Search Unity

  1. How can our website serve you better? Give us your feedback. Take our survey and let us know.
    Dismiss Notice

Time.smoothDeltaTime ???

Discussion in 'Scripting' started by DGuy, Apr 20, 2008.

  1. DGuy

    DGuy

    Joined:
    Feb 7, 2007
    Posts:
    187
    (This was asked in the WishList Topic, but I feel my reply would be more helpful, and more properly put, here.)

    Code (csharp):
    1.  
    2.   smoothDeltaTime = (current delta time + (sum of 'n' previous delta times)) / ('n' + 1)
    3.  
    Using smoothDeltaTime limits the effect of sudden fluctuations in deltaTime.

    Example:
    Code (csharp):
    1.  
    2.     deltaTime          smoothDeltaTime ('n' = 2)
    3.     --------           -------------------------
    4.     ...               ...
    5.     0.25               0.25
    6.     0.25               0.25
    7.     0.25               0.25
    8.     0.95 (+380%)       0.4833 (+193%)
    9.     0.25 (-73%)        0.4833
    10.     0.25               0.4833
    11.     0.25               0.25   (-51%)
    12.     0.25               0.25
    13.     ...                ...
    14.  
    In the table above, you see deltaTime is going along steadly at 0.25, but then suddenly it spikes up 380% to 0.95 then drop down 73% to 0.25. The visual effect of the spike would be everything suddenly moving 380% more units than the previous frame.

    The smoothDeltaTime, since the values are averaged over several frames, only sees a jump of 193%. And when it does drop back down to normal, the drop is only 51%. Now, while there will still probably be a visually noticible "hitch", it won't be as drastic as the deltaTime hitch. The hitch will have been spread, or "smoothed", out over several frames.

    I agree, it would be nice if some of the descriptions where expanded on, but hopefully my little description will help in the mean time (and is not too far off the actual mark)
     
  2. 5vdc

    5vdc

    Joined:
    Dec 14, 2010
    Posts:
    2
    Thank you for sharing.
    The documentation hasn't been updated yet :-/
     
  3. CgRobot

    CgRobot

    Joined:
    Jan 4, 2011
    Posts:
    175
    I didn't even know there was such a thing as smoothDeltaTime. Thanks for explaining this. I'm sure I will use this at some point.
     
  4. n0mad

    n0mad

    Joined:
    Jan 27, 2009
    Posts:
    3,732
    Just for information if any docs manager comes by this section :
    April 2012 (4 years later), and the docs about it are still vague. I had to Google search "smoothDeltaTime" to find out further explanations here.
     
    Last edited: Apr 9, 2012
    Hunter_Bobeck likes this.
  5. Morning

    Morning

    Joined:
    Feb 4, 2012
    Posts:
    1,141
    The doc is quite hurting in some parts. I wonder why they didn't make a wiki version where anyone could contribute to keep things updated.
     
  6. Tseng

    Tseng

    Joined:
    Nov 29, 2010
    Posts:
    1,217
    Cause the docs also have to be accessible offline and it doesn't work with Wikis? :p
     
  7. Morning

    Morning

    Joined:
    Feb 4, 2012
    Posts:
    1,141
    You can have manual for offline use + an online version and a separate wiki just for online where users can add things. Many applications use this sort of method and it works wonderfully.
     
    Walter_Hulsebos and SparrowGS like this.
  8. Demigiant

    Demigiant

    Joined:
    Jan 27, 2011
    Posts:
    3,234
    Thanks DGuy for the useful information, and thanks n0mad for bumping this :)

    Unity docs should really be taken more seriously by the Unity guys. The amount of time that a Unity user has to lose to get information about things that should be well documented is absurd. Online searches, forum help, direct trial and error testing. And all this not to find solutions to issues, but just to understand undocumented (or badly documented) API. If we count these hours as working hours, Unity costs a LOT more than 1,500$.
     
  9. virror

    virror

    Joined:
    Feb 3, 2012
    Posts:
    2,961
    Should it not be possible to have a community driven wiki then?
    Someone just have to set one up and I'm sure many will contribute to it.
     
    Last edited: Apr 10, 2012
    Raptosauru5 and SparrowGS like this.
  10. col000r

    col000r

    Joined:
    Mar 27, 2008
    Posts:
    684
    why can't they just add a comments section to the docs like they have on php.net?! everyone could add clarifications and examples and everyone on the planet would benefit greatly!
     
  11. Bezzy

    Bezzy

    Joined:
    Apr 1, 2009
    Posts:
    75
    This. All day, this.

    Not that the resources aren't great as they are, but having something a bit more expanded than this kinda (understandably) rushed doc note would be great.
     
    Naphier likes this.
  12. Essential

    Essential

    Joined:
    Sep 8, 2011
    Posts:
    265
    Ahh, thanks for the explanation. The current doc's explanation—"Smooths the delta time"—could mean a hundred different things.
     
    Rockaso and ModLunar like this.
  13. maynoam

    maynoam

    Joined:
    Dec 24, 2014
    Posts:
    9
    From my experimentation Unity calculates it as a linear combination of current frame time and previous smoothed time like this:

    newSmoothDeltaTime = f*Time.deltaTime+(1-f)*prevSmoothDeltaTime

    Where they seem to use f=0.2 (In Unity 5 at least)

    In the end I had to implement it myself because Time.smoothDeltaTime seems to be updated also when the timeScale is zero causing it to be nearly zero after my game was unpaused.
     
