Physics.Simulate

Unity Engine
1

Hello there,

Today I was experimenting with this and I got a bit confused.
I thought this would allow us to call the simulation whenever we would like and with a variable time step and have it fully sync with the normal update.

I’m currently calling it like this:

void Update () {
        Physics.Simulate(Time.time - m_timer);
        m_timer = Time.time;
    }
}

And most of the times I get 2 calls to the FixedUpdates inside my scripts for each normal Update loop.
Is that the expected behaviour? Is there a possibility of syncing this with the normal Update so the FixedUpdates are called only once?

Thanks!

2

Unity - Scripting API: Physics.autoSimulation Turned off auto fixed update?

3

yep, sorry for to mention that in my previous email. This is the Start of the same component of the update:

void Start () {
        Physics.autoSimulation = false;
        m_timer = Time.time;

    }

EDIT: The funny thing is that if I modify the Fixed TimeStep in the Project Setting it seems to modify the number of times FixedUpdate is called.

4

Physics2D.Simulate has nothing whatsoever to do with FixedUpdate callbacks as it says in the docs.

FixedUpdate is just a callback, it isn’t directly related to physics; it just gives you a callback with a fixed timestep. It’ll still get called more than once if it needs to catch up. FixedUpdate will still happen even if you turn off the auto-simulation. It’s just that physics won’t be called during the fixed update and you are left to call it yourself either in the fixed update or per-frame as you’re doing here.

Doing it per-frame will be more expensive as you’re potentially calling it much more often and you’ll not get a consistent simulation as it’s a variable time so calculations will be different each time depending on frame-rate. If that’s not a problem then you can do this.

Also, if you’re doing it per-frame then don’t use interpolation on Rigidbody as it’s a waste as interpolation is not used.

2 Likes
5

Thanks for the clarification, that explains things!
I’ve always seen the FixedUpdate tighted to the Physics so I thought that would be still connected.

So, if you don´t update Physics.Simulate at a Time.fixedDeltaTime rate, then FixedUpdate is a completely disconnected callback with no specific purpose, am I correct assuming that?

6

It’s more subtle than that. What you think of as FixedUpdate is simply the engine calling “MonoBehaviour.FixedUpdate” (scripts) each fixed interval. After those have been called, with “autoSimulate=true” then the engine implicitly simulates physics with the fixed time interval. If you set “autoSimulate=false” then the engine still calls “MonoBehaviour.FixedUpdate” but it doesn’t simulate physics at all and leaves that to you. You are free to call it during the “MonoBehaviour.FixedUpdate” yourself with a fixed time interval or during “MonoBehaviour.Update” (per-frame) with the frame time interval.

You can see this here as a diagram: Unity - Manual: Order of execution for event functions