swift - 如何在 RealityKit 中实现广告牌效果(LookAt 相机)？

Question

我想在RealityKit中实现billboard效果(飞机一直看着摄像头)，我使用了Entity.Look()方法，但是结果很奇怪，我什至做不到看到飞机，我用的脚本如下，请问有什么问题吗？

struct ARViewContainer: UIViewRepresentable {
    
    func makeUIView(context: Context) -> ARView {
        
        let arView = ARView(frame: .zero)
        let config = ARWorldTrackingConfiguration()
        config.planeDetection = .horizontal
        arView.session.run(config, options:[ ])
        arView.session.delegate = arView
        arView.createPlane()
        return arView        
    }
    
    func updateUIView(_ uiView: ARView, context: Context) { }
}

var planeMesh = MeshResource.generatePlane(width: 0.15, height: 0.15)
var planeMaterial = SimpleMaterial(color:.white,isMetallic: false)
var planeEntity = ModelEntity(mesh:planeMesh,materials:[planeMaterial])
var arCameraPostion : SIMD3<Float>!
var isPlaced = false

extension ARView : ARSessionDelegate{
    func createPlane(){
        let planeAnchor = AnchorEntity(plane:.horizontal)
        planeAnchor.addChild(planeEntity)
        self.scene.addAnchor(planeAnchor)
        //planeAnchor.transform.rotation = simd_quatf(angle: .pi, axis: [0,1,0])

    }
  
    public func session(_ session: ARSession, didUpdate frame: ARFrame){
        guard let arCamera = session.currentFrame?.camera else { return }
        if isPlaced {
            arCameraPostion = SIMD3(arCamera.transform.columns.3.x,0,arCamera.transform.columns.3.z)
            planeEntity.look(at: arCameraPostion, from: planeEntity.position, upVector: [0, 1, 0],relativeTo: nil)
        }
    }
   
    public func session(_ session: ARSession, didAdd anchors: [ARAnchor]) {
        isPlaced = true
    }
}

Answer 1

session(_:didUpdate:) 方法

尝试以下逻辑为 RealityKit 相机实现“广告牌”行为。您可以使用此代码作为起点。它根据相机位置生成模型围绕其局部 Y 轴的旋转。

import RealityKit
import ARKit

class ViewController: UIViewController {

    @IBOutlet var arView: ARView!
    var model = Entity()

    override func viewDidLoad() {
        super.viewDidLoad()

        arView.session.delegate = self
        
        let config = ARWorldTrackingConfiguration()
        arView.session.run(config)

        self.model = try! ModelEntity.load(named: "drummer")
        let anchor = AnchorEntity(world: [0, 0, 0])
        anchor.addChild(self.model)
        arView.scene.anchors.append(anchor)
    }
}

模型的枢轴点必须位于模型的中心(而不是距模型一定距离)。

extension ViewController: ARSessionDelegate {

    func session(_ session: ARSession, didUpdate frame: ARFrame) {

        let camTransform: float4x4 = arView.cameraTransform.matrix

        let alongXZPlane: simd_float4 = camTransform.columns.3

        let yaw: Float = atan2(alongXZPlane.x - model.position.x,
                               alongXZPlane.z - model.position.z)    
        print(yaw)

        // Identity matrix 4x4
        var positionAndScale = float4x4()
        
        // position
        positionAndScale.columns.3.z = -0.25
        
        // scale
        positionAndScale.columns.0.x = 0.01
        positionAndScale.columns.1.y = 0.01
        positionAndScale.columns.2.z = 0.01
        
        // orientation matrix
        let orientation = Transform(pitch: 0, yaw: yaw, roll: 0).matrix

        // matrices multiplication
        let transform = simd_mul(positionAndScale, orientation)   
        
        self.model.transform.matrix = transform
    }
}

订阅(to:on:_:)方法

或者，您可以实现对事件流的订阅。

import RealityKit
import Combine

class ViewController: UIViewController {

    @IBOutlet var arView: ARView!
    var model = Entity()
    var subs: [AnyCancellable] = []

    override func viewDidLoad() {
        super.viewDidLoad()    
        self.model = try! ModelEntity.load(named: "drummer")
        let anchor = AnchorEntity(world: [0, 0, 0])
        anchor.addChild(self.model)
        arView.scene.anchors.append(anchor)
        
        arView.scene.subscribe(to: SceneEvents.Update.self) { _ in
            let camTransform: float4x4 = self.arView.cameraTransform.matrix
            let alongXZPlane: simd_float4 = camTransform.columns.3    
            let yaw: Float = atan2(alongXZPlane.x - self.model.position.x,
                                   alongXZPlane.z - self.model.position.z)
    
            var positionAndScale = float4x4()    
            positionAndScale.columns.3.z = -0.25    
            positionAndScale.columns.0.x = 0.01
            positionAndScale.columns.1.y = 0.01
            positionAndScale.columns.2.z = 0.01    
            let orientation = Transform(pitch: 0, yaw: yaw, roll: 0).matrix
            let transform = simd_mul(positionAndScale, orientation)    
            self.model.transform.matrix = transform               
        }.store(in: &subs)
    }
}