An example of using OpenCV (version 4+) in your Godot game through GDNative API. It works with Godot 3.1+, the example allows you to control a ball initialized using face detection using optical flow in the camera (e.g. by waving hand).
You must have OpenCV installed and discoverable. Then, compile the native code in the native directory, this will produce library bouncy, which you have to copy to the native folder and, if needed, fix the path to it in bouncy.gdnlib file. Then you can open the project in Godot.