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Update vs Fixed Update vs Late Update

Discussion in 'Physics' started by ChrisX, Mar 5, 2015.

  1. ChrisX

    ChrisX

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    Hi all.

    If this isn't in the proper forum, please move it to the proper one. I'm still new here...

    Anyway, I'd like to know about the fundamentals behind Update, Fixed Update and Late Update. The definitions, I get already, Update is executed every frame, Fixed Update is executed in a fixed time per frame, Late Update is executed after the Update circle is done. My questions are

    1. When should you use Update and when should you use Fixed Update and when should you use Late Update? And... what happens when you misplace codes (e.g: Put what should be in Fixed Update in Update instead, put what should be in Update in Fixed Update instead)
    2. How do we know the fixed time used in Fixed Update, and can we manipulate it?
    3. How important is Late Update? Why do we need it when we can put it within Update?

    Thanks!
     
  2. DanielQuick

    DanielQuick

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    1. a
    Update: Most things
    Fixed Update: Physics things
    LateUpdate: Things that need to happen after update/right before the camera renders

    1. b
    If you added a force every Update instead of FixedUpdate, your game would run differently depending on the frame-rate.

    2. Edit -> Project Settings -> Time
    From scripting, check out the Time class.

    3. A common example is for a camera controller that should be centered on a moving object. If both of them are moving in Update there is a chance that the camera may move first, then the object will move making it no longer centered. Moving the camera in LateUpdate makes sure that the object is moved first, then the camera centers on it.
     
  3. ChrisX

    ChrisX

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    1. b. And what happens if you do the reverse, that is, putting what should be in Update in Fixed Update instead?
     
  4. DanielQuick

    DanielQuick

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    If you put movement code such as translate into FixedUpdate rather than Update the movement will appear jagged.
     
  5. azurvii

    azurvii

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    @DanielQuick Thanks. Re:3.: What is the difference between using LateUpdate() and put the camera movement code at the bottom of Update()?
    I think the rendering does not start after the whole Update() finishes, right?
     
  6. screenname_taken

    screenname_taken

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    What if you have something that is moving in Update, but is not in the same script as the one moving the camera?
    Having the camera movement in the bottom of the update loop doesn't mean it will run at the very end, after all the other stuff stopped moving as different scripts may run in different times. There is a limit to a computer's resources and so scripts are not executed all at the same time. (There is even a place in unity where you can force a whole script to run after a certain other script)
    LateUpdate will ensure that what is in that loop will run after every single other Update loop in all the other scripts.
     
  7. duck

    duck

    Unity Technologies

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    This sounds like a good candidate for a new page in the scripting section of the manual. I'll see what I can do.
     
  8. screenname_taken

    screenname_taken

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    @duck and please add a note in the documentation regarding JS and SceneManagement that you need to add
    import UnityEngine.SceneManagement at the start of said scripts.
     
  9. azurvii

    azurvii

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    Thanks @screenname_taken, I was also reading http://docs.unity3d.com/Manual/ExecutionOrder.html, and got a little bit more confused over the order. From the flowchart at the bottom of the article, the "Screen rendering" stage takes place after LateUpdate, which is after Update.
    So does it mean that the camera kicks off rendering when it's moved, and does not follow the illustrated order? or does it take a snapshot of its view and then do the rendering after LateUpdate?
    My confusion here is my unclear understanding of the order of events: I assume the camera "takes a shot" only in the rendering stage, but it seems I'm wrong?
     
  10. screenname_taken

    screenname_taken

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    It depends when you move the camera. If you move it in LateUpdate or Update, then it'll move and then render.
    The order of execution is the one in the flowchart.
    Rendering to the screen happens after LateUpdate.
     
  11. ookk47oo

    ookk47oo

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    So does this mean camera render may happen between update functions?
     
  12. DWriedt

    DWriedt

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    I'm still new, but the way that I understand it is this. You want to do your physics checks in FixedUpdate, because if you did it in Update, one frame later you may be past a collision. Fixed update might repeat 1-20 times in the time one Update runs. You don't want to be past a collision and it not reporting there is a collision, because it's overshot it's target.
     
  13. Edy

    Edy

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    Update and FixedUpdate may be seen as two separate execution paths, not necessarily "after" or "before" each other.
    • Update executes on each frame. When you see 134 FPS in the stats it means 134 Update cycles per second. If VSync is enabled then Update is called at the display refresh rate. Frames may be skipped sometimes if the cpu/gpu load can't keep the rate.
    • FixedUpdate executes at a fixed rate (50 Hz default, defined in Project Settings > Time > Fixed Timestep). Frames won't be ever skipped here: instead, the game time may be "slowed down" and/or Update calls will be skipped in order to perform every single FixedUpdate call.
    There are situations where multiple FixedUpdate calls are performed between each Update call. Most frequently, Update is called several times between each FixedUpdate call.

    Rule of thumb:
    • Update: visual stuff, effects, things that may be adapted to varying delta time, and skipped for saving CPU/GPU.
    • FixedUpdate: physics, gameplay, things that depend on precise timing and/or would affect gameplay if skipping them.
    Putting everything in FixedUpdate is incorrect. The game should be able to skip things in the case of intensive CPU/GPU usage. Otherwise, gameplay will just slow down in those cases.
     
    Last edited: Oct 29, 2019