public double getHullDistance(SphereCluster other) {
		double distance = 0.0;
		//get the center through getCenter so subclass have a chance
		double[] center0 = getCenter();
		double[] center1 = other.getCenter();
		distance = distance(center0, center1);

		distance = distance - getRadius() - other.getRadius();
		return distance;
	}
public double getHullDistance ( SphereCluster other ) { double distance = 0.0 ; double [ ] center0 = getCenter ( ) ; double [ ] center1 = other . getCenter ( ) ; distance = distance ( center0 , center1 ) ; distance = distance - getRadius ( ) - other . getRadius ( ) ; return distance ; }
/*
the minimal distance between the surface of two clusters.
is negative if the two clusters overlap