168 lines
8.3 KiB
XML
168 lines
8.3 KiB
XML
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<title>Libv4l Userspace Library</title>
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<section id="libv4l-introduction">
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<title>Introduction</title>
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<para>libv4l is a collection of libraries which adds a thin abstraction
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layer on top of video4linux2 devices. The purpose of this (thin) layer
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is to 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 the
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same class.</para>
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<para>An example of using libv4l is provided by
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<link linkend='v4l2grab-example'>v4l2grab</link>.
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</para>
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<para>libv4l consists of 3 different libraries:</para>
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<section>
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<title>libv4lconvert</title>
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<para>libv4lconvert is a library that converts several
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different pixelformats found in V4L2 drivers into a few common RGB and
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YUY formats.</para>
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<para>It currently accepts the following V4L2 driver formats:
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<link linkend="V4L2-PIX-FMT-BGR24"><constant>V4L2_PIX_FMT_BGR24</constant></link>,
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<link linkend="V4L2-PIX-FMT-HM12"><constant>V4L2_PIX_FMT_HM12</constant></link>,
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<link linkend="V4L2-PIX-FMT-JPEG"><constant>V4L2_PIX_FMT_JPEG</constant></link>,
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<link linkend="V4L2-PIX-FMT-MJPEG"><constant>V4L2_PIX_FMT_MJPEG</constant></link>,
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<link linkend="V4L2-PIX-FMT-MR97310A"><constant>V4L2_PIX_FMT_MR97310A</constant></link>,
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<link linkend="V4L2-PIX-FMT-OV511"><constant>V4L2_PIX_FMT_OV511</constant></link>,
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<link linkend="V4L2-PIX-FMT-OV518"><constant>V4L2_PIX_FMT_OV518</constant></link>,
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<link linkend="V4L2-PIX-FMT-PAC207"><constant>V4L2_PIX_FMT_PAC207</constant></link>,
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<link linkend="V4L2-PIX-FMT-PJPG"><constant>V4L2_PIX_FMT_PJPG</constant></link>,
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<link linkend="V4L2-PIX-FMT-RGB24"><constant>V4L2_PIX_FMT_RGB24</constant></link>,
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<link linkend="V4L2-PIX-FMT-SBGGR8"><constant>V4L2_PIX_FMT_SBGGR8</constant></link>,
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<link linkend="V4L2-PIX-FMT-SGBRG8"><constant>V4L2_PIX_FMT_SGBRG8</constant></link>,
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<link linkend="V4L2-PIX-FMT-SGRBG8"><constant>V4L2_PIX_FMT_SGRBG8</constant></link>,
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<link linkend="V4L2-PIX-FMT-SN9C10X"><constant>V4L2_PIX_FMT_SN9C10X</constant></link>,
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<link linkend="V4L2-PIX-FMT-SN9C20X-I420"><constant>V4L2_PIX_FMT_SN9C20X_I420</constant></link>,
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<link linkend="V4L2-PIX-FMT-SPCA501"><constant>V4L2_PIX_FMT_SPCA501</constant></link>,
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<link linkend="V4L2-PIX-FMT-SPCA505"><constant>V4L2_PIX_FMT_SPCA505</constant></link>,
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<link linkend="V4L2-PIX-FMT-SPCA508"><constant>V4L2_PIX_FMT_SPCA508</constant></link>,
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<link linkend="V4L2-PIX-FMT-SPCA561"><constant>V4L2_PIX_FMT_SPCA561</constant></link>,
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<link linkend="V4L2-PIX-FMT-SQ905C"><constant>V4L2_PIX_FMT_SQ905C</constant></link>,
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<constant>V4L2_PIX_FMT_SRGGB8</constant>,
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<link linkend="V4L2-PIX-FMT-UYVY"><constant>V4L2_PIX_FMT_UYVY</constant></link>,
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<link linkend="V4L2-PIX-FMT-YUV420"><constant>V4L2_PIX_FMT_YUV420</constant></link>,
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<link linkend="V4L2-PIX-FMT-YUYV"><constant>V4L2_PIX_FMT_YUYV</constant></link>,
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<link linkend="V4L2-PIX-FMT-YVU420"><constant>V4L2_PIX_FMT_YVU420</constant></link>,
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and <link linkend="V4L2-PIX-FMT-YVYU"><constant>V4L2_PIX_FMT_YVYU</constant></link>.
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</para>
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<para>Later on libv4lconvert was expanded to also be able to do
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various video processing functions to improve webcam video quality.
