Multiple Delta Time

In games is very common to has multiples Delta Time for multiple purposes:

• Scaled Delta Time: for gameplay logic
• Unscaled Delta Time: for UI logic
• Physics Delta: Time: for physics logic

In PlayCanvas the update method pass the scaled Delta Time (dt) has a parameter. That is perfect, but sometimes we need an unscaled Delta Time.

The problem

1. Pause game and set scale time in zero.
   this.app.timeScale = 0;

2. Pause Menu logic can't use Delta Time to update some elements, because it is in zero.
   UI.prototype.update = function(dt) {
console.log(dt) // log: 0
};

The solution (walkaround)

To resolve the problem I calculate the unscaled Delta Time:
this.unscaledDt = (pc.app._time - this.oldTime) / 1000;

How did Unity solve it?

In Unity the update method doesn't pass a Delta Time, it is because the class Time provides multiple Delta Time:

• Time.deltaTime: The time in seconds it took to complete the last frame (Read Only).
• Time.unscaledDeltaTime: The timeScale-independent interval in seconds from the last frame to the current one (Read Only).
• Time.fixedDeltaTime: The interval in seconds at which physics and other fixed frame rate updates (like MonoBehaviour's FixedUpdate) are performed.

https://docs.unity3d.com/ScriptReference/Time.html

The real solution

I believed it could be a good idea to expose the current Delta Time and an unscaled Delta Time in some instance of the PlayCanvas engine, something like this:

• pc.app.unscaledDeltaTime
• pc.app.deltaTime

What do you think?

Start by tracing the application update flow around the update(dt) callback, app.timeScale, and the internal app._time value shown in the issue. Check how scaled frame time is computed and passed to scripts, then determine the scope of exposing both scaled and unscaled values. Done should include the proposed app-level timing values and behavior that remains usable when timeScale is zero.