98 lines
4.0 KiB
Markdown
98 lines
4.0 KiB
Markdown
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Video I/O with OpenCV Overview {#videoio_overview}
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===================================
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### See also:
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- @ref videoio "Video I/O Code Reference"
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- Tutorials: @ref tutorial_table_of_content_app
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General Information
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===================
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The OpenCV @ref videoio module is a set of classes and functions to read and write video or images sequence.
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Basically, the module provides the cv::VideoCapture and cv::VideoWriter classes as 2-layer interface to many video
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I/O APIs used as backend.
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Some backends such as Direct Show (DSHOW), Microsoft Media Foundation (MSMF),
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Video 4 Linux (V4L), etc... are interfaces to the video I/O library provided by the operating system.
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Some others backends like OpenNI2 for Kinect, Intel Perceptual Computing SDK, GStreamer,
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XIMEA Camera API, etc... are interfaces to proprietary drivers or to external library.
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See the list of supported backends here: cv::VideoCaptureAPIs
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@warning Some backends are experimental use them at your own risk
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@note Each backend supports devices properties (cv::VideoCaptureProperties) in a different way or might not support any property at all.
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Select the backend at runtime
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-----------------------------
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OpenCV automatically selects and uses first available backend (`apiPreference=cv::CAP_ANY`).
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As advanced usage you can select the backend to use at runtime. Currently this option is
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available only with %VideoCapture.
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For example to grab from default camera using Direct Show as backend
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```cpp
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//declare a capture object
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cv::VideoCapture cap(0, cv::CAP_DSHOW);
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//or specify the apiPreference with open
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cap.open(0, cv::CAP_DSHOW);
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```
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If you want to grab from a file using the Direct Show as backend:
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```cpp
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//declare a capture object
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cv::VideoCapture cap(filename, cv::CAP_DSHOW);
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//or specify the apiPreference with open
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cap.open(filename, cv::CAP_DSHOW);
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```
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@sa cv::VideoCapture::open() , cv::VideoCapture::VideoCapture()
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#### How to enable backends
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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.
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To enable built-in videoio backends:
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1. Enable corresponding CMake option, e.g. `-DWITH_GSTREAMER=ON`
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2. Rebuild OpenCV
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To enable dynamically-loaded videoio backend (currently supported: GStreamer and FFmpeg on Linux, MediaSDK on Linux and Windows):
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1. Enable backend and add it to the list of plugins: `-DWITH_GSTREAMER=ON -DVIDEOIO_PLUGIN_LIST=gstreamer` CMake options
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2. Rebuild OpenCV
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3. Check that `libopencv_videoio_gstreamer.so` library exists in the `lib` directory
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@note Don't forget to clean CMake cache when switching between these two modes
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#### Use 3rd party drivers or cameras
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Many industrial cameras or some video I/O devices don't provide standard driver interfaces
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for the operating system. Thus you can't use VideoCapture or VideoWriter with these devices.
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To get access to their devices, manufactures provide their own C++ API and library that you have to
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include and link with your OpenCV application.
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It is a common case that these libraries read/write images from/to a memory buffer. If it so, it is
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possible to make a `Mat` header for memory buffer (user-allocated data) and process it
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in-place using OpenCV functions. See cv::Mat::Mat() for more details.
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The FFmpeg library
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------------------
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OpenCV can use the FFmpeg library (http://ffmpeg.org/) as backend to record, convert and stream audio and video.
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FFmpeg is a complete, cross-reference solution. If you enable FFmpeg while configuring OpenCV than
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CMake will download and install the binaries in `OPENCV_SOURCE_CODE/3rdparty/ffmpeg/`. To use
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FFmpeg at runtime, you must deploy the FFmpeg binaries with your application.
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@note FFmpeg is licensed under the GNU Lesser General Public License (LGPL) version 2.1 or later.
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See `OPENCV_SOURCE_CODE/3rdparty/ffmpeg/readme.txt` and http://ffmpeg.org/legal.html for details and
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licensing information
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