ofxCv getting position of flow

I’m trying to get the trend in the flow with ofVideoGrabber using ofxCV.

If there is motion in the frame I want to track where that motion goes to and store that in a variable. I have been looking at the optical flow stuff in ofxCv particularly getFlowPosition but this seems to just get the flow at that x y position rather than tracking where the motion goes too.

Any ideas would be great.
Thanks