smoothStep function
Do a smooth step (hermite interpolation) interpolation with edge0 and
edge1 by amount. The computation is equivalent to the GLSL function
smoothstep.
Implementation
double smoothStep(double edge0, double edge1, double amount) {
final t = ((amount - edge0) / (edge1 - edge0)).clamp(0.0, 1.0).toDouble();
return t * t * (3.0 - 2.0 * t);
}