SceneKit素材被填充随机模式，为什么？

我有一个简单的触摸检测方法，应该改变被触摸节点的颜色。SceneKit素材被填充随机模式，为什么？

override func touchesBegan(touches: Set<NSObject>, withEvent event: UIEvent) { 
    super.touchesBegan(touches, withEvent: event) 

    let touch = touches.first as! UITouch 
    let point = touch.locationInView(view) 

    let options: [NSObject : AnyObject] = [ 
     SCNHitTestFirstFoundOnlyKey: NSNumber(bool: true), 
     SCNHitTestSortResultsKey: NSNumber(bool: true) 
    ] 

    if let results = sceneView.hitTest(point, options: options) as? [SCNHitTestResult] { 
     if let result = results.first { 
      // Red color material 
      let material = SCNMaterial() 
      material.diffuse.contents = UIColor.redColor() 

      // Assign it to the node 
      result.node.geometry?.firstMaterial = material 
     } 
    } 
}

我的节点层次结构包含使用自定义SCNGeometry并用常规SCNBox几何8个节点做出一个节点。

let boxGeometry = SCNBox(width: 1, height: 1, length: 1, chamferRadius: 0) 
let boxNode = SCNNode(geometry: boxGeometry) 
boxNode.position = SCNVector3(x: vector.x, y: vector.y, z: vector.z)

下面是我触摸一个盒子后颜色变化情况的截图。图案在每次旋转时闪烁并改变。

enter image description here