Portal rendering [1] was actually a well-known hidden-surface removal method long before the game Portal, right there with Quake-style BSP trees. It's actually

Portal rendering [1] was actually a well-known hidden-surface removal method long before the game Portal, right there with Quake-style BSP trees. It's actually somewhat surprising that it took so long for someone to turn them into an actual game mechanic rather than just an implementation detail, though I do believe some games used them to create non-Euclidean topologies… [1] https://en.wikipedia.org/wiki/Portal_rendering https://en.wikipedia.org/wiki/Portal_rendering