deepin-ocr/3rdparty/opencv-4.5.4/modules/videoio/doc/videoio_overview.markdown

Video I/O with OpenCV Overview

See also:

• @ref videoio "Video I/O Code Reference"
• Tutorials: @ref tutorial_table_of_content_app

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.

Some backends such as Direct Show (DSHOW), 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

//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:

//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.

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.

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 FFmpeg 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