In the Fortune's algorithm for 2D Voronoi Diagram, while adding circle events, we check whether the tangent to the circle will be in future of the current position of sweepline. I am trying to visualize a situation in which it will not be.
There are two scenarios when circle events can be added:
- Post a site event.
- Post a deletion of parabola.
I guess that for circle event post a site event the tangent to the circle will always be in future (or present). Correct me if I am wrong here.
I need help visualizing a scenario in which the the tangent to the circle is in the 'past' of sweepline. Any reference which gives such a scenario will be equally useful.