     private func createShapeLayer() -> CAShapeLayer {
         let layer = CAShapeLayer()
         layer.contentsScale = UIScreen.mainScreen().scale
-        layer.lineWidth = 10
+        layer.lineWidth = 30 / xform.a
         layer.fillColor = UIColor.clearColor().CGColor
         layer.strokeColor = UIColor(red: 1, green: 0, blue: 1, alpha: 1).CGColor
         layer.shadowRadius = 2.0

    }
    
    @IBAction func undo() {
        print("undo", index)
        index -= 1
        maskView.image = image
        renderLayers(0..<index)

    let image = UIImage(named: "dog.jpg")!
    var layers = [CALayer]()
    var index = 0
    var xform = CGAffineTransformIdentity {
        didSet {
            shapeLayer.lineWidth = 30 / xform.a
            builder.minSegment = 8.0 / xform.a
        }
    }

    var builder = SNPathBuilder(minSegment: 8.0)
    lazy var shapeLayer:CAShapeLayer = {
