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diff --git a/Documentation/userspace-api/media/v4l/libv4l-introduction.rst b/Documentation/userspace-api/media/v4l/libv4l-introduction.rst new file mode 100644 index 000000000..05690f235 --- /dev/null +++ b/Documentation/userspace-api/media/v4l/libv4l-introduction.rst @@ -0,0 +1,180 @@ +.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later +.. c:namespace:: V4L + +.. _libv4l-introduction: + +************ +Introduction +************ + +libv4l is a collection of libraries which adds a thin abstraction layer +on top of video4linux2 devices. The purpose of this (thin) layer is to +make it easy for application writers to support a wide variety of +devices without having to write separate code for different devices in +the same class. + +An example of using libv4l is provided by +:ref:`v4l2grab <v4l2grab-example>`. + +libv4l consists of 3 different libraries: + +libv4lconvert +============= + +libv4lconvert is a library that converts several different pixelformats +found in V4L2 drivers into a few common RGB and YUY formats. + +It currently accepts the following V4L2 driver formats: +:ref:`V4L2_PIX_FMT_BGR24 <V4L2-PIX-FMT-BGR24>`, +:ref:`V4L2_PIX_FMT_HM12 <V4L2-PIX-FMT-HM12>`, +:ref:`V4L2_PIX_FMT_JPEG <V4L2-PIX-FMT-JPEG>`, +:ref:`V4L2_PIX_FMT_MJPEG <V4L2-PIX-FMT-MJPEG>`, +:ref:`V4L2_PIX_FMT_MR97310A <V4L2-PIX-FMT-MR97310A>`, +:ref:`V4L2_PIX_FMT_OV511 <V4L2-PIX-FMT-OV511>`, +:ref:`V4L2_PIX_FMT_OV518 <V4L2-PIX-FMT-OV518>`, +:ref:`V4L2_PIX_FMT_PAC207 <V4L2-PIX-FMT-PAC207>`, +:ref:`V4L2_PIX_FMT_PJPG <V4L2-PIX-FMT-PJPG>`, +:ref:`V4L2_PIX_FMT_RGB24 <V4L2-PIX-FMT-RGB24>`, +:ref:`V4L2_PIX_FMT_SBGGR8 <V4L2-PIX-FMT-SBGGR8>`, +:ref:`V4L2_PIX_FMT_SGBRG8 <V4L2-PIX-FMT-SGBRG8>`, +:ref:`V4L2_PIX_FMT_SGRBG8 <V4L2-PIX-FMT-SGRBG8>`, +:ref:`V4L2_PIX_FMT_SN9C10X <V4L2-PIX-FMT-SN9C10X>`, +:ref:`V4L2_PIX_FMT_SN9C20X_I420 <V4L2-PIX-FMT-SN9C20X-I420>`, +:ref:`V4L2_PIX_FMT_SPCA501 <V4L2-PIX-FMT-SPCA501>`, +:ref:`V4L2_PIX_FMT_SPCA505 <V4L2-PIX-FMT-SPCA505>`, +:ref:`V4L2_PIX_FMT_SPCA508 <V4L2-PIX-FMT-SPCA508>`, +:ref:`V4L2_PIX_FMT_SPCA561 <V4L2-PIX-FMT-SPCA561>`, +:ref:`V4L2_PIX_FMT_SQ905C <V4L2-PIX-FMT-SQ905C>`, +:ref:`V4L2_PIX_FMT_SRGGB8 <V4L2-PIX-FMT-SRGGB8>`, +:ref:`V4L2_PIX_FMT_UYVY <V4L2-PIX-FMT-UYVY>`, +:ref:`V4L2_PIX_FMT_YUV420 <V4L2-PIX-FMT-YUV420>`, +:ref:`V4L2_PIX_FMT_YUYV <V4L2-PIX-FMT-YUYV>`, +:ref:`V4L2_PIX_FMT_YVU420 <V4L2-PIX-FMT-YVU420>`, and +:ref:`V4L2_PIX_FMT_YVYU <V4L2-PIX-FMT-YVYU>`. + +Later on libv4lconvert was expanded to also be able to do various video +processing functions to improve webcam video quality. The video +processing is split in to 2 parts: libv4lconvert/control and +libv4lconvert/processing. + +The control part is used to offer video controls which can be used to +control the video processing functions made available by +libv4lconvert/processing. These controls are stored application wide +(until reboot) by using a persistent shared memory object. + +libv4lconvert/processing offers the actual video processing +functionality. + +libv4l1 +======= + +This library offers functions that can be used to quickly make v4l1 +applications work with v4l2 devices. These functions work exactly like +the normal open/close/etc, except that libv4l1 does full emulation of +the v4l1 api on top of v4l2 drivers, in case of v4l1 drivers it will +just pass calls through. + +Since those functions are emulations of the old V4L1 API, it shouldn't +be used for new applications. + +libv4l2 +======= + +This library should be used for all modern V4L2 applications. + +It provides handles