ARKit 指南

ARKit简介

ARKit是Apple开发的增强现实框架，它让开发者能够轻松创建沉浸式的AR体验。通过结合设备的摄像头、传感器和机器学习能力，ARKit可以实现以下核心功能：

• 世界追踪（World Tracking）：理解设备在3D空间中的位置和方向
• 平面检测（Plane Detection）：识别水平和垂直表面
• 光照估计（Light Estimation）：分析真实世界的光照条件
• 遮挡处理（Occlusion）：让虚拟对象被真实物体遮挡
• 人体检测（Body Tracking）：追踪人体姿态和动作

核心概念

1. ARSession

ARSession是ARKit的核心，负责管理AR体验的生命周期：

import ARKit

class ViewController: UIViewController {
    @IBOutlet var sceneView: ARSCNView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // 设置场景视图的代理
        sceneView.delegate = self
        
        // 创建新场景
        let scene = SCNScene()
        sceneView.scene = scene
    }
    
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        
        // 创建会话配置
        let configuration = ARWorldTrackingConfiguration()
        configuration.planeDetection = [.horizontal, .vertical]
        
        // 运行会话
        sceneView.session.run(configuration)
    }
}

2. ARCamera和世界追踪

ARCamera提供了设备在3D空间中的位置和方向信息：

func session(_ session: ARSession, didUpdate frame: ARFrame) {
    let camera = frame.camera
    let cameraTransform = camera.transform
    
    // 获取相机位置
    let cameraPosition = SCNVector3(
        cameraTransform.columns.3.x,
        cameraTransform.columns.3.y,
        cameraTransform.columns.3.z
    )
    
    print("Camera position: \(cameraPosition)")
}

3. 平面检测与锚点

ARKit可以检测现实世界中的平面并创建锚点：

func renderer(_ renderer: SCNSceneRenderer, 
              didAdd node: SCNNode, 
              for anchor: ARAnchor) {
    
    guard let planeAnchor = anchor as? ARPlaneAnchor else { return }
    
    // 创建平面几何体
    let plane = SCNPlane(width: CGFloat(planeAnchor.extent.x),
                        height: CGFloat(planeAnchor.extent.z))
    
    plane.materials.first?.diffuse.contents = UIColor.blue.withAlphaComponent(0.5)
    
    let planeNode = SCNNode(geometry: plane)
    planeNode.position = SCNVector3(planeAnchor.center.x, 0, planeAnchor.center.z)
    planeNode.eulerAngles.x = -.pi / 2
    
    node.addChildNode(planeNode)
}

实例：AR家具摆放

一个简单的AR家具摆放应用，用户可以在检测到的平面上放置3D模型。

项目设置

首先，在项目中添加必要的权限和框架：

<!-- Info.plist -->
<key>NSCameraUsageDescription</key>
<string>This app needs camera access to provide AR functionality</string>

主要代码实现

import UIKit
import ARKit
import SceneKit

class FurniturePlacementViewController: UIViewController {
    
    @IBOutlet weak var sceneView: ARSCNView!
    @IBOutlet weak var addButton: UIButton!
    
    var selectedModel: SCNNode?
    var furnitureModels = ["chair", "table", "sofa"] // 3D模型名称
    
    override func viewDidLoad() {
        super.viewDidLoad()
        setupARView()
        setupUI()
    }
    
    func setupARView() {
        sceneView.delegate = self
        sceneView.autoenablesDefaultLighting = true
        
        // 添加手势识别
        let tapGesture = UITapGestureRecognizer(target: self, 
                                              action: #selector(handleTap(_:)))
        sceneView.addGestureRecognizer(tapGesture)
    }
    
    func setupUI() {
        addButton.layer.cornerRadius = 25
        addButton.backgroundColor = UIColor.systemBlue
    }
    
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        
        let configuration = ARWorldTrackingConfiguration()
        configuration.planeDetection = [.horizontal]
        configuration.environmentTexturing = .automatic
        
        sceneView.session.run(configuration)
    }
    
    @objc func handleTap(_ gesture: UITapGestureRecognizer) {
        let touchLocation = gesture.location(in: sceneView)
        
