 
     }
     
-    public func generate(containers: [String : Container], options: [String : Any]) -> MDLType {
+    public func generate(containers: [String : Container] = [:], options: [String : Any] = [:]) -> MDLType {
         return self.copy()
     }
 

    
    public var physicalLens = PhysicalLensNode()
    public var physicalImagingSurface = PhysicalImagingSurfaceNode()
    
    public func parseXML(nodes: Container, elem: XMLElement) {
        if let id = elem.attributes["id"] {

    public static let flash: vector_float3 = float3(0.0, 0.0, 0.0)
    public static let exposure: vector_float3 = float3(1.0, 1.0, 1.0)
    public static let exposureCompression: vector_float2 = float2(1.0, 0.0)
    
    public required init() {
        
    }

    public func parseXML(nodes: Container, elem: XMLElement) {
        if let id = elem.attributes["id"] {
