# Intersection of centroid and line/rectangle

I'm currently making a traffic count system using opencv and python and what I exactly intend to do is that,to increment the counter as soon as the centroid of the blob passes or crosses a line or a small rectangle. My question is how to do this as I don't know how to implement cv2.lines or cv2.rectangle in this. Please see this video and if anybody could tell how exactly the intersection or counting is working in this video: https://www.youtube.com/watch?v=z1Cvn...

Thanks in advance.