x is the horizontal axis, and increases when moving from left to right.
y is the vertical axis, and increases from bottom to top.
(touch_x, touch_y) is the
point selected by the user.
(center_x, center_y) is the point at the center of the
theta is measured counter-clockwise from the
+x axis. Then:
delta_x = touch_x - center_x delta_y = touch_y - center_y theta_radians = atan2(delta_y, delta_x)
Edit: you mentioned in a comment that y increases from top to bottom. In that
delta_y = center_y - touch_y
But it would be more correct to describe this as expressing
in polar coordinates relative to
(center_x, center_y). As ChrisF mentioned,
the idea of taking an “angle between two points” is not well defined.