Comment by BSTRhino
2 days ago
Hi everyone,
I'm making a game engine that uses rollback netcode for its multiplayer architecture. As far as I can tell, no physics engine supports incremental rollback thus far. This means the entire physics engine state has to be snapshotted every frame, which basically means it's infeasible to have large worlds with rollback netcode. I've made a physics engine which only snapshots the changes, and so now I think you can have large worlds, as long as most of the world is static. I think that's true in most cases, like when you're walking around a big spaceship for example, all the walls, tables, control panels etc don't really move. I wrote up a bit of a post to describe some of the cool things I discovered while making my own physics engine.
Have you considered the opportunity of using delta compression on snapshots? Like the internal state of the physics simulation, most of the gamestate itself don't change between frames. Using delta compression on the whole structure is doable.
I was also curious about this, and I don't think the other replies understood what was being suggested.
If I understood correctly, the aim of the engine is to lower the in-memory size of the history of game states, by only snapshotting the delta. I'm also curious what would happen if, instead, you'd just run any deterministic snapshottable physics engine, and delta compressed the history on the fly. I think this is how, for example, Braid works.
Might be that it doesn't work, that running the delta check on two big enough snapshots would be too slow, and that's what this engine fixes. But would love to hear if it was considered.
The thing the author is trying to solve for here is reducing the amount of CPU used on the client when it rolls back the simulation and re-simulates to keep server authority.
He does this by only rolling back and re-simulating only a subset of the world, greatly reducing the amount of CPU required. It's cool that he's approaching this from the point of view of adding support for it in the physics engine itself, vs. making it something that the game has to do themselves.
Delta compression is an unrelated technique which reduce the amount of bandwidth sent from server to client, by sending only the differences between the snapshot at baseline frame n and the current snapshot frame m on the server.
Just want to clear this up for anybody trying to follow along. Bringing in delta compression is an unrelated thing (but somewhat similar conceptually). It might confuse people to talk about these things at the same time, if they're really just trying to understand what the author is doing in the article.
cheers
- Glenn
No, in this context I meant delta-compression in the case of local roll-back.
The entire gamestate has to be rolled back when using this style of netcode, regardless of bandwidth, reducing the size of snapshots in memory can also reduce make it faster to rebuild.
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Yes thank you, you understand perfectly and thank you for explaining. Also, I LOVE your series of blog posts, thank you for making them!
Just to add to the general discussion for everyone following along - rollback netcode only sends inputs around, not state, so it doesn't really have much to send. I think I'm doing about 1.5 KB per second. When you point your mouse it sends that data in 10 bytes. There's not a lot to delta compress.
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How does it compare with deterministic physics engines, given that their appeal for multiplayer is that they can perform rollback?
This physics engine is a deterministic physics engine. It has to be to make sure that when it rolls back and resimulates forward, it gets the same answer on all machines.
The determinism is partly possible because WebAssembly is deterministic (except for a few known cases https://github.com/WebAssembly/design/blob/main/Nondetermini...), and partly because I’m making sure to use my own trigonometric functions, and the entire game simulation is executed single threaded with a known order of execution.
If you meant to ask how does it compare to non deterministic physics engines, I’m sure they might be faster on the physics but would be slower on the rollback, and I think on most reasonably-sized games the slow rollback would dominate and so they would be slower overall. But, you wouldn’t make a rollback netcode game with that size of world anyway, at least maybe not until now, so it’s a bit of a false comparison. They’re good at their different use cases.
Planetary Annihilation did it and wrote and gave talks about it.
How big is the state that you want to rollback?
I think this is a cool idea. Well done!
Thank you so much!