smootherstep

https://gyazo.com/665835d8d88dd005ed7736c631123f4a

2回微分しても連続

$ 6t^5 - 15t^4 + 10t^3

code:cpp

float smootherstep(float a, float b, float t) {

t = saturate((t - a) / (b - a));

return t * t * t * (t * (t * 6.0 - 15.0) + 10.0);

}