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Synchronizing physics, when physics is ran client-side?

Started by pjuke Apr 24, 2013 at 12:45 PM 2 replies 3k views

Original Post

Apr 24, 2013 at 12:45 PM

I'm developing a web-based networked game using HTML5 and Box2D as physics engine.

The network communication is by Web Sockets.

Player A can spawn objects in the world, while Player B controlls an actor that can interact with them (jumping on them and such).

But here's the problem I've come across:

How do I synchronize the spawning objects from Player A, to Player B?

I mean. Imagine if Player B is running across the screen, and Player A spawns an object that is falling to the ground.

Now, due to the latency, the actor is running a bit behind on Player A's screen, and therefore, the spawning object might collide with the actor on Player A's screen. But in reality, the actor on Player B's screen have already run pass that point, and therefore, no collission.

It's a very basic game, but now I just realized I'm stuck and don't know how to solve it really.

I guess I have to do something with the server.

At the moment, the server only relays the messages between the clients, no physics or so is ran at the server-side. All physics is on client-side.

Thanks in advance! :-)

hplus0603

Apr 24, 2013 at 6:10 PM

There's no perfect solution, because the web introduces latency.
One common work-around is to make player movements have lag that depends on server round-trip time.
Thus, the player doesn't start (or stop) moving on the local screen until it gets the command from the server to do so.
This way, you know that the state on the server is always correct, and the state on the client simply mirrors the state on the server.

pjuke

Apr 24, 2013 at 6:44 PM

There's no perfect solution, because the web introduces latency.
One common work-around is to make player movements have lag that depends on server round-trip time.
Thus, the player doesn't start (or stop) moving on the local screen until it gets the command from the server to do so.
This way, you know that the state on the server is always correct, and the state on the client simply mirrors the state on the server.

So if I got this correct, both players needs the server to tell the player to move. That is, even when the user press' the forward key, the actor wont start to move until it gets the response from the server? Like the first Quake game, with no client-side prediction?

I see that solution would work, but I need immediate response from inputs tho. Especially from the player which spawns objects.

I'm currently using a jitter-buffer on the receiving side, so it is already a little bit of excessive latency going on.

Thank you very much for replying anyway.

Do you know any other 'common' solution?

hplus0603

Apr 25, 2013 at 6:43 PM

The other common solution is to detect inconsistencies and "snap" the player.

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