Lerp() vs LerpStable(): What's the practical difference?

Development Programming & Scripting
1

I saw something about LerpStable() in the 4.6 notes and I was wondering what exactly it does. Does it return anything different than what Lerp returns? Is there any advantage to using one over the other?

2

Hi Jared,

I think from mathematical point of view these functions are equivalent. However, Lerp function can lead to errors on large numbers. For example:

int a = -1000000000, b = -1.2*a;
float alpha = 0.5;

auto r1 = FMath::Lerp(a, b, alpha); // Incorrect result, cause integer overflow
auto r2 = FMath::LerpStable(a, b, alpha);  // OK

Best regards,

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3

Thank you for that breakdown, . I imagine then that LerpStable must require a bit more memory or work a tick slower than Lerp in exchange for the stability. I appreciate your answer

4

Exactly. Lerp compiled to i386 assembler without any optimization is just 4 instructions (not counting function parameter handling), while LerpStable translates to 6 instructions (one additional push to the FPU register stack, and one additional multiplication).

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