double Smoothstep (double value, double start, double end )

Return Value: given a value between 0.0 and 1.0 inclusive a value between start and end will be returned. Rather than being linearly interpolated, the return value will "ease-in" and "ease-out" as it approaches either end of the interval defined by start and end.

Arguments:

This function works just like PercentStep( ) with the following exceptions: the input value is clamped, and the return value eases in and out as it approaches start and  end.