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How would you calculate the degrees between two points on the edge of a circle in Swift.

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PoKoBros
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  • Do you know for sure this is a circle? Like, you know the center of the circle? Because two points does not a circle make, since an infinite number of different circles can be desecribed by two points - you need a third, either on the circle or the origin. – corsiKa Feb 21 '15 at 02:39
  • i know the origin also @corsiKa – PoKoBros Feb 21 '15 at 02:45
  • So then you can just use arctan(delta y/delta x). I don't know what swift sprite key is, so I would not be sure of the format for the code for that. Edit: most programming languages using trig uses radians, so your output might be in radians. If there's a function to convert it to degrees, just use that; otherwise, use the formula to convert it to degrees which is rads*180/pi – Muhatashim Feb 21 '15 at 03:12

1 Answers1

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Given points p1, p2 on a circle with center center, you would compute the difference vectors first:

let v1 = CGVector(dx: p1.x - center.x, dy: p1.y - center.y)
let v2 = CGVector(dx: p2.x - center.x, dy: p2.y - center.y)

Then 

let angle = atan2(v2.dy, v2.dx) - atan2(v1.dy, v1.dx)

is the (directed) angle between those vectors in radians, and

var deg = angle * CGFloat(180.0 / M_PI)

the angle in degrees. The computed value can be in the range -360 .. 360, so you might want to normalize it to the range 0 <= deg < 360 with

if deg < 0 { deg += 360.0 }
Martin R
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  • will this code work on an iPad? all that is going to happen is that the circle will enlarge a little bit. – PoKoBros Feb 22 '15 at 04:15
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    @PoKoBros: This is just math, it works on any device. It only computes an angle, so what do you mean by "the circle will enlarge a bit" ? – Martin R Feb 22 '15 at 07:01
  • I just mean the radius of the circle will double ok an iPad – PoKoBros Feb 22 '15 at 15:10
  • @PoKoBros: Sorry, I don't get it. This question was about computing an angle from three points (center, p1, p2). The radius is not even involved in the formula. So how/where does the *"radius double on an iPad"*? – Martin R Feb 22 '15 at 15:22
  • For some reason when I try the same exact code on the iPad, it does not work. – PoKoBros Feb 22 '15 at 16:24
  • @PoKoBros: Can you provide a concrete example of input values, actual output and expected output? – Martin R Feb 22 '15 at 16:25
  • I actually only needed to calculate up to radians not degrees so I stopped at the let angle line in your answer. I guess what I am trying to ask is that shouldn't the radians outputted be the same in all circles? – PoKoBros Feb 22 '15 at 16:36
  • Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/71433/discussion-between-pokobros-and-martin-r). – PoKoBros Feb 22 '15 at 17:35