A Physics Engine with Incremental Rollback for Multiplayer Games

maxbendick•about 2 hours ago

As a long-time web/app developer getting into game dev, it feels like I'm entering "the big leagues" of software engineering. Tougher problems, more problems, more _interesting_ problems, and problems without prebuilt solutions. Much more fun than making yet another dashboard.

Combining player control, multiplayer, non-player control, and physics is one of the tougher problems. I got it handled (enough) for my project, but I'd be very interested to read the source if Easel's physics engine gets open-sourced.

BSTRhino•1 day 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.

stephc_int13•about 2 hours ago

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.

gafferongames•about 2 hours ago

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

BSTRhino•about 2 hours ago

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.

ornitorrincos•about 2 hours ago

How does it compare with deterministic physics engines, given that their appeal for multiplayer is that they can perform rollback?

gafferongames•about 2 hours ago

I think this is a cool idea. Well done!

BSTRhino•about 2 hours ago

Thank you so much!

mncharity•about 2 hours ago

Fwiw, one case where I've wanted rollback has been input fusion over interface devices with diverse latencies. You might have 10 ms for a keypress, 100 ms for optical tracking, and 1000 ms for speech. So given click+"the red one"(spoken), you might start running click+"the one in front"(default), and almost a second later rollback and rerun with "the red one". Or for real example, keypress event handling might branch on optical "pressed where on the keycap" and "by which finger", which won't become available for several frames.

riidom•about 3 hours ago

Can the games only be played on the easel site, or can I upload to e.g. itch.io as well (probably with the limitation of only singleplayer?)

BSTRhino•about 3 hours ago

You can upload to itch.io yes: https://easel.games/docs/learn/publishing/export

The export basically creates a page with an HTML IFRAME in it that embeds the hosted version of your game on easel.games so that all the multiplayer and leaderboards continue to work.

Thanks for your interest!

A Physics Engine with Incremental Rollback for Multiplayer Games

⚡ Community Insights

Discussion (18 Comments)Read Original on HackerNews