Question

I’m creating a simple 2D game in javascript/canvas.
I need to figure out the angle of a certain object relative to my position.

So: say I’m at (10,10) and the object is at (10,5) – that would result in 90 degrees (as positive Y is down, negative Y is up)
(10,10) vs (10,15) would be 270 degrees.

How would I go about this?

Answer 1

Suppose you’re at (a, b) and the object is at (c, d). Then the relative position of the object to you is (x, y) = (c – a, d – b).

Then you could use the Math.atan2() function to get the angle in radians.

var theta = Math.atan2(-y, x);

note that the result is in the range [-π, π]. If you need nonnegative numbers, you have to add

if (theta < 0)
   theta += 2 * Math.PI;

and convert radians to degrees, multiply by 180 / Math.PI.