int GetAngleABC(Point a, Point b, Point c)
{
Point ab = { b.x - a.x, b.y - a.y };
Point cb = { b.x - c.x, b.y - c.y };

float dot = (ab.x * cb.x + ab.y * cb.y); // dot product
float cross = (ab.x * cb.y - ab.y * cb.x); // cross product

float alpha = atan2(cross, dot);

return (int)floor(alpha * 180.0 / CV_PI + 0.5);
}