Как я могу делать тени в RealityKit для файлов USDZ?

Освещение в RealityKit имеет некоторые особенности:

• DirectionalLight ()

  • Position is not important
  • Ориентация важна
  • Есть тени

• SpotLight ()

  • Position is important
  • Ориентация важна
  • Есть тени

• PointLight ()

  • Position is important
  • Ориентация не важна
  • Без теней

Вот как эти типы источников света выглядят в коде:

@IBOutlet var arView: ARView!

override func viewDidLoad() {
    super.viewDidLoad()
    arView.backgroundColor = .black
    
    let directLight = DirectionalLight()
    directLight.light.color = .red
    directLight.light.intensity = 10000
    directLight.position.x = 0
    directLight.orientation = simd_quatf(angle: Float.pi/5,
                                          axis: [0, 1, 0])
    directLight.shadow = .init(DirectionalLightComponent.Shadow(maximumDistance: 5, 
                                                                      depthBias: 1))
    
    let spotLight = SpotLight()
    spotLight.light.color = .green
    spotLight.light.intensity = 450000
    spotLight.position.x = -1.1
    spotLight.shadow = .init()
    
    let pointLight = PointLight()
    pointLight.light.color = .blue
    pointLight.light.intensity = 700000
    pointLight.position.x = 3.0
    // pointLight has no shadows
    // pointLight's intensity is much higher than Directional's or Spot's one


    // SPHERE
    let sphere = MeshResource.generateSphere(radius: 0.4)
    let simpleModel = ModelEntity(mesh: sphere,
            materials: [SimpleMaterial(color: .lightGray,
                                  isMetallic: true)])
    let usdzEntity = simpleModel
    usdzEntity.generateCollisionShapes(recursive: true)

    // PLANE
    let plane: MeshResource = .generatePlane(width: 2.0,
                                             depth: 2.0)
    let material = SimpleMaterial(color: .lightGray,
                             isMetallic: false)
    let entity = ModelEntity(mesh: plane,
                        materials: [material])
    entity.orientation = simd_quatf(angle: Float.pi/4,
                                     axis: [1, 0, 0])
    usdzEntity.addChild(entity)

    let usdzAnchorEntity = AnchorEntity()
    usdzAnchorEntity.addChild(usdzEntity)
    arView.scene.addAnchor(usdzAnchorEntity)

    let lightAnchor = AnchorEntity(world: [0, 0, 2.5])
    lightAnchor.addChild(directLight)
    lightAnchor.addChild(spotLight)
    lightAnchor.addChild(pointLight)
    arView.scene.addAnchor(lightAnchor)
}