Deterministic physics, same platform

Questions & Answers
1

I'm aware that unity physics aren't deterministic on different platforms, but what about a consistent platform like IOS? When answering this question, please can you confirm whether this applies to the latest Unity engine (3 at this point) or whether it is information gleaned from older posts.

2

I would be very wary of basing my project/application on the assumption that physics are deterministic, even on a given platform. For if it eventually reveals to be untrue, if you believe 'some guy on the internetz who told you it was', you'll end up with a massive problem on your hands.

In your situation, I'd assume that it isn't and build your project on this decision.

Plus, in physics engines, there's no telling when one person's machine might do a little error and end up being out-of-sync after a check.

3

Beside the hardware limitations like floating point precision that will accumulate over time there are some things, even in the "real" world, that can't be calculated/determined.

Best example the double pendulum.

4

Unity physics is not deterministic even in the same machine.
You can disable coan friction and do some other tricks to make it more deterministic but at the end it’s not at all.

We tried to make a replay based on physX but could not because of this fact.

5

We made Unity physics deterministic between Windows running PCs’. It was not deterministic even on the same machine at first but after making tremendous tests we finally made a setup that is almost deterministic. I say almost because we still not able to make it cross-platform deterministic. But I may say it is fully deterministic between PC-Windows machines.

Our findings to achieve windows platform determinism:

Magical Settings:

  • Rigidbody.useConeFriction = false
  • Make extreme caution to not to overlap or pass-through colliders in the scene. Only physics engine should handle collisions and transform operations.

For all rigidbody objects in the scene:

  • Rigidbody Collision detection = Discrete
  • Rigidbody Interpolate = None

And when moving physics objects by code, we came up with this, make sure it is called in FixedUpdate():

IEnumerator SyncCallback()
{
    if (rigidbody.isKinematic == false)
    {
        rigidbody.velocity = Vector3.zero;
        rigidbody.angularVelocity = Vector3.zero;
    }

    rigidbody.isKinematic = true;

    yield return new WaitForFixedUpdate();

    rigidbody.position = mSyncData.Position;
    rigidbody.rotation = mSyncData.Rotation;

    yield return new WaitForFixedUpdate();

    State = mSyncData.State;
    rigidbody.isKinematic = false;
}
6

Yes! But not by default. Here’s my implementation that will be on the Asset Store: http://redd.it/358hl6.