        // 进行命中测试，查找平面
        let hitTestResults = sceneView.hitTest(touchLocation, 
                                             types: .existingPlaneUsingExtent)
        
        if let hitResult = hitTestResults.first {
            placeFurniture(at: hitResult)
        }
    }
    
    func placeFurniture(at hitResult: ARHitTestResult) {
        guard let selectedModelName = furnitureModels.randomElement(),
              let scene = SCNScene(named: "\(selectedModelName).scn"),
              let modelNode = scene.rootNode.childNodes.first else {
            return
        }
        
        // 设置模型位置
        let position = SCNVector3(
            hitResult.worldTransform.columns.3.x,
            hitResult.worldTransform.columns.3.y,
            hitResult.worldTransform.columns.3.z
        )
        
        modelNode.position = position
        modelNode.scale = SCNVector3(0.01, 0.01, 0.01) // 缩放模型
        
        // 添加动画效果
        let scaleAction = SCNAction.scale(to: 0.05, duration: 0.3)
        scaleAction.timingMode = .easeOut
        
        sceneView.scene.rootNode.addChildNode(modelNode)
        modelNode.runAction(scaleAction)
    }
    
    @IBAction func addButtonTapped(_ sender: UIButton) {
        // 可以在这里添加更多交互逻辑
        showModelSelectionAlert()
    }
    
    func showModelSelectionAlert() {
        let alert = UIAlertController(title: "选择家具", 
                                    message: "请选择要添加的家具类型", 
                                    preferredStyle: .actionSheet)
        
        for model in furnitureModels {
            alert.addAction(UIAlertAction(title: model.capitalized, style: .default) { _ in
                // 设置选中的模型
                print("Selected model: \(model)")
            })
        }
        
        alert.addAction(UIAlertAction(title: "取消", style: .cancel))
        present(alert, animated: true)
    }
}

// MARK: - ARSCNViewDelegate
extension FurniturePlacementViewController: ARSCNViewDelegate {
    
    func renderer(_ renderer: SCNSceneRenderer, 
                  didAdd node: SCNNode, 
                  for anchor: ARAnchor) {
        
        guard let planeAnchor = anchor as? ARPlaneAnchor else { return }
        
        // 创建平面可视化
        let planeGeometry = SCNPlane(width: CGFloat(planeAnchor.extent.x),
                                   height: CGFloat(planeAnchor.extent.z))
        
        let material = SCNMaterial()
        material.diffuse.contents = UIColor.white.withAlphaComponent(0.3)
        planeGeometry.materials = [material]
        
        let planeNode = SCNNode(geometry: planeGeometry)
        planeNode.position = SCNVector3(planeAnchor.center.x, 0, planeAnchor.center.z)
        planeNode.eulerAngles.x = -.pi / 2
        
        node.addChildNode(planeNode)
    }
    
    func renderer(_ renderer: SCNSceneRenderer, 
                  didUpdate node: SCNNode, 
                  for anchor: ARAnchor) {
        
        guard let planeAnchor = anchor as? ARPlaneAnchor,
              let planeNode = node.childNodes.first,
              let plane = planeNode.geometry as? SCNPlane else { return }
        
        // 更新平面大小
        plane.width = CGFloat(planeAnchor.extent.x)
        plane.height = CGFloat(planeAnchor.extent.z)
        planeNode.position = SCNVector3(planeAnchor.center.x, 0, planeAnchor.center.z)
    }
}

性能优化建议

1. 合理管理AR会话

override func viewWillDisappear(_ animated: Bool) {
    super.viewWillDisappear(animated)
    sceneView.session.pause()
}

2. 优化3D资源

• 使用适当的多边形数量
• 压缩纹理
• 使用LOD（Level of Detail）技术

3. 内存管理

func resetSession() {
    let configuration = ARWorldTrackingConfiguration()
    sceneView.session.run(configuration, 
                         options: [.resetTracking, .removeExistingAnchors])
}

调试技巧

1. 启用调试选项

sceneView.debugOptions = [
    .showFeaturePoints,
    .showWorldOrigin,
    .showBoundingBoxes
]

2. 监控会话状态

func session(_ session: ARSession, 
             cameraDidChangeTrackingState camera: ARCamera) {
    switch camera.trackingState {
    case .normal:
        print("Tracking is normal")
    case .notAvailable:
        print("Tracking is not available")
    case .limited(let reason):
        print("Tracking is limited: \(reason)")
    }
}

开发中需要注意的点

设备兼容性、光照条件、错误处理、性能监控