OpenCV
3.4.16
Open Source Computer Vision
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The OpenCV Video I/O 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.
Some backends such as Direct Show (DSHOW), Video For Windows (VFW), 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
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
If you want to grab from a file using the Direct Show as backend:
Backends are available only if they have been built with your OpenCV binaries.
Check in opencv2/cvconfig.h
to know which APIs are currently available (e.g. HAVE_MSMF, HAVE_VFW, HAVE_LIBV4L
, etc...).
To enable/disable APIs, you have to:
-DWITH_MSMF=ON -DWITH_VFW=ON ...
) or checking related switch in cmake-guiMany 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.
It is a common case that these 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.
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 FFmpeg binaries with your application.
OPENCV_SOURCE_CODE/3rdparty/ffmpeg/readme.txt
and http://ffmpeg.org/legal.html for details and licensing information