The eigen library calculates the angle between two vectors

One 、 Distance from a point to the origin ,pt(x,y)

Vector2f v1(x,y);
float  res1= v1.norm();              //    be equal to  sqrt(x^2+y^2) ,  Distance 
float  res2 = v1.squaredNorm();     //    (x^2+y^2)

Two 、 The distance between two points

	CPoint pt1(10, 10), pt2(5, 5);
	Vector2f v4(pt1.x, pt1.y), v5(pt2.x, pt2.y);
	float  len = (v4 - v5).norm();

3、 ... and 、 The angle between two vectors

Vector2f v1,Vector2f v2;
double cosValNew=v1.dot(v2) /(v1.norm()*v2.norm()); // angle cos value 
double angleNew = acos(cosValNew) * 180 / M_PI;     // Radian angle