Converting radians to degrees

Question

I've got the following code that transforms a imageView and rotates it in its superview on touch:

override func touchesMoved(touches: NSSet, withEvent event: UIEvent) {                              
    var touch: AnyObject? = event.allTouches()?.anyObject()
    var location = touch!.locationInView(touch!.view)

    var dx: Float = Float(location.x - hsbWheel250ImageView.center.x)
    var dy: Float = Float(location.y - hsbWheel250ImageView.center.y)
    var radians = atan2f(dy, dx)

    //deltaAngle is set in touchesBegan
    var angleDiff = deltaAngle - radians
    hsbWheel250ImageView.layer.transform = CATransform3DMakeRotation(-angleDiffCGFloat, 0, 0, 1)

   //This is where my problem is

   var angleDiffCGFloat = CGFloat(deltaAngle - radians)
   var angleInDegrees = angleDiffCGFloat * (180.0 / Float(M_PI))
   println(angleInDegrees)

   //For the first 180 degrees of the rotation angleInDegrees returns the correct 
   //number i.e. 0-180. But then after 180, where it should return 181, 
   //it returns -179. Where it should return 270 it returns -90 and so on 
   //and so forth..

I've been trying to google my way around, but the only solution people are giving (when converting from radians to degrees) is the one above... I think this is simply a maths problem, but I'm afraid my brain has switched off for this one.

Any help is greatly appreciated.

Answer 1

atan2f returns the principal angle, which has a range from -π to +π (-180 to +180 degrees). You can either deal with the negative values explicitly or use fmodf to force the angles into the required range, e.g. change:

var angleInDegrees = angleDiffCGFloat * (180.0 / Float(M_PI))

to:

var angleInDegrees = fmodf(360.0 + angleDiffCGFloat * (180.0 / Float(M_PI)), 360.0);