Thanks to you; I never tested the zero/zero radius case - fixed that. It actually caused a division by zero.

@Superku: Indeed, there are visible artifacts when the ring is closed; but this will never be the case in the game, so that is acceptable for me. I don't see the 'lighting differences' you are talking about, the top surface is always clean to me. I'll add a PNG screenshot function for the next POC, that is a good idea