Running GetWorld()->Tick() multiple times per frame to facilitate rollbacks for netplay

[ITNoetic]

@TheJamsh;389732

P2P isn’t possible in UE4, and it won’t be, probably for a very long time - if ever. just because it’s on the roadmap doesn’t mean Epic is either working on or even considering it, it’s there for someone to look at later. I have no idea what GGPO stands for, but UE4’s CharacterMovementComponent already does Prediction and Reconciliation so you can use that for reference, and Unreal Tournament wrote their own slightly more optimized system as well. However if your using UE4 characters, you don’t need to do anything at all - it’s already done for you.

Unreals Server/Client model is a much more proven system than GGPO.

You may also want to know that UE4 isn’t deterministic internally, not even slightly, so even if you use the same input and somehow manage to sync up the timestamps, the results will drift over extended periods of time. You absolutely have to do server-authoritative corrections in Unreal, or your connections will go out of sync.

P2P has already been done in UE4. Rising Thunder was developed by the creator of GGPO, so I know that what I’m trying to do can work. I just don’t know quite how to finagle the engine into doing what I want, nor do I know exactly how much of the engine I need to avoid in order to do so. If at all possible, I would prefer not to have to either convert Character.cpp and CharacterMovement.cpp

@TheJamsh;389732

Replaying old moves is easy to understand but very hard to actually program in. I’m doing my own version for physics-based movement based on Glenn Fielders work (wish that dude hung around here!). You don’t tick the actor multiple times (that will cause you all kindzzz of problems), you simply apply the input multiple times in one frame for each out-of-date move, and update the results accordingly. You store a circular buffer of all moves with input, and when you receive a move from the server, you find out which one corresponds to it locally, delete any older moves, then play them back over a single frame using the stored delta time for that move and essentially ‘correct’ the move buffer. You move the collision of the object first, and offset the mesh so that it slowly blends back to the collision and doesn’t ‘snap’ on the clients view. This is roughly how CMC works.

This method sounds promising. Would you please elaborate? It sounds like every tick, I would store, say, the values I’m sending to the MoveForward and MoveRight functions, along with the frame number. Then, when it’s time to do a rollback, in GameMode’s tick function, I would reset each actor to their position on that frame, then iterate over each rolled-back frame for each PlayerController and call their MoveForward and MoveRight functions each frame with the same old data. Do I have that right?

@TheJamsh;389732

I suggest taking a look at this work by Glenn Fielder, since the premises are basically the same: Introduction to Networked Physics | Gaffer On Games
And, I’d also suggest looking at how CMC works and checking out UE4’s documentation: Understanding Networked Movement in the Character Movement Component for Unreal Engine | Unreal Engine 5.3 Documentation

Bottom line is, there’s a lot to be said for using the tools the way they are designed to be used. There’s no reason that a fast-paced game can’t used a Server-Authoritative model - which is much safer than P2P and far easier to program. The solidity of the latest Unreal Tournament is a testament to how well these models can work, even in high-latency situations. You don’t want to make your game optimized for a low-latency connection, you want it to work in ALL types of latency situations.

I’ve already made up my mind about this. To me, there is value in using GGPO that is difficult to articulate or justify to most people.