Havok strange behaviour when switching tabs on last versions

Bugs
4

If _useDeltaForWorldStep is set to false, then a fixed timestep of 1/60 seconds is used for every Physics step.

If you’re running Physics on the main thread (like in this case) and switch tabs, I believe that the tab process freezes (to some extent?) for browser performance, which freezes rendering and Physics. So when you return to the Babylon JS window, the Physics has 2 options:

  • Attempt to “catch up” to the current time by passing the large amount of time elapsed while you were on a different tab as deltaTime to the next Physics step calculation. You’ll see a transient speed-up in the Physics, e.g. an object falling 10X faster for a short period of time or an impulse
  • Forget about the time elapsed while you were on a different tab and just continue simulating Physics steps at a fixed timestep of 1/60 seconds

In summary:

  • _useDeltaForWorldStep: true is more aligned with rendering frames. But, if rendering is disrupted by switching tabs, then Physics will face a disruption too
  • _useDeltaForWorldStep: false can yield a smoother Physics simulation with less speed-ups and unexpected impulses, but is less aligned with rendering

Here’s a great article on this: Fix Your Timestep! | Gaffer On Games

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