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1 | <title>Libv4l Userspace Library</title> |
2 | <section id="libv4l-introduction"> | |
3 | <title>Introduction</title> | |
4 | ||
5 | <para>libv4l is a collection of libraries which adds a thin abstraction | |
6 | layer on top of video4linux2 devices. The purpose of this (thin) layer | |
7 | is to make it easy for application writers to support a wide variety of | |
8 | devices without having to write separate code for different devices in the | |
9 | same class.</para> | |
10 | <para>An example of using libv4l is provided by | |
11 | <link linkend='v4l2grab-example'>v4l2grab</link>. | |
12 | </para> | |
13 | ||
14 | <para>libv4l consists of 3 different libraries:</para> | |
15 | <section> | |
16 | <title>libv4lconvert</title> | |
17 | ||
18 | <para>libv4lconvert is a library that converts several | |
19 | different pixelformats found in V4L2 drivers into a few common RGB and | |
20 | YUY formats.</para> | |
21 | <para>It currently accepts the following V4L2 driver formats: | |
22 | <link linkend="V4L2-PIX-FMT-BGR24"><constant>V4L2_PIX_FMT_BGR24</constant></link>, | |
23 | <link linkend="V4L2-PIX-FMT-HM12"><constant>V4L2_PIX_FMT_HM12</constant></link>, | |
24 | <link linkend="V4L2-PIX-FMT-JPEG"><constant>V4L2_PIX_FMT_JPEG</constant></link>, | |
25 | <link linkend="V4L2-PIX-FMT-MJPEG"><constant>V4L2_PIX_FMT_MJPEG</constant></link>, | |
26 | <link linkend="V4L2-PIX-FMT-MR97310A"><constant>V4L2_PIX_FMT_MR97310A</constant></link>, | |
27 | <link linkend="V4L2-PIX-FMT-OV511"><constant>V4L2_PIX_FMT_OV511</constant></link>, | |
28 | <link linkend="V4L2-PIX-FMT-OV518"><constant>V4L2_PIX_FMT_OV518</constant></link>, | |
29 | <link linkend="V4L2-PIX-FMT-PAC207"><constant>V4L2_PIX_FMT_PAC207</constant></link>, | |
30 | <link linkend="V4L2-PIX-FMT-PJPG"><constant>V4L2_PIX_FMT_PJPG</constant></link>, | |
31 | <link linkend="V4L2-PIX-FMT-RGB24"><constant>V4L2_PIX_FMT_RGB24</constant></link>, | |
32 | <link linkend="V4L2-PIX-FMT-SBGGR8"><constant>V4L2_PIX_FMT_SBGGR8</constant></link>, | |
33 | <link linkend="V4L2-PIX-FMT-SGBRG8"><constant>V4L2_PIX_FMT_SGBRG8</constant></link>, | |
34 | <link linkend="V4L2-PIX-FMT-SGRBG8"><constant>V4L2_PIX_FMT_SGRBG8</constant></link>, | |
35 | <link linkend="V4L2-PIX-FMT-SN9C10X"><constant>V4L2_PIX_FMT_SN9C10X</constant></link>, | |
36 | <link linkend="V4L2-PIX-FMT-SN9C20X-I420"><constant>V4L2_PIX_FMT_SN9C20X_I420</constant></link>, | |
37 | <link linkend="V4L2-PIX-FMT-SPCA501"><constant>V4L2_PIX_FMT_SPCA501</constant></link>, | |
38 | <link linkend="V4L2-PIX-FMT-SPCA505"><constant>V4L2_PIX_FMT_SPCA505</constant></link>, | |
39 | <link linkend="V4L2-PIX-FMT-SPCA508"><constant>V4L2_PIX_FMT_SPCA508</constant></link>, | |
40 | <link linkend="V4L2-PIX-FMT-SPCA561"><constant>V4L2_PIX_FMT_SPCA561</constant></link>, | |
41 | <link linkend="V4L2-PIX-FMT-SQ905C"><constant>V4L2_PIX_FMT_SQ905C</constant></link>, | |
42 | <constant>V4L2_PIX_FMT_SRGGB8</constant>, | |
43 | <link linkend="V4L2-PIX-FMT-UYVY"><constant>V4L2_PIX_FMT_UYVY</constant></link>, | |
44 | <link linkend="V4L2-PIX-FMT-YUV420"><constant>V4L2_PIX_FMT_YUV420</constant></link>, | |
45 | <link linkend="V4L2-PIX-FMT-YUYV"><constant>V4L2_PIX_FMT_YUYV</constant></link>, | |
46 | <link linkend="V4L2-PIX-FMT-YVU420"><constant>V4L2_PIX_FMT_YVU420</constant></link>, | |
