Comment by olaulaja

It can be rather difficult, mostly due to the occlusion calculations having to be conservative (must count visible things as visible, allowed to count invisible as visible, or things pop) and latency (must account for every possible position within max move speed * max latency, or things pop)

The naive raycast from player camera to other player would be fine for perf but may count partially visible as invisible, so its unacceptable. You'd have to raycast every pixel of the potentially visible player model to stay conservative. With movement + latency this expands to every pixel the player model could potentially occupy during your max latency period, and you need to consider the viewer moving too!

In practice this expands to a visibility test between two spheres with radius max_latency*max_movespeed + player_model_radius. Now, you could theoretically do a bunch of random raycasts between the spheres and get an answer that is right some of the time, but it would be a serious violation of our conservativeness criteria and the performance would get worse with more rays/better results. Also keep in mind that we need to do this for every single player/player pair a few dozen times per second, so it needs to be fast!

To do this, you need a dedicated data structure that maps volumes to other volumes visible from said volume. There are a few, and they are all non-trivial and/or slow to build well. (google for eg. potentially visible set, cell-portal graph + occlusion). You also trade performance for precision, and in practice you walls might become 'transparent' a bit too early. With all this being done, we can actually "do occlusion calculations server-side".

There's just one problem with this that I still don't know a solution for, namely precision. With fast players and imprecise conservative visibility, things you care about are going to count as visible pretty often, including stuff like enemies peeking from behind a corner (because they could have moved at full sprint for 100ms and the end of the wall is rounded away in your acceleration structure anyway) so all this complexity might not get you that much, particularly if your game is fast paced. You'd prevent some wallhacks but not the ones that really matter.

TLDR yes, it's actually hard and might not be good enough anyway