code: public static double crossRatios( Point3D_F64 a0 , Point3D_F64 a1 , Point3D_F64 a2 , Point3D_F64 a3) {
  		double d01 = a0.distance(a1);
  		double d23 = a2.distance(a3);
  		double d02 = a0.distance(a2);
  		double d13 = a1.distance(a3);
  
  		return (d01*d23)/(d02*d13);
  	}
docs: Computes the cross-ratio between 4 points. This is an invariant under projective geometry.
  @param a0 Point
  @param a1 Point
  @param a2 Point
  @param a3 Point
  @return cross ratio