docs: public static double ncc(NccFeature a, NccFeature b) {
  		double top = 0;
  
  		final int N = a.value.length;
  		for( int i = 0; i < N; i++ ) {
  			top += a.value[i]*b.value[i];
  		}
  
  		return top/(N*a.sigma * b.sigma);
  	}