SceneKit Quaternion from Axis + Angle

Quaternion.swift

import SceneKit

extension SCNVector3 {
    func normalized() -> SCNVector3 {
        let magnitude = ((self.x * self.x) + (self.y * self.y) + (self.z * self.z)).squareRoot()
        return SCNVector3(self.x / magnitude, self.y / magnitude, self.z / magnitude)
    }
}

func QuaternionFromAxisAngle(_ inAxis: SCNVector3, radians: CGFloat) -> SCNQuaternion {
    // See:
    // https://www.youtube.com/watch?v=zjMuIxRvygQ
    // http://sacredsoftware.net/tutorials/quaternion.html
    
    // For a better explanation as to why the angle is halved, see:
    // https://eater.net/quaternions/
    let halfAngle = radians * 0.5
    let sinAngle: CGFloat = sin(halfAngle)
    
    let axis = inAxis.normalized()
    
    // Recall that a quaternion can be expressed as:
    // cos(angle) + sin(angle) * (x + y + z)
    // Which expands to:
    // cos(a) + sin(a)x + sin(a)y + sin(a)z
    // The cos(a) is our real component (W)
    
    return SCNQuaternion(x: axis.x * sinAngle,
                         y: axis.y * sinAngle,
                         z: axis.z * sinAngle,
                         w: cos(halfAngle))
}