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libv4l is a collection of libraries which adds a thin abstraction layer on
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top of video4linux2 devices. The purpose of this (thin) layer is to make it
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easy for application writers to support a wide variety of devices without
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having to write seperate code for different devices in the same class.
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All libv4l components are licensed under the GNU Lesser General Public
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License version 2 or (at your option) any later version.
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libv4l consists of 3 different libraries:
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libv4lconvert started as a library to convert from any (known) pixelformat to
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V4l2_PIX_FMT_BGR24, RGB24, YUV420 or YVU420.
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The list of know source formats is large and continually growing, so instead
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of keeping an (almost always outdated) list here in the README, I refer you
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to the source, see the list of defines at the top of
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libv4lconvert/libv4lconvert.c for the full list.
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For more details on the v4lconvert_ functions see libv4lconvert.h.
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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 on a software basis. So
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the name no longer 100% covers the functionality. The video processing is
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split in to 2 parts, libv4lconvert/control and libv4lconvert/processing.
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The control part is used to offer video controls which can be used to control
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the video processing functions made available by libv4lconvert/processing.
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These controls are stored application wide (until reboot) by using a
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persistent shared memory object.
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libv4lconvert/processing offers the actual video processing functionality.
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This offers functions like v4l1_open, v4l1_ioctl, etc. which can by used to
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quickly make v4l1 applications work with v4l2 devices. These functions work
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exactly like the normal open/close/etc, except that libv4l1 does full emulation
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of the v4l1 api on top of v4l2 drivers, in case of v4l1 drivers it will just
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pass calls through. For more details on the v4l1_ functions see libv4l1.h .
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This offers functions like v4l2_open, v4l2_ioctl, etc. which can by used to
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quickly make v4l2 applications work with v4l2 devices with weird formats.
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libv4l2 mostly passes calls directly through to the v4l2 driver. When the
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app does a TRY_FMT / S_FMT with a not supported format libv4l2 will get in
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the middle and emulate the format (if an app wants to know which formats the
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hardware can _really_ do it should use ENUM_FMT, not randomly try a bunch of
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S_FMT's). For more details on the v4l2_ functions see libv4l2.h .
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The functionality provided by libv4l1 for v4l1 apps and libv4l2 for v4l2 apps
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can also be used by existing apps without modifying them. For this purpose
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2 wrapper libraries are provided which can be preloaded before starting the
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application using the LD_PRELOAD environment variable. These wrappers will
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then intercept calls to open/close/ioctl/etc. and if these calls directed
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towards a video device the wrapper will redirect the call to the libv4lX
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The preloadable libv4l1 wrapper which adds v4l2 device compatibility to v4l1
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applications is called v4l1compat.so. The preloadable libv4l2 wrapper which
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adds support for various pixelformats to v4l2 applications is called
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Example usage (after install in default location):
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$ export LD_PRELOAD=/usr/local/lib/libv4l/v4l1compat.so
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libv4l requires shmem file system support in the kernel (CONFIG_SHMEM).
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Installation Instructions
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-------------------------
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Simple type the following commands from the libv4l-x.y.z directory
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(adjusting PREFIX as desired):
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make install PREFIX=/usr/local
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Note: make install also supports the DESTDIR=... parameter for installation
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If you require static libraries to also be built, these can be compiled
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along with the dynamic equivalents by defining LINKTYPE to 'static', e.g.:
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make install LINKTYPE=static
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Q: Why libv4l, whats wrong with directly accessing v4l2 devices ?
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Q: Do we really need yet another library ?
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A: Current webcam using applications like ekiga contain code to handle many
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different specific pixelformats webcam's use, but that code only supports a
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small subset of all native webcam (compressed) pixelformats. Other current
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v4l2 applications do not support anything but rgb pixelformats (xawtv for
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example) and this will not work with most webcams at all.
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With gspca being ported to v4l2 and thus decoding to normal formats being
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removed from the device driver as this really belongs in userspace, ekiga
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would need to be extended with many more often chip dependent formats, like
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the bayer compression used by the spca561 and the (different) compression used
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by the pac207 and the (again different) compression used by the sn9c102. Adding
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support for all these formats should not be done at the application level, as
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then it needs to be written for each application seperately. Licensing issues
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with the decompressors will then also become a problem as just cut and pasting
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from one application to another is bound to hit license incompatibilities.
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So clearly this belongs in a library, and in a library with a license which
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allows this code to be used from as many different applications as possible.
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Hence libv4l was born.
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Q: Under which license may I use and distribute libv4l?
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A: The libv4l libraries are licensed under the GNU Library General Publishing
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License version 2 or (at your option) any later version. See the included
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COPYING.LIB file. The decompression helpers are licensed under the GNU
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Library Publishing License version 2 (as they are derived from kernel code)
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Q: Okay so I get the use of having a libv4lconvert, but why libv4l1 ?
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A: Many v4l2 drivers do not offer full v4l1 compatibility. They often do not
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implemented the CGMBUF ioctl and v4l1 style mmap call. Adding support to all
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these drivers for this is a lot of work and more importantly unnecessary
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adds code to kernel space.
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Also even if the CGMBUF ioctl and v4l1 style mmap are supported, then most
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cams still deliver pixelformats which v4l1 applications do not understand.
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This libv4l1 was born as an easy way to get v4l1 applications to work with
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v4l2 devices without requiring full v4l1 emulation (including format
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conversion) in the kernel, and without requiring major changes to the
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Q: Why should I use libv4l2 in my app instead of direct device access
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combined with libv4lconvert?
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A: libv4l2 is mainly meant for quickly and easily adding support for more
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pixelformats to existing v4l2 applications. So if you feel better directly
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accessing the device in combination with libv4lconvert thats fine too.
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Notice that libv4l2 also does emulation of the read() call on devices which
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do not support it in the driver. In the background this uses mmap buffers
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(even on devices which do support the read call). This mmap gives libv4lconvert
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zero-copy access to the captured frame, and then it can write the converted
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data directly to the buffer the application provided to v4l2_read(). Thus
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another reason to use liv4l2 is to get the no memcpy advantage of the mmap
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capture method combined with the simplicity of making a simple read() call.
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Q: Where to send bugreports / questions?
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A: Please send libv4l questions / bugreports to the:
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Linux Media Mailing List <linux-media@vger.kernel.org>
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Subscription is not necessary to send mail to this list. If you're not
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subscribed please put yourself in the CC of your original mail so you
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will receive replies.
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README.lib