  14. bitbutter

    bitbutter

    Joined:
    Jul 19, 2012
    Posts:
    60
    Wow. 7 years(!) later the docs page for this hasn't changed.
     
    ModLunar likes this.
  15. DonLoquacious

    DonLoquacious

    Joined:
    Feb 24, 2013
    Posts:
    1,667
    The documentation almost never has the exact math that goes into any of its built-in methods and members, so this is just one of a million different things that have some magic going on behind the scenes, and no reason (or at least, no more than normal) to update the documentation. Anyways, they've been more focused on those promotional videos that they call tutorials.
     
    Raptosauru5 likes this.
  16. A.Killingbeck

    A.Killingbeck

    Joined:
    Feb 21, 2014
    Posts:
    483
    Smooth delta time does exactly what it says.If there is a significant jump in the framerate you're running, SmoothDelta time will report values closer to your targetFPS but weighted by the amount of frame loss. deltaTime will give you the actualy time difference.
     
  17. hdtnl

    hdtnl

    Joined:
    Jul 28, 2016
    Posts:
    5
    Or might the reason about silly of their document for a big community :D then you all are here
     
  18. Legion4

    Legion4

    Joined:
    Apr 18, 2017
    Posts:
    4
    2018 and still had to google it since the Docs weren't very helpful
     
    ModLunar likes this.
  19. sean244

    sean244

    Joined:
    Nov 4, 2017
    Posts:
    77
    So shouldn't we always use smoothDeltaTime rather than just deltaTime? Is it ever preferable to just use deltaTime?
     
    Last edited: Jun 29, 2018
  20. kdgalla

    kdgalla

    Joined:
    Mar 15, 2013
    Posts:
    2,764
    Seems like a matter of preference. What should happen if someone's machine lags? Should it skip (deltatime) and be caught-up the next frame, or should it go into slow-mo for a few frames(SmoothDeltaTime)? I've never tried SmoothDeltaTime, though.
     
  21. AndersMalmgren

    AndersMalmgren

    Joined:
    Aug 31, 2014
    Posts:
    5,363
    Using smoothdelta you will run slower in the spike frames so a slow computer will lose against a faster one in multiplayer
     
    FlightOfOne likes this.
  22. psorger

    psorger

    Joined:
    Dec 6, 2016
    Posts:
    1
    Not what I was looking for exactly, but this kind of implements a simple low pass filter, effectively filtering the time step so it doesn't change so much each frame. Simple DSP (digital signal processing)

    This:
    newSmoothDeltaTime = f*Time.deltaTime+(1-f)*prevSmoothDeltaTime
    Where they seem to use f=0.2 (In Unity 5 at least)
    Implements a simple "one pole" filter with a "cutoff frequency (knee) coefficient" of 0.2.

    As you increase the number of 'previous delta times' the 'curve' of fps over frames gets smoother. Of course, to really do it correctly minimizing distortion you have to multiply those previous delta times by their own coefficients...
     
    senkal_ likes this.
  23. WeakRaider29

    WeakRaider29

    Joined:
    Nov 15, 2020
    Posts:
    4
    I am sorry to disappoint you sir; but the documentation of Time.smoothDeltaTime is still vague as ever
     
    sterneck_moritz likes this.
  24. APSchmidt

    APSchmidt

    Joined:
    Aug 8, 2016
    Posts:
    4,172
  25. berniegp

    berniegp

    Unity Technologies

    Joined:
    Sep 9, 2020
    Posts:
    19
    I'm happy to tell you that I'm updating this very part of the documentation (in fact the whole Time API doc) as I'm writing this! I don't know exactly when exactly, but it will be published soon.
     
  26. DeloitteCam

    DeloitteCam

    Joined:
    Jan 23, 2018
    Posts:
    22
    @berniegp When you update the docs, can you perhaps poke some devs to update the implementation so that smoothDeltaTime compensates for changes in deltatime when Time.timeScale is changed. Currently, if you were to create a component for a missile seeking a target and you used smoothDeltaTime then paused your game using Time.timeScale = 0; the missile would keep tracking and slowly decelerate for about 2 seconds after the game has been "paused".

    Given the intent of smoothDeltaTime this feels like an unexpected side effect that should probably be patched.
     
    Last edited: Mar 20, 2021
  27. berniegp

    berniegp

    Unity Technologies

    Joined:
    Sep 9, 2020
    Posts:
    19
    smoothDeltaTime currently has a very simple logic where it interpolates between the current frame's "real" deltaTime and the previous smoothDeltaTime with a weight of about 20% for the "real" deltaTime. In other words, it tracks deltaTime with a delay to, as the name says, smooth out its variations. While I see your point, the fact is that the current behavior has existed for a long time now and both the current behavior and your suggestion are valid choices for a smoothed deltaTime value in different contexts. Therefore this is not likely to change.

    However, the upshot is that you can pretty easily make your own smoothed time since it's so simple. Just add a bit of logic to a singleton class (e.g. MyTime) and zero-out your smoothed time when the timeScale is 0.
     
  28. berniegp

    berniegp

    Unity Technologies

    Joined:
    Sep 9, 2020
    Posts:
    19
    mikeohc likes this.
unityunity