public static double getRotationAngle(Vector3D a, Vector3D b)
	{
		if(a.equals(b))
			return 0.0;
		Vector3D aNormalized = new Vector3D(a);
		aNormalized.normalize();
		Vector3D bNormalized = new Vector3D(b);
		bNormalized.normalize();
		double dot = getDotProduct(aNormalized, bNormalized);
		double angle = Math.acos(dot);
		if(Double.isNaN(angle))
			angle = 0.0;
		return angle;
	}