Assumptions: `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

screen. `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

case,

```
delta_y = center_y - touch_y
```

But it would be more correct to describe this as expressing `(touch_x, touch_y)`

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.