Multiplayer Push

Development Programming & Scripting Multiplayer & Networking
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Hey!

Me and a couple of friends are working on a party game in Unreal Engine. We understand replication pretty well and are able to replicate certain mechanics. But we cant seem te replicate our Push. We have been stuck on our Push for Months now.

It works well for the server. After we tried to replicate it, the clients could also Push other players but is was more a teleport then a push. In engine with no simulated lag it does work. But when we simulate having like 20 or 30 ping the client just teleports.

We also included our code and a link to the video of our issue,

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Thank you,

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Player/Player interactions are always trouble in multiplayer games.

The way to get them “perfect,” is to use the input-synchronous, delayed-command pattern. Every command has to go to the server first, and is only executed once it comes back from the server. That way, all commands can be executed in the same way on all clients.
Ideally you do this with a deterministic, constant step size simulation engine, too.

The draw-back of that model, is that you have to live with a server round-trip time for every input. This is totally acceptable for RTS games, but not so good for twitch games.

In your game, you have to make up your mind on who is “authority” of a “push” operation. Either the pushed, or the pusher, will be “authority” and their position will “win.” The other side will very likely see a correction, because the time of contact doesn’t happen the same in the delayed frame, as it does in the original frame. Note that two characters colliding may have different opinion about who is the “pusher!”

If you want it to look similar for everybody, perhaps one option is to actually allow pawn/pawn overlap (so physics doesn’t push,) and instead detect pawn/pawn overlap, and send a command that separates the two pawns with a pre-determined position animation/vector. Because this command will be executed with the same amount of separation over the same amount of time on both clients, it will look more similar than a physics-based collision solution. You’ll typically want to manage this as bidirectional – “player A and B are separating” – with no difference in ordering, rather than trying to implement “A pushes B so A stands still” because B will see something different on their side.

Of course, you will still run into cases where player 1 runs ahead of player 2 on their own client, and sees themselves pushing player 2, whereas player 2 runs ahead of player 1 on their client, and thinks that player 1 misses. If player 2 starts being pushed there, they might be surprised at that outcome. This is not a “physics” problem, this is a “speed of light” problem – people will have separate world views because of latency, and separate (split) worldviews cannot possibly be reconciled.

1 Like
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Thank you for Sharing your sight of view and how to solve this

but as a game development student what do you prefer to use to fix this problem?
Wich technique?

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If command latency is acceptable (RTS style) then I’d use deterministic command queues.

if command latency is not acceptable, then I’d implement a “push” event that pushes both players equally hard in opposite direction, and make a rule that the same player pair can only have one push event per half-second or so. I would detect the need to push probably on collision, and send it to the server, but also locally pre-execute it for instant feedback. I would separately just stop a player when they (locally) run into another player, and send a “player stopped” message to the server.

Another option is to let a player use a “push” action that launches the other player but not themselves, like an explicit attack. Make it succeed if they players are close enough. Make it send an event that the “attack” has happened, and play it the same way on both clients and on the server.