Physics not synchronizing correctly over the network when using Bullet

Question

I'm trying to implement a client/server physics system using Bullet however I'm having problems getting things to sync up.

I've implemented a custom motion state which reads and writes the transform from my game objects and it works locally but I've tried two different approaches for networked games:

1. Dynamic objects on the client that are also on the server (eg not random debris and other unimportant stuff) are made kinematic. This works correctly but the objects don't move very smoothly
2. Objects are dynamic on both but after each message from the server that the object has moved I set the linear and angular velocity to the values from the server and call btRigidBody::proceedToTransform with the transform on the server. I also call btCollisionObject::activate(true); to force the object to update.

My intent with method 2 was to basically do method 1 but hijacking Bullet to do a poor-man's prediction instead of doing my own to smooth out method 1, but this doesn't seem to work (for reasons that are not 100% clear to me even stepping through Bullet) and the objects sometimes end up in different places.

Am I heading in the right direction? Bullet seems to have it's own interpolation code built-in. Can that help me make method 1 work better? Or is my method 2 code not working because I am accidentally stomping that?

EDIT: Another problem with method 1 I just noticed is that the collision response will be way off for collisions against non-synchronized objects. Kinetic bodies kind of shoot stuff to infinity sometimes since they can't be knocked back.

Answer 1

You need proper client-side prediction.

You should really read in details the link Roy T. provided you in his comment. It describes what to do with player input & character physics, but the principle stays the same for "server-driven physics".

This isn't trivial to implement but in a few words, for game objects that need to be in sync:

• Run the physics on both the server and the client;
• Server sends updates regularly;
• Client continuously & smoothly re-adjusts its physics world to the server values.

So yes, you're heading in the right direction with your method 2. Just overriding the values isn't enough though, you will get jumps on the client, what you need to do is smoothly & continuously interpolate to the server values.

For your actual bug, I'm not familiar with Bullet, but you're probably missing some values e.g. you've set the linear and angular velocities, but have you set the accelerations?

Answer 2

What I do personally is whoever is hosting the game creates the phsyics world and sync objects with the clients. Even if its a p2p network scheme I still base the physics engine on one of the players clients.

Other physics that I use that is purely eye candy doesn't even need to be synchronized.

In a prototype I made a while back called "boilerzerker", I ran physics on the host and particle effects(also using physics) were not synced across a network but independent for each client because they were eye candy.

Answer 3

It is impossible to implement network synchronous physics worlds. Small difference in step N course much more bigger difference in step N+1 You can't apply forces or impulses to keep it sync and look realistic.

Solutions:-

1. You can consider to sync only few objects like characters or racing cars especially if they are kinematic. But most part of the world would not be in sync to look realistic.

2. You can have one physics worlds on server and broadcast object positions and velocities to clients.