An example of using OpenCV 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.
For macOS, you can use Brew (beware of the many dependencies!):
brew install opencv
Then, compile the native code in the gdnative/src directory:
cd gdnative/src && cmake . && make
This will produce library bouncy and will automatically put it in gdnative/bouncy.
The gdnlib file is ready for this setup for Windows, macOS and Linux 64-bit libraries. If you need more, you can edit the file gdnative/bouncy/bouncy.gdnlib with your editor of preference, or with Godot directly.