Video I/O with OpenCV Overview {#videoio_overview}
===================================

### See also:
  - @ref videoio "Video I/O Code Reference"
  - Tutorials: @ref tutorial_table_of_content_videoio

General Information
===================

The OpenCV @ref videoio module is a set of classes and functions to read and write video or images sequence.

Basically, the module provides the cv::VideoCapture and cv::VideoWriter classes as 2-layer interface to many video
I/O APIs used as backend.

![Video I/O with OpenCV](pics/videoio_overview.svg)

Some backends such as (DSHOW) Direct Show, Microsoft Media Foundation (MSMF),
Video 4 Linux (V4L), etc... are interfaces to the video I/O library provided by the operating system.

Some others backends like OpenNI2 for Kinect, Intel Perceptual Computing SDK, GStreamer,
XIMEA Camera API, etc...  are interfaces to proprietary drivers or to external library.

See the list of supported backends here: cv::VideoCaptureAPIs

@warning Some backends are experimental use them at your own risk
@note Each backend supports devices properties (cv::VideoCaptureProperties) in a different way or might not support any property at all.


Select the backend at runtime
-----------------------------

OpenCV automatically selects and uses first available backend (`apiPreference=cv::CAP_ANY`).

As advanced usage you can select the backend to use at runtime. Currently this option is
available only with %VideoCapture.

For example to grab from default camera using Direct Show as backend

```cpp
//declare a capture object
cv::VideoCapture cap(0, cv::CAP_DSHOW);

//or specify the apiPreference with open
cap.open(0, cv::CAP_DSHOW);
```

If you want to grab from a file using the Direct Show as backend:

```cpp
//declare a capture object
cv::VideoCapture cap(filename, cv::CAP_DSHOW);

//or specify the apiPreference with open
cap.open(filename, cv::CAP_DSHOW);
```

@sa cv::VideoCapture::open() , cv::VideoCapture::VideoCapture()

#### How to enable backends

There are two kinds of videoio backends: built-in backends and plugins which will be loaded at runtime (since OpenCV 4.1.0). Use functions cv::videoio_registry::getBackends, cv::videoio_registry::hasBackend and cv::videoio_registry::getBackendName to check actual presence of backend during runtime.

To enable built-in videoio backends:
  1. Enable corresponding CMake option, e.g. `-DWITH_GSTREAMER=ON`
  2. Rebuild OpenCV

To enable dynamically-loaded videoio backend (currently supported: GStreamer and FFmpeg on Linux, MediaSDK on Linux and Windows):
  1. Enable backend and add it to the list of plugins: `-DWITH_GSTREAMER=ON -DVIDEOIO_PLUGIN_LIST=gstreamer` CMake options
  2. Rebuild OpenCV
  3. Check that `libopencv_videoio_gstreamer.so` library exists in the `lib` directory

@note Don't forget to clean CMake cache when switching between these two modes

#### Use 3rd party drivers or cameras

Many industrial cameras or some video I/O devices don't provide standard driver interfaces
for the operating system. Thus you can't use  VideoCapture or VideoWriter with these devices.

To get access to their devices, manufactures provide their own C++ API and library that you have to
include and link with your OpenCV application.

Is common case that this libraries read/write images from/to a memory buffer. If it so, it is
possible to make a `Mat` header for memory buffer (user-allocated data) and process it
in-place using OpenCV functions. See cv::Mat::Mat() for more details.

The FFmpeg library
------------------

OpenCV can use the FFmpeg library (http://ffmpeg.org/) as backend to record, convert and stream audio and video.
FFMpeg is a complete, cross-reference solution. If you enable FFmpeg while configuring OpenCV than
CMake will download and install the binaries in `OPENCV_SOURCE_CODE/3rdparty/ffmpeg/`. To use
FFMpeg at runtime, you must deploy the FFMepg binaries with your application.

@note FFmpeg is licensed under the GNU Lesser General Public License (LGPL) version 2.1 or later.
See `OPENCV_SOURCE_CODE/3rdparty/ffmpeg/readme.txt` and http://ffmpeg.org/legal.html for details and
licensing information
