Cylinder Orientation between two points on a sphere, Scenekit, Quaternions IOS

Question

I\'ve been trying to draw a cylinder between two points on the outer edge of a sphere using SceneKit. I have already produced a line between these two points using primitive

Answer 1

Thank you, Rickster! I have taken it a little further and made a class out of it:

class LineNode: SCNNode
{
    init( parent: SCNNode,     // because this node has not yet been assigned to a parent.
              v1: SCNVector3,  // where line starts
              v2: SCNVector3,  // where line ends
          radius: CGFloat,     // line thicknes
      radSegmentCount: Int,    // number of sides of the line
        material: [SCNMaterial] )  // any material.
    {
        super.init()
        let  height = v1.distance(v2)

        position = v1

        let ndV2 = SCNNode()

        ndV2.position = v2
        parent.addChildNode(ndV2)

        let ndZAlign = SCNNode()
        ndZAlign.eulerAngles.x = Float(M_PI_2)

        let cylgeo = SCNCylinder(radius: radius, height: CGFloat(height))
        cylgeo.radialSegmentCount = radSegmentCount
        cylgeo.materials = material

        let ndCylinder = SCNNode(geometry: cylgeo )
        ndCylinder.position.y = -height/2
        ndZAlign.addChildNode(ndCylinder)

        addChildNode(ndZAlign)

        constraints = [SCNLookAtConstraint(target: ndV2)]
    }

    override init() {
        super.init()
    }
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }
 }

I have tested this class successfully in an iOS app, using this function, which draws 100 lines (oops cylinders :o).

    func linesTest3()
    {
        let mat = SCNMaterial()
        mat.diffuse.contents  = UIColor.whiteColor()
        mat.specular.contents = UIColor.whiteColor()

        for _ in 1...100    // draw 100 lines (as cylinders) between random points.
        {
            let v1 =  SCNVector3( x: Float.random(min: -50, max: 50),
                                  y: Float.random(min: -50, max: 50),
                                  z: Float.random(min: -50, max: 50) )

            let v2 =  SCNVector3( x: Float.random(min: -50, max: 50),
                                  y: Float.random(min: -50, max: 50),
                                  z: Float.random(min: -50, max: 50) )

            // Just for testing, add two little spheres to check if lines are drawn correctly:
            // each line should run exactly from a green sphere to a red one:

            root.addChildNode(makeSphere(v1, radius: 0.5, color: UIColor.greenColor()))
            root.addChildNode(makeSphere(v2, radius: 0.5, color: UIColor.redColor()))

            // Have to pass the parentnode because 
            // it is not known during class instantiation of LineNode.

            let ndLine = LineNode(
                         parent: scene.rootNode, // ** needed
                             v1: v1,    // line (cylinder) starts here
                             v2: v2,    // line ends here
                         radius: 0.2,   // line thickness
                radSegmentCount: 6,     // hexagon tube
                       material: [mat] )  // any material

            root.addChildNode(ndLine)
        }
    }

Regards. (btw. I can only see 3D objects.. I have never seen a "line" in my life :o)