The Open Source Computer Vision Library has >2500 algorithms, extensive documentation and sample code for real-time computer vision. It works on Windows, Linux, Mac OS X, Android and iOS.
Q&A forum: answers.opencv.org
Please pay special attention to our tutorials! http://docs.opencv.org/3.2.0/d9/df8/tutorial_root.html
Books about the OpenCV are described here: http://opencv.org/books.html
A SourceForge treasure.