handles to call V4L2 open/ioctl/close/poll methods. Instead 86of just providing the raw output of the device, it enhances the calls in 87the sense that it will use libv4lconvert to provide more video formats 88and to enhance the image quality. 89 90In most cases, libv4l2 just passes the calls directly through to the 91v4l2 driver, intercepting the calls to 92:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>`, 93:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, 94:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`, 95:ref:`VIDIOC_ENUM_FRAMESIZES <VIDIOC_ENUM_FRAMESIZES>` and 96:ref:`VIDIOC_ENUM_FRAMEINTERVALS <VIDIOC_ENUM_FRAMEINTERVALS>` in 97order to emulate the formats 98:ref:`V4L2_PIX_FMT_BGR24 <V4L2-PIX-FMT-BGR24>`, 99:ref:`V4L2_PIX_FMT_RGB24 <V4L2-PIX-FMT-RGB24>`, 100:ref:`V4L2_PIX_FMT_YUV420 <V4L2-PIX-FMT-YUV420>`, and 101:ref:`V4L2_PIX_FMT_YVU420 <V4L2-PIX-FMT-YVU420>`, if they aren't 102available in the driver. :ref:`VIDIOC_ENUM_FMT <VIDIOC_ENUM_FMT>` 103keeps enumerating the hardware supported formats, plus the emulated 104formats offered by libv4l at the end. 105 106.. _libv4l-ops: 107 108Libv4l device control functions 109------------------------------- 110 111The common file operation methods are provided by libv4l. 112 113Those functions operate just like the gcc function ``dup()`` and 114V4L2 functions 115:c:func:`open()`, :c:func:`close()`, 116:c:func:`ioctl()`, :c:func:`read()`, 117:c:func:`mmap()` and :c:func:`munmap()`: 118 119.. c:function:: int v4l2_open(const char *file, int oflag, ...) 120 121 operates like the :c:func:`open()` function. 122 123.. c:function:: int v4l2_close(int fd) 124 125 operates like the :c:func:`close()` function. 126 127.. c:function:: int v4l2_dup(int fd) 128 129 operates like the libc ``dup()`` function, duplicating a file handler. 130 131.. c:function:: int v4l2_ioctl (int fd, unsigned long int request, ...) 132 133 operates like the :c:func:`ioctl()` function. 134 135.. c:function:: int v4l2_read (int fd, void* buffer, size_t n) 136 137 operates like the :c:func:`read()` function. 138 139.. c:function:: void v4l2_mmap(void *start, size_t length, int prot, int flags, int fd, int64_t offset); 140 141 operates like the :c:func:`munmap()` function. 142 143.. c:function:: int v4l2_munmap(void *_start, size_t length); 144 145 operates like the :c:func:`munmap()` function. 146 147Those functions provide additional control: 148 149.. c:function:: int v4l2_fd_open(int fd, int v4l2_flags) 150 151 opens an already opened fd for further use through v4l2lib and possibly 152 modify libv4l2's default behavior through the ``v4l2_flags`` argument. 153 Currently, ``v4l2_flags`` can be ``V4L2_DISABLE_CONVERSION``, to disable 154 format conversion. 155 156.. c:function:: int v4l2_set_control(int fd, int cid, int value) 157 158 This function takes a value of 0 - 65535, and then scales that range to the 159 actual range of the given v4l control id, and then if the cid exists and is 160 not locked sets the cid to the scaled value. 161 162.. c:function:: int v4l2_get_control(int fd, int cid) 163 164 This function returns a value of 0 - 65535, scaled to from the actual range 165 of the given v4l control id. when the cid does not exist, could not be 166 accessed for some reason, or some error occurred 0 is returned. 167 168v4l1compat.so wrapper library 169============================= 170 171This library intercepts calls to 172:c:func:`open()`, :c:func:`close()`, 173:c:func:`ioctl()`, :c:func:`mmap()` and 174:c:func:`munmap()` 175operations and redirects them to the libv4l counterparts, by using 176``LD_PRELOAD=/usr/lib/v4l1compat.so``. It also emulates V4L1 calls via V4L2 177API. 178 179It allows usage of binary legacy applications that still don't use 180libv4l. 181