Physics (Bullet) Client-side prediction

[Snowcrash5]

@0lento;547108

Substepping isn’t running at fully fixed intervals: it’s still synced to your existing framerate in the end. What it does is that it guarantees you’ll have more physics steps in case your framerate drops low (when without substepping you’d be in trouble). This doesn’t however give you fixed way to save same physics tick data on server and client because they may still run substepping at slightly different speeds. With fixed timestep, you’ll feed physx the exact same values on both client and server simulation and it should give you a lot more deterministic results. Ideally, you’d also apply and read your input on these fixed physics steps and also use same compressed input values even at local simulation in case you you compress floats on replication. This would guarantee that both simulations would get same input data.

Resimulating everything after receiving new input package from client would be total overkill. Also obviously you can’t just run the whole physics simulation in the past to make sure the slowest link is getting taken into account. Although depending on game type, this could actually be acceptable if you don’t need to do fast direction changes etc (for example something like boat/ship simulator could work just fine). I don’t have any magic bullets here, but in the nutshell, you’d probably want to only ever resimulate parts that really need to be simulated again for simulation to be accurate.

In this case, you could simulate every client’s rigidbody pawn separately on physics scene from it’s previous update to the current update and only if it happens to collide with some other clients rigidbody in past, then you’d resimulate them both to see the true collision between them. This can be pretty tricky to implement, but once done, it should give correct results and probably with least amount of CPU overhead (this would also help to reduce the workload on clients CPUs).

How can you simulate a physics tick on a Physics Scene? I’ve digged into the code and if I’m not wrong, at the start of the game tick, the game goes to the physics scene and calls StartFrame(), and when the end of the game tick comes (or the start of the next one?), the game gets the physic scene’s completion event, a graph event, with GetCompletionEvent(), and checks if its complete, if it is, it calls EndFrame(), and if not, it attaches a delegate to it that will be called when the physics tick ends, and will call EndFrame(), if I’ve understood that right.

I am probably missing the point completely, but what I’ve tried to do is do the same thing, call StartFrame(), and then get the completion event and attach a delegate to it, which will call EndFrame(), and do it a few times to resimulate my physics, but I just get a crash that says the completion event graph event does not exist yet. This is probably because I tried to get it right after starting the frame, and not later on.

I have no idea where to look from here, any help would be greatly appreciated