to call V4L2 open/ioctl/close/poll methods. Instead +of just providing the raw output of the device, it enhances the calls in +the sense that it will use libv4lconvert to provide more video formats +and to enhance the image quality. + +In most cases, libv4l2 just passes the calls directly through to the +v4l2 driver, intercepting the calls to +:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>`, +:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, +:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`, +:ref:`VIDIOC_ENUM_FRAMESIZES <VIDIOC_ENUM_FRAMESIZES>` and +:ref:`VIDIOC_ENUM_FRAMEINTERVALS <VIDIOC_ENUM_FRAMEINTERVALS>` in +order to emulate the formats +:ref:`V4L2_PIX_FMT_BGR24 <V4L2-PIX-FMT-BGR24>`, +:ref:`V4L2_PIX_FMT_RGB24 <V4L2-PIX-FMT-RGB24>`, +:ref:`V4L2_PIX_FMT_YUV420 <V4L2-PIX-FMT-YUV420>`, and +:ref:`V4L2_PIX_FMT_YVU420 <V4L2-PIX-FMT-YVU420>`, if they aren't +available in the driver. :ref:`VIDIOC_ENUM_FMT <VIDIOC_ENUM_FMT>` +keeps enumerating the hardware supported formats, plus the emulated +formats offered by libv4l at the end. + +.. _libv4l-ops: + +Libv4l device control functions +------------------------------- + +The common file operation methods are provided by libv4l. + +Those functions operate just like the gcc function ``dup()`` and +V4L2 functions +:c:func:`open()`, :c:func:`close()`, +:c:func:`ioctl()`, :c:func:`read()`, +:c:func:`mmap()` and :c:func:`munmap()`: + +.. c:function:: int v4l2_open(const char *file, int oflag, ...) + + operates like the :c:func:`open()` function. + +.. c:function:: int v4l2_close(int fd) + + operates like the :c:func:`close()` function. + +.. c:function:: int v4l2_dup(int fd) + + operates like the libc ``dup()`` function, duplicating a file handler. + +.. c:function:: int v4l2_ioctl (int fd, unsigned long int request, ...) + + operates like the :c:func:`ioctl()` function. + +.. c:function:: int v4l2_read (int fd, void* buffer, size_t n) + + operates like the :c:func:`read()` function. + +.. c:function:: void v4l2_mmap(void *start, size_t length, int prot, int flags, int fd, int64_t offset); + + operates like the :c:func:`munmap()` function. + +.. c:function:: int v4l2_munmap(void *_start, size_t length); + + operates like the :c:func:`munmap()` function. + +Those functions provide additional control: + +.. c:function:: int v4l2_fd_open(int fd, int v4l2_flags) + + opens an already opened fd for further use through v4l2lib and possibly + modify libv4l2's default behavior through the ``v4l2_flags`` argument. + Currently, ``v4l2_flags`` can be ``V4L2_DISABLE_CONVERSION``, to disable + format conversion. + +.. c:function:: int v4l2_set_control(int fd, int cid, int value) + + This function takes a value of 0 - 65535, and then scales that range to the + actual range of the given v4l control id, and then if the cid exists and is + not locked sets the cid to the scaled value. + +.. c:function:: int v4l2_get_control(int fd, int cid) + + This function returns a value of 0 - 65535, scaled to from the actual range + of the given v4l control id. when the cid does not exist, could not be + accessed for some reason, or some error occurred 0 is returned. + +v4l1compat.so wrapper library +============================= + +This library intercepts calls to +:c:func:`open()`, :c:func:`close()`, +:c:func:`ioctl()`, :c:func:`mmap()` and +:c:func:`munmap()` +operations and redirects them to the libv4l counterparts, by using +``LD_PRELOAD=/usr/lib/v4l1compat.so``. It also emulates V4L1 calls via V4L2 +API. + +It allows usage of binary legacy applications that still don't use +libv4l. |