Prevent Cycles

[shaunlebron]

From Slime0 at reddit

I have drawn the base of boxes A and B. You can't determine the order they need to be drawn in unless you consider the boxes inbetween; if box C is between them, they must be drawn in order A, C, B. If box D is between them, they must be drawn in order B, D, A. If your algorithm adds the constraint that A must be drawn before B, when in reality the order must be B, D, A, you will end up with a cycle in your graph and the topological ordering can't be determined.

The drawing beneath that shows why you can't solve this problem for all cases in 3D: each box is obscured by one of the other boxes, creating a cycle.