| .. -*- coding: utf-8; mode: rst -*- |
| |
| .. _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>`, |
| ``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` |
| :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` |
| :ref:`VIDIOC_ENUM_FRAMESIZES` and |
| :ref:`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` |
| 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 glibc |
| open/close/dup/ioctl/read/mmap/munmap: |
| |
| - int v4l2_open(const char *file, int oflag, ...) - operates like the |
| standard :ref:`open() <func-open>` function. |
| |
| - int v4l2_close(int fd) - operates like the standard |
| :ref:`close() <func-close>` function. |
| |
| - int v4l2_dup(int fd) - operates like the standard dup() function, |
| duplicating a file handler. |
| |
| - int v4l2_ioctl (int fd, unsigned long int request, ...) - operates |
| like the standard :ref:`ioctl() <func-ioctl>` function. |
| |
| - int v4l2_read (int fd, void* buffer, size_t n) - operates like the |
| standard :ref:`read() <func-read>` function. |
| |
| - void v4l2_mmap(void *start, size_t length, int prot, int flags, int |
| fd, int64_t offset); - operates like the standard |
| :ref:`mmap() <func-mmap>` function. |
| |
| - int v4l2_munmap(void *_start, size_t length); - operates like the |
| standard :ref:`munmap() <func-munmap>` function. |
| |
| Those functions provide additional control: |
| |
| - 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. |
| |
| - 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. |
| |
| - 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 open/close/ioctl/mmap/mmunmap |
| 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. |
| |
| |
| .. ------------------------------------------------------------------------------ |
| .. This file was automatically converted from DocBook-XML with the dbxml |
| .. library (https://github.com/return42/sphkerneldoc). The origin XML comes |
| .. from the linux kernel, refer to: |
| .. |
| .. * https://github.com/torvalds/linux/tree/master/Documentation/DocBook |
| .. ------------------------------------------------------------------------------ |