what are the most widely used applications based on opencv using c++??

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As the name suggests, OpenCV does Computer Vision, which is not often used in user applications.

I think OpenCV is used mostly in robotics (the ROS and libmrpt are strongly integrated with OpenCV).

As OpenCV is very low-level, it is not directly used by applications, rather integrated in libraries that provide a simplified interface for specific tasks and can be used by apps as plugins or modules to provide extra functionnality.

For example video and image filters provided by the libavfilter and gmic libraries, face detection/recognition library libkf5kface for KDE apps, license plate recognition (openalpr), cell counting (opencfu) and so on.

There are also some PHP and Python libraries that can be used for server-side image recognition and automatic processing.

Some user applications that use OpenCV directly are Siril (for astronomical image processing) and Saga-GIS (for geological image processing).

OpenCV is used for a wide-range of Image Processing tasks - from smoothing images to stitching them together to form a panoramic images or processing images on a higher level and for instance finding all of the cats and dogs in a image. If you're getting started a good place to start is the excellent Pyimagesearch blog or possibly of more relevance a post of his that might be helpful. Most applications will have both a python and c++ implementation, the latter being used for computationally intensive applications - even so, there should be an implementation in both