I always thought this was a super interesting problem, because you're balancing a bunch of stuff - emergent behavior, latency, and server load. Latency is the

I always thought this was a super interesting problem, because you're balancing a bunch of stuff - emergent behavior, latency, and server load. Latency is the obvious one - you have to be able to tell not only if a client can see another client, but whether they might be able to see each other in, say, 500ms. That's pretty hard and computationally intensive, at-least compared to just checking whether bounding boxes have line-of-sight. Where things get interesting is when we come to emergent behavior tied to rendering. A good example is shadows - you can have no LOS to a player, and not be in a position where you could get LOS quickly, but be able to see their shadow. So your client needs to know where the other player is so the shadow can be rendered correctly. Sound is also hard, and forces you to balance server load and client trust. Lets say someone really far away takes a step that's audible within 10m. You can't hear that, but the server would have to check distance against every player to confirm that. If you have a directional sound system (where sounds from inside a room will originate from the doorway for those outside) it's even harder, as the server is having to calculate the shortest path not blocked by the enviroment. Even if you do all this, you're still screwed - since when you determine the dishonest player can hear something so you tell their client client to play a sound, you have to send them the coordinates of the origin, so you're telling them where the other player is anyways!