Recently, I came across the website https://www.pyimagesearch.com/ which has some of the greatest tutorials on OpenCV. I highly recommend to check it out.
OpenCV is the leading open-source library for computer vision, image processing, and machine learning and features GPU acceleration for real-time operation. It has C++, C, Python and Java interfaces and supports Windows, Linux, Mac OS, iOS and Android. OpenCV was designed for computational efficiency and with a strong focus on real-time applications. Written in optimized C/C++, the library can take advantage of multi-core processing.