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Physics.Simulate

Discussion in '2017.1 Beta' started by Misscelan, Jun 9, 2017.

  1. Misscelan

    Misscelan

    Joined:
    Mar 8, 2013
    Posts:
    176
    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:
    Code (CSharp):
    1. void Update () {
    2.         Physics.Simulate(Time.time - m_timer);
    3.         m_timer = Time.time;
    4.     }
    5. }
    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!
     
    Misscelan, Jun 9, 2017
    #1

  2. hippocoder

    hippocoder

    Digital Ape

    Joined:
    Apr 11, 2010
    Posts:
    29,723
    hippocoder, Jun 9, 2017
    #2

  3. Misscelan

    Misscelan

    Joined:
    Mar 8, 2013
    Posts:
    176
    yep, sorry for to mention that in my previous email. This is the Start of the same component of the update:

    Code (CSharp):
    1. void Start () {
    2.         Physics.autoSimulation = false;
    3.         m_timer = Time.time;
    4.  
    5.     }
    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.
     
    Misscelan, Jun 9, 2017
    #3

  4. MelvMay

    MelvMay

    Unity Technologies

    Joined:
    May 24, 2013
    Posts:
    11,481
    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.
     
    Last edited: Jun 9, 2017
    MelvMay, Jun 9, 2017
    #4
    Peter77 and Misscelan like this.

  5. Misscelan

    Misscelan

    Joined:
    Mar 8, 2013
    Posts:
    176
    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?
     
    Misscelan, Jun 9, 2017
    #5

  6. MelvMay

    MelvMay

    Unity Technologies

    Joined:
    May 24, 2013
    Posts:
    11,481
    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: https://docs.unity3d.com/Manual/ExecutionOrder.html
     
    MelvMay, Jun 9, 2017
    #6