47 | and <link linkend="V4L2-PIX-FMT-YVYU"><constant>V4L2_PIX_FMT_YVYU</constant></link>. | |
48 | </para> | |
49 | <para>Later on libv4lconvert was expanded to also be able to do | |
50 | various video processing functions to improve webcam video quality. | |
51 | The video processing is split in to 2 parts: libv4lconvert/control and | |
52 | libv4lconvert/processing.</para> | |
53 | ||
54 | <para>The control part is used to offer video controls which can | |
55 | be used to control the video processing functions made available by | |
56 | libv4lconvert/processing. These controls are stored application wide | |
57 | (until reboot) by using a persistent shared memory object.</para> | |
58 | ||
59 | <para>libv4lconvert/processing offers the actual video | |
60 | processing functionality.</para> | |
61 | </section> | |
62 | <section> | |
63 | <title>libv4l1</title> | |
64 | <para>This library offers functions that can be used to quickly | |
65 | make v4l1 applications work with v4l2 devices. These functions work exactly | |
66 | like the normal open/close/etc, except that libv4l1 does full emulation of | |
67 | the v4l1 api on top of v4l2 drivers, in case of v4l1 drivers it | |
68 | will just pass calls through.</para> | |
69 | <para>Since those functions are emulations of the old V4L1 API, | |
70 | it shouldn't be used for new applications.</para> | |
71 | </section> | |
72 | <section> | |
73 | <title>libv4l2</title> | |
74 | <para>This library should be used for all modern V4L2 | |
75 | applications.</para> | |
76 | <para>It provides handles to call V4L2 open/ioctl/close/poll | |
77 | methods. Instead of just providing the raw output of the device, it enhances | |
78 | the calls in the sense that it will use libv4lconvert to provide more video | |
79 | formats and to enhance the image quality.</para> | |
80 | <para>In most cases, libv4l2 just passes the calls directly | |
81 | through to the v4l2 driver, intercepting the calls to | |
82 | <link linkend='vidioc-g-fmt'><constant>VIDIOC_TRY_FMT</constant></link>, | |
83 | <link linkend='vidioc-g-fmt'><constant>VIDIOC_G_FMT</constant></link> | |
84 | <link linkend='vidioc-g-fmt'><constant>VIDIOC_S_FMT</constant></link> | |
85 | <link linkend='vidioc-enum-framesizes'><constant>VIDIOC_ENUM_FRAMESIZES</constant></link> | |
86 | and <link linkend='vidioc-enum-frameintervals'><constant>VIDIOC_ENUM_FRAMEINTERVALS</constant></link> | |
87 | in order to emulate the formats | |
88 | <link linkend="V4L2-PIX-FMT-BGR24"><constant>V4L2_PIX_FMT_BGR24</constant></link>, | |
89 | <link linkend="V4L2-PIX-FMT-RGB24"><constant>V4L2_PIX_FMT_RGB24</constant></link>, | |
90 | <link linkend="V4L2-PIX-FMT-YUV420"><constant>V4L2_PIX_FMT_YUV420</constant></link>, | |
91 | and <link linkend="V4L2-PIX-FMT-YVU420"><constant>V4L2_PIX_FMT_YVU420</constant></link>, | |
92 | if they aren't available in the driver. | |
93 | <link linkend='vidioc-enum-fmt'><constant>VIDIOC_ENUM_FMT</constant></link> | |
94 | keeps enumerating the hardware supported formats, plus the emulated formats | |
95 | offered by libv4l at the end. | |
96 | </para> | |
97 | <section id="libv4l-ops"> | |
98 | <title>Libv4l device control functions</title> | |
99 | <para>The common file operation methods are provided by | |
100 | libv4l.</para> | |
101 | <para>Those functions operate just like glibc | |
102 | open/close/dup/ioctl/read/mmap/munmap:</para> | |
