When dealing with a multi-touch rotation I was searching for a simple method to calculate the angle of two fingers. Well, actually more precisely the angle between the vector from one finger to the second finger and the horizontal X Axis.
While I was still searching my friend Christian Pfandler came up with this function:
The regular atan(x) function seem to have a limitation by only returning values between 0 and +/- PI. Because of this there is also the atan2(y,x) function which is orders of magnitude more useful. To get a continuos angle for the full circle we have to treat the upper half and the lower half seperately though. I found this here.
I don’t quite understand it, but it seems to work. 🙂 Please keep in mind though that like all angles in Cocoa Touch this is in radians as well. So not 0…360, but 0…2*PI. If you know an even simpler method, please let me know.