Metal SCNProgram - 无法渲染具有视频内容的 SpriteKit 场景

我（拼命地）尝试使用视频作为 SCNScene 中的纹理以及一些奇特的着色器修改器。我想使用 SCNProgram 来完成该部分。我刚刚拿了一个here https://stackoverflow.com/a/37738386/2054629:

#include <metal_stdlib>

using namespace metal;

#include <SceneKit/scn_metal>

struct MyNodeBuffer {
    float4x4 modelTransform;
    float4x4 modelViewTransform;
    float4x4 normalTransform;
    float4x4 modelViewProjectionTransform;
};

typedef struct {
    float3 position [[ attribute(SCNVertexSemanticPosition) ]];
    float2 texCoords [[ attribute(SCNVertexSemanticTexcoord0) ]];
} MyVertexInput;

struct SimpleVertex
{
    float4 position [[position]];
    float2 texCoords;
};

vertex SimpleVertex myVertex(MyVertexInput in [[ stage_in ]],
                             constant SCNSceneBuffer& scn_frame [[buffer(0)]],
                             constant MyNodeBuffer& scn_node [[buffer(1)]])
{
    SimpleVertex vert;
    vert.position = scn_node.modelViewProjectionTransform * float4(in.position, 1.0);
    vert.texCoords = in.texCoords;

    return vert;
}

fragment half4 myFragment(SimpleVertex in [[stage_in]],
                          texture2d<float, access::sample> diffuseTexture [[texture(0)]])
{
    constexpr sampler sampler2d(coord::normalized, filter::linear, address::repeat);
    float4 color = diffuseTexture.sample(sampler2d, in.texCoords);
    return half4(color);
}

它只是在几何体上显示纹理。我现在正在创建一个 SceneKit 场景，添加一个使用 SCNProgram 的简单立方体，创建一个包含简单正方形的 SpriteKit 场景，并将此 SpriteKit 场景作为纹理传递到我的立方体。它工作正常：

class ViewController: UIViewController {
  @IBOutlet weak var scnView: SCNView!

  let fragmentModifier = try! String(contentsOfFile: Bundle.main.path(forResource: "fragment", ofType: "shader")!)
  var videoLayer: AVPlayerLayer!
  var videoPlayer: AVPlayer!

  override func viewDidLoad() {
    super.viewDidLoad()

    setupVideo()

    // Do any additional setup after loading the view, typically from a nib.
    scnView.showsStatistics = true
    scnView.allowsCameraControl = true

    let scnScene = SCNScene()
    scnView.scene = scnScene

    // setup SceneKit scene
    let cameraNode = SCNNode()
    cameraNode.camera = SCNCamera()
    cameraNode.position = SCNVector3(x: 0, y: 0, z: 25)
    scnScene.rootNode.addChildNode(cameraNode)

    let cubeNode = SCNNode()
    cubeNode.geometry = SCNBox(width: 5, height: 5, length: 5, chamferRadius: 0)
    scnScene.rootNode.addChildNode(cubeNode)

    // setup SpriteKit Scene
    let skScene = SKScene()
    skScene.backgroundColor = UIColor.black
    skScene.size = CGSize(width: 100, height: 100)

    // a simple green square
    let skNode = SKShapeNode(rect: CGRect(x: 0, y: 0, width: 20, height: 20))
    skNode.fillColor = UIColor.green
    skNode.position = CGPoint(x: 5, y: 5)

    // a node to hold a video
    let skVideoNode = SKVideoNode(avPlayer: videoPlayer)
    skVideoNode.size = CGSize(width: 100, height: 100)
    skVideoNode.position = CGPoint(x: 5, y: 5)

    skScene.addChild(skNode)
    // skScene.addChild(skVideoNode)

    let material = cubeNode.geometry!.firstMaterial!
    createProgram(for: material)

    let textureProperty = SCNMaterialProperty(contents: skScene)
    material.setValue(textureProperty, forKey: "diffuseTexture")
  }

  func setupVideo() {
    let item = AVPlayerItem(url: URL(fileURLWithPath: Bundle.main.path(forResource: "test", ofType: "mp4")!))
    let player = AVPlayer(playerItem: item)
    let layer = AVPlayerLayer(player: player)
    videoLayer = layer
    videoPlayer = player
    player.play()
  }

  func createProgram(for material: SCNMaterial) {
    let program = SCNProgram()
    program.fragmentFunctionName = "myFragment"
    program.vertexFunctionName = "myVertex"

    material.program = program
  }
}

现在，如果我用精灵套件场景中的视频节点替换正方形，即我现在要做的：

// skScene.addChild(skNode)
skScene.addChild(skVideoNode)

输出只是纯黑色。视频确实正在播放并输出声音。如果相关的话，我使用的是 iOS 10.0.1。