You are correct that you cannot see the line behind the ball. But the line is just that... it's a line, not a single point. You CAN see the line emerging from both sides behind the ball. Your eye/brain makes the computation that the diameter of the ball is past the line (in your example). I'm not saying that its not a tough call, or people don't make mistakes. It's just that your example has been used before in other threads and it is valid only if the line is not a line. There is a perspective factor which our brains take into account.