如何在SceneKit中实现淡出平面对象？

Question

我目前正在SceneKit / Swift中开发一种建筑保护AR应用程序，该应用程序可检测目标图像（骆驼的图像）并将其覆盖在具有更新的骆驼图像纹理的平面上。我希望更新的图像平面在第一次检测到图像时淡入，而在不再检测到目标图像时淡出。

我通过camelNode跟踪场景中渲染节点的数量。

我的渐入渐出机制在起作用。最初，我在func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor)中触发了渐变操作，但将其更改为func renderer(_ renderer: SCNSceneRenderer, willUpdate node: SCNNode, for anchor: ARAnchor)。但是，淡出功能不起作用。我还尝试了didUpdate函数，并认为它没有达到imageAnchor.isTracked。当不再检测到图像时，渲染的平面将消失而不会褪色。

这是willUpdate函数的代码：

func renderer(_ renderer: SCNSceneRenderer, willUpdate node: SCNNode, for anchor: ARAnchor) { 
    guard anchor is ARImageAnchor else {return}
    if let imageAnchor = anchor as? ARImageAnchor {
         if (imageAnchor.isTracked == false) { // No longer tracking the object so trigger fade out
             let imageSize = imageAnchor.referenceImage.physicalSize
             let plane = SCNPlane(width: CGFloat(imageSize.width), height: CGFloat(camelHeight))
             plane.firstMaterial?.diffuse.contents = toRenderImgs[0]
             let camelToRenderNode = SCNNode(geometry: plane)
             camelToRenderNode.name = "CamelNodeFront"
             camelToRenderNode.position.z = -Float((imageSize.height-CGFloat(camelHeight))/2)
             camelToRenderNode.position.x = -((Float(imageSize.width)-Float(camelWidth))/2)
             camelToRenderNode.eulerAngles.x = -.pi / 2
             camelToRenderNode.opacity = 1

             node.addChildNode(camelToRenderNode)

             let fadeOutAnimation = SCNAction.fadeOut(duration: 0.5)

             camelToRenderNode.runAction(fadeOutAnimation, completionHandler: {
                  camelNodes[0].removeFromParentNode()
                  camelNodes.removeAll()
                  camelToRenderNode.removeFromParentNode()
            })
         } else {
              if (camelNodes.count == 0) {
                  let imageSize = imageAnchor.referenceImage.physicalSize
                  let plane = SCNPlane(width: CGFloat(imageSize.width), height: CGFloat(camelHeight))
                  plane.firstMaterial?.diffuse.contents = camelToRenderImgs[0]
                  let camelToRenderNode = SCNNode(geometry: plane)
                  camelToRenderNode.name = "CamelNodeFront"
                  camelToRenderNode.position.z = -Float((imageSize.height-CGFloat(camelHeight))/2)
                  camelToRenderNode.position.x = -((Float(imageSize.width)-Float(camelWidth))/2)
                  camelToRenderNode.eulerAngles.x = -.pi / 2
                  camelToRenderNode.opacity = 0
                  let fadeInAnimation = SCNAction.fadeIn(duration: 0.5)
                  node.addChildNode(camelToRenderNode)
                  camelNodes.append(camelToRenderNode)

                  camelToRenderNode.runAction(fadeInAnimation)
              }
         }
    }
}