Calculating angle a segment forms with a ray I am given a point C and a ray r starting there. I know the coordinates (xc, yc) of the point C and the angle theta the ray r forms with the horizontal, theta in ( pi, pi . I am also given another point P of which I know the coordinates (xp, yp) how do I calculate the angle alpha that the segment CP forms with the ray r, alpha in ( pi, pi ? Some examples follow I can use the the atan2 function.