Getting the rotation of device around the world origin's y axis in ARKit

Question

I am trying to calculate the rotation of my device when I rotate it around the y-axis in ARKit. For clarification the y-axis in ARKit is the axis pointing upwards perpendicular to the ground.

I used eulerangles to get the rotation of the camera like this:

var alpha = sceneView.pointOfView?.eulerAngles.y

This approximately works when 0=<alpha<pi/2 and when -pi/2<alpha<=0, but for what is supposed to be other angles I get the wrong readings. I suspect this has to do with gimbal lock and that I have to somehow use quaternions to be able to get the correct angle regardless of quadrant. Any help is much appreciated!

Answer 1

Solved it by using quaternions instead of eulerangles. This is the code I used:

  guard let cameraNode = sceneView.pointOfView else { return }

  //Get camera orientation expressed as a quaternion
  let q = cameraNode.orientation

  //Calculate rotation around y-axis (heading) from quaternion and convert angle so that
  //0 is along -z-axis (forward in SceneKit) and positive angle is clockwise rotation.
  let alpha = Float.pi - atan2f( (2*q.y*q.w)-(2*q.x*q.z), 1-(2*pow(q.y,2))-(2*pow(q.z,2)) )

I used the quaternion to heading/euler conversion listed on this website.