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 ) {

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

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

}