DelaTime as speed

[dicUdeMP]

Okay so in the example by the teacher we set the current yaw to be lerped between the current and the target yaw, with the speed of DeltaTime * 1.

CurrentYaw = FMath::Lerp(CurrentYaw, TargetYaw, DeltaTime * 1.f);

Now reading that, I don’t understand why this results in frame-rate independent speed.

I get 120 fps in the editor, which means a frame time of 8.33 repeating. I capped my FPS to 30 which means a frame time of 3.33 repeating.

Which means that the code might as well be

CurrentYaw = FMath::Lerp(CurrentYaw, TargetYaw, 8.33f);

CurrentYaw = FMath::Lerp(CurrentYaw, TargetYaw, 3.33f);

How could those two lines reproduce the same result?

I thought it might be that DeltaTime is something else and I misunderstood it, so I begun printing it with.

UE_LOG(LogTemp, Warning, TEXT("Deltatime is %f"), DeltaTime);

Results were:

120 fps == Deltatime is 0.008334
30 fps == Deltatime is 0.033333

So, a bit off by a few decimal points, but the initial conundrum stands, how do two different numbers produce the same result?

Then I thought it has something to do with Lerp itself. I tried finding the actual code for Lerp but Visual Studio basically hanged trying to find it. The unreal documentation for it isn’t particularly helpful either.

“Performs a linear interpolation between two values, Alpha ranges from 0-1”

So yeah. How does this actually work?

[DanM]

At 120fps Tick is going to be called 4x as much within the same time period so it would need a value 4x less.

0.03333 / 4 = 0.008333

[dicUdeMP]

Okay, that’s actually way simpler than I thought it would be.

Thank you for the quick response @DanM