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059b1c5b | 1 | .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later |
407e84cd | 2 | .. c:namespace:: V4L |
82559ac0 | 3 | |
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4 | .. _libv4l-introduction: |
5 | ||
6 | ************ | |
7 | Introduction | |
8 | ************ | |
9 | ||
10 | libv4l is a collection of libraries which adds a thin abstraction layer | |
11 | on top of video4linux2 devices. The purpose of this (thin) layer is to | |
12 | make it easy for application writers to support a wide variety of | |
13 | devices without having to write separate code for different devices in | |
14 | the same class. | |
15 | ||
16 | An example of using libv4l is provided by | |
17 | :ref:`v4l2grab <v4l2grab-example>`. | |
18 | ||
19 | libv4l consists of 3 different libraries: | |
20 | ||
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21 | libv4lconvert |
22 | ============= | |
23 | ||
24 | libv4lconvert is a library that converts several different pixelformats | |
25 | found in V4L2 drivers into a few common RGB and YUY formats. | |
26 | ||
27 | It currently accepts the following V4L2 driver formats: | |
28 | :ref:`V4L2_PIX_FMT_BGR24 <V4L2-PIX-FMT-BGR24>`, | |
be25b043 | 29 | :ref:`V4L2_PIX_FMT_NV12_16L16 <V4L2-PIX-FMT-NV12-16L16>`, |
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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>`, | |
bb2ade02 | 48 | :ref:`V4L2_PIX_FMT_SRGGB8 <V4L2-PIX-FMT-SRGGB8>`, |
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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 | ||
55 | Later on libv4lconvert was expanded to also be able to do various video | |
56 | processing functions to improve webcam video quality. The video | |
57 | processing is split in to 2 parts: libv4lconvert/control and | |
58 | libv4lconvert/processing. | |
59 | ||
60 | The control part is used to offer video controls which can be used to | |
61 | control the video processing functions made available by | |
62 | libv4lconvert/processing. These controls are stored application wide | |
63 | (until reboot) by using a persistent shared memory object. | |
64 | ||
65 | libv4lconvert/processing offers the actual video processing | |
66 | functionality. | |
67 | ||
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68 | libv4l1 |
69 | ======= | |
70 | ||
71 | This library offers functions that can be used to quickly make v4l1 | |
72 | applications work with v4l2 devices. These functions work exactly like | |
73 | the normal open/close/etc, except that libv4l1 does full emulation of | |
74 | the v4l1 api on top of v4l2 drivers, in case of v4l1 drivers it will | |
75 | just pass calls through. | |
76 | ||
77 | Since those functions are emulations of the old V4L1 API, it shouldn't | |
78 | be used for new applications. | |
79 | ||
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80 | libv4l2 |
81 | ======= | |
82 | ||
83 | This library should be used for all modern V4L2 applications. | |
84 | ||
85 | It provides handles to call V4L2 open/ioctl/close/poll methods. Instead | |
86 | of just providing the raw output of the device, it enhances the calls in | |
87 | the sense that it will use libv4lconvert to provide more video formats | |
88 | and to enhance the image quality. | |
89 | ||
90 | In most cases, libv4l2 just passes the calls directly through to the | |
91 | v4l2 driver, intercepting the calls to | |
af4a4d0d | 92 | :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>`, |
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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 | |
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97 | order 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 | |
bb2ade02 | 102 | available in the driver. :ref:`VIDIOC_ENUM_FMT <VIDIOC_ENUM_FMT>` |
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103 | keeps enumerating the hardware supported formats, plus the emulated |
104 | formats offered by libv4l at the end. | |
105 | ||
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106 | .. _libv4l-ops: |
107 | ||
108 | Libv4l device control functions | |
109 | ------------------------------- | |
110 | ||
111 | The common file operation methods are provided by libv4l. | |
112 | ||
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113 | Those functions operate just like the gcc function ``dup()`` and |
114 | V4L2 functions | |
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115 | :c:func:`open()`, :c:func:`close()`, |
116 | :c:func:`ioctl()`, :c:func:`read()`, | |
117 | :c:func:`mmap()` and :c:func:`munmap()`: | |
5377d91f | 118 | |
9281f251 | 119 | .. c:function:: int v4l2_open(const char *file, int oflag, ...) |
5377d91f | 120 | |
407e84cd | 121 | operates like the :c:func:`open()` function. |
5377d91f | 122 | |
9281f251 | 123 | .. c:function:: int v4l2_close(int fd) |
5377d91f | 124 | |
407e84cd | 125 | operates like the :c:func:`close()` function. |
5377d91f | 126 | |
9281f251 | 127 | .. c:function:: int v4l2_dup(int fd) |
5377d91f | 128 | |
e452134c | 129 | operates like the libc ``dup()`` function, duplicating a file handler. |
5377d91f | 130 | |
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131 | .. c:function:: int v4l2_ioctl (int fd, unsigned long int request, ...) |
132 | ||
407e84cd | 133 | operates like the :c:func:`ioctl()` function. |
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134 | |
135 | .. c:function:: int v4l2_read (int fd, void* buffer, size_t n) | |
136 | ||
407e84cd | 137 | operates like the :c:func:`read()` function. |
9281f251 | 138 | |
5cd5f134 | 139 | .. c:function:: void *v4l2_mmap(void *start, size_t length, int prot, int flags, int fd, int64_t offset); |
9281f251 | 140 | |
5cd5f134 | 141 | operates like the :c:func:`mmap()` function. |
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142 | |
143 | .. c:function:: int v4l2_munmap(void *_start, size_t length); | |
e452134c | 144 | |
407e84cd | 145 | operates like the :c:func:`munmap()` function. |
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146 | |
147 | Those functions provide additional control: | |
148 | ||
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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 | |
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152 | modify libv4l2's default behavior through the ``v4l2_flags`` argument. |
153 | Currently, ``v4l2_flags`` can be ``V4L2_DISABLE_CONVERSION``, to disable | |
9281f251 | 154 | format conversion. |
5377d91f | 155 | |
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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 | |
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160 | not locked sets the cid to the scaled value. |
161 | ||
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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. | |
5377d91f | 167 | |
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168 | v4l1compat.so wrapper library |
169 | ============================= | |
170 | ||
e452134c | 171 | This library intercepts calls to |
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172 | :c:func:`open()`, :c:func:`close()`, |
173 | :c:func:`ioctl()`, :c:func:`mmap()` and | |
174 | :c:func:`munmap()` | |
5377d91f | 175 | operations and redirects them to the libv4l counterparts, by using |
e452134c | 176 | ``LD_PRELOAD=/usr/lib/v4l1compat.so``. It also emulates V4L1 calls via V4L2 |
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177 | API. |
178 | ||
179 | It allows usage of binary legacy applications that still don't use | |
180 | libv4l. |