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The video processing is split in to 2 parts: libv4lconvert/control and
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libv4lconvert/processing.</para>
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<para>The control part is used to offer video controls which can
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be used to 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.</para>
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<para>libv4lconvert/processing offers the actual video
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processing functionality.</para>
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</section>
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<section>
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<title>libv4l1</title>
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<para>This library offers functions that can be used to quickly
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make v4l1 applications work with v4l2 devices. These functions work exactly
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like 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
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will just pass calls through.</para>
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<para>Since those functions are emulations of the old V4L1 API,
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it shouldn't be used for new applications.</para>
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</section>
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<section>
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<title>libv4l2</title>
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<para>This library should be used for all modern V4L2
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applications.</para>
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<para>It provides handles to call V4L2 open/ioctl/close/poll
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methods. Instead of just providing the raw output of the device, it enhances
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the calls in the sense that it will use libv4lconvert to provide more video
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formats and to enhance the image quality.</para>
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<para>In most cases, libv4l2 just passes the calls directly
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through to the v4l2 driver, intercepting the calls to
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<link linkend='vidioc-g-fmt'><constant>VIDIOC_TRY_FMT</constant></link>,
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<link linkend='vidioc-g-fmt'><constant>VIDIOC_G_FMT</constant></link>
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<link linkend='vidioc-g-fmt'><constant>VIDIOC_S_FMT</constant></link>
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<link linkend='vidioc-enum-framesizes'><constant>VIDIOC_ENUM_FRAMESIZES</constant></link>
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and <link linkend='vidioc-enum-frameintervals'><constant>VIDIOC_ENUM_FRAMEINTERVALS</constant></link>
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in order to emulate the formats
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<link linkend="V4L2-PIX-FMT-BGR24"><constant>V4L2_PIX_FMT_BGR24</constant></link>,
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<link linkend="V4L2-PIX-FMT-RGB24"><constant>V4L2_PIX_FMT_RGB24</constant></link>,
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<link linkend="V4L2-PIX-FMT-YUV420"><constant>V4L2_PIX_FMT_YUV420</constant></link>,
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and <link linkend="V4L2-PIX-FMT-YVU420"><constant>V4L2_PIX_FMT_YVU420</constant></link>,
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if they aren't available in the driver.
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<link linkend='vidioc-enum-fmt'><constant>VIDIOC_ENUM_FMT</constant></link>
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keeps enumerating the hardware supported formats, plus the emulated formats
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offered by libv4l at the end.
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</para>
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<section id="libv4l-ops">
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<title>Libv4l device control functions</title>
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<para>The common file operation methods are provided by
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libv4l.</para>
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<para>Those functions operate just like glibc
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open/close/dup/ioctl/read/mmap/munmap:</para>
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<itemizedlist><listitem>
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<para>int v4l2_open(const char *file, int oflag,
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...) -
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operates like the standard <link linkend='func-open'>open()</link> function.
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</para></listitem><listitem>
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<para>int v4l2_close(int fd) -
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operates like the standard <link linkend='func-close'>close()</link> function.
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</para></listitem><listitem>
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<para>int v4l2_dup(int fd) -
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operates like the standard dup() function, duplicating a file handler.
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</para></listitem><listitem>
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<para>int v4l2_ioctl (int fd, unsigned long int request, ...) -
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operates like the standard <link linkend='func-ioctl'>ioctl()</link> function.
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</para></listitem><listitem>
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<para>int v4l2_read (int fd, void* buffer, size_t n) -
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operates like the standard <link linkend='func-read'>read()</link> function.
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</para></listitem><listitem>
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<para>void v4l2_mmap(void *start, size_t length, int prot, int flags, int fd, int64_t offset); -
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operates like the standard <link linkend='func-mmap'>mmap()</link> function.
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</para></listitem><listitem>
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<para>int v4l2_munmap(void *_start, size_t length); -
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operates like the standard <link linkend='func-munmap'>munmap()</link> function.
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</para></listitem>
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</itemizedlist>
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<para>Those functions provide additional control:</para>
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<itemizedlist><listitem>
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<para>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 <constant>V4L2_DISABLE_CONVERSION</constant>,
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to disable format conversion.
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</para></listitem><listitem>
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<para>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
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the actual range of the given v4l control id, and then if the cid exists
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and is not locked sets the cid to the scaled value.
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</para></listitem><listitem>
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<para>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 occured 0 is returned.
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</para></listitem>
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</itemizedlist>
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</section>
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</section>
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<section>
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<title>v4l1compat.so wrapper library</title>
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<para>This library intercepts calls to
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open/close/ioctl/mmap/mmunmap operations and redirects them to the libv4l
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counterparts, by using LD_PRELOAD=/usr/lib/v4l1compat.so. It also
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emulates V4L1 calls via V4L2 API.</para>
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<para>It allows usage of binary legacy applications that
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still don't use libv4l.</para>
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</section>
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</section>
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<!--
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Local Variables:
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mode: sgml
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sgml-parent-document: "v4l2.sgml"
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indent-tabs-mode: nil
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End:
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-->
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