103 | <itemizedlist><listitem> | |
104 | <para>int v4l2_open(const char *file, int oflag, | |
105 | ...) - | |
106 | operates like the standard <link linkend='func-open'>open()</link> function. | |
107 | </para></listitem><listitem> | |
108 | <para>int v4l2_close(int fd) - | |
109 | operates like the standard <link linkend='func-close'>close()</link> function. | |
110 | </para></listitem><listitem> | |
111 | <para>int v4l2_dup(int fd) - | |
112 | operates like the standard dup() function, duplicating a file handler. | |
113 | </para></listitem><listitem> | |
114 | <para>int v4l2_ioctl (int fd, unsigned long int request, ...) - | |
115 | operates like the standard <link linkend='func-ioctl'>ioctl()</link> function. | |
116 | </para></listitem><listitem> | |
117 | <para>int v4l2_read (int fd, void* buffer, size_t n) - | |
118 | operates like the standard <link linkend='func-read'>read()</link> function. | |
119 | </para></listitem><listitem> | |
120 | <para>void v4l2_mmap(void *start, size_t length, int prot, int flags, int fd, int64_t offset); - | |
121 | operates like the standard <link linkend='func-mmap'>mmap()</link> function. | |
122 | </para></listitem><listitem> | |
123 | <para>int v4l2_munmap(void *_start, size_t length); - | |
124 | operates like the standard <link linkend='func-munmap'>munmap()</link> function. | |
125 | </para></listitem> | |
126 | </itemizedlist> | |
127 | <para>Those functions provide additional control:</para> | |
128 | <itemizedlist><listitem> | |
129 | <para>int v4l2_fd_open(int fd, int v4l2_flags) - | |
130 | opens an already opened fd for further use through v4l2lib and possibly | |
131 | modify libv4l2's default behavior through the v4l2_flags argument. | |
132 | Currently, v4l2_flags can be <constant>V4L2_DISABLE_CONVERSION</constant>, | |
133 | to disable format conversion. | |
134 | </para></listitem><listitem> | |
135 | <para>int v4l2_set_control(int fd, int cid, int value) - | |
136 | This function takes a value of 0 - 65535, and then scales that range to | |
137 | the actual range of the given v4l control id, and then if the cid exists | |
138 | and is not locked sets the cid to the scaled value. | |
139 | </para></listitem><listitem> | |
140 | <para>int v4l2_get_control(int fd, int cid) - | |
141 | This function returns a value of 0 - 65535, scaled to from the actual range | |
142 | of the given v4l control id. when the cid does not exist, could not be | |
25985edc | 143 | accessed for some reason, or some error occurred 0 is returned. |
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144 | </para></listitem> |
145 | </itemizedlist> | |
146 | </section> | |
147 | </section> | |
148 | <section> | |
149 | ||
150 | <title>v4l1compat.so wrapper library</title> | |
151 | ||
152 | <para>This library intercepts calls to | |
153 | open/close/ioctl/mmap/mmunmap operations and redirects them to the libv4l | |
154 | counterparts, by using LD_PRELOAD=/usr/lib/v4l1compat.so. It also | |
155 | emulates V4L1 calls via V4L2 API.</para> | |
156 | <para>It allows usage of binary legacy applications that | |
157 | still don't use libv4l.</para> | |
158 | </section> | |
159 | ||
160 | </section> | |
161 | <!-- | |
162 | Local Variables: | |
163 | mode: sgml | |
164 | sgml-parent-document: "v4l2.sgml" | |
165 | indent-tabs-mode: nil | |
166 | End: | |
167 | --> |