启用SCNPhysicsShape后SceneKit中SCNPhysicsVehicle车辆无法移动

当手动设置body.physicsShape = SCNPhysicsShape(node: chassie)时，默认会把chassie节点下所有子节点（包括四个wheelLocator定位器节点）的碰撞形状合并进底盘的物理形状中。这会导致车轮的物理约束（SCNPhysicsVehicleWheel）和底盘的碰撞体产生冲突——相当于车轮被“卡”在底盘碰撞体内部，无法通过引擎力推动车辆移动。

而不设置自定义物理形状时，SceneKit自动生成的底盘碰撞体尺寸、位置和视觉模型不匹配（通常高度偏矮），所以会出现车轮陷入地面的视觉问题。

1. 创建物理形状时排除车轮定位器节点 #

生成SCNPhysicsShape时，通过excludeNodes选项排除四个wheelLocator节点，避免它们的碰撞体干扰车轮的物理约束：

func setupVeicles(){
        let chassie = loadAssetWithName(nameFile: "rc_car", nameNode: "rccarBody", type: "scn", scale: SCNVector3(1, 1, 1))
        chassie.position = SCNVector3(0, 1, 0)

        // 先获取车轮定位器节点
        let wheelFL = chassie.childNode(withName: "wheelLocator_FL", recursively: true)!
        let wheelFR = chassie.childNode(withName: "wheelLocator_FR", recursively: true)!
        let wheelBL = chassie.childNode(withName: "wheelLocator_RL", recursively: true)!
        let wheelBR = chassie.childNode(withName: "wheelLocator_RR", recursively: true)!

        let body = SCNPhysicsBody.dynamic()
        // 创建排除定位器的物理形状
        let shapeOptions = [SCNPhysicsShape.Option.excludeNodes: [wheelFL, wheelFR, wheelBL, wheelBR]] as [SCNPhysicsShape.Option : Any]
        body.physicsShape = SCNPhysicsShape(node: chassie, options: shapeOptions)
        
        body.categoryBitMask = BodyType.car.rawValue
        body.collisionBitMask = BodyType.floor.rawValue
        body.contactTestBitMask = BodyType.floor.rawValue
        body.allowsResting = false
        body.mass = 1
        body.restitution = 0.1
        body.friction = 0.5
        body.rollingFriction = 0
        chassie.physicsBody = body

        // 后续车轮创建逻辑不变
        let phywheelFL = createPhysicsVehicleWheel(wheelNode: wheelFL, position: SCNVector3(3, 1.31, 2.753))
        let phywheelFR = createPhysicsVehicleWheel(wheelNode: wheelFR, position: SCNVector3(-3, 1.271, 2.753))
        let phywheelBL = createPhysicsVehicleWheel(wheelNode: wheelBL, position: SCNVector3(4, 1.31, -3.566))
        let phywheelBR = createPhysicsVehicleWheel(wheelNode: wheelBR, position: SCNVector3(-4, 1.31, -3.566))
        
        physicsVehicle = SCNPhysicsVehicle(chassisBody: chassie.physicsBody!, wheels: [phywheelFL,phywheelFR,phywheelBL,phywheelBR])
        self.scene.physicsWorld.addBehavior(physicsVehicle)
        self.scene.rootNode.addChildNode(chassie)
    }

2. 选择合适的物理形状类型 #

根据车辆模型复杂度，选择更适配的物理形状类型，优化碰撞检测精度和性能：

let shapeOptions = [
    SCNPhysicsShape.Option.excludeNodes: [wheelFL, wheelFR, wheelBL, wheelBR],
    SCNPhysicsShape.Option.type: SCNPhysicsShape.ShapeType.convexHull // 复杂动态物体推荐用凸包
] as [SCNPhysicsShape.Option : Any]

3. 调整车轮物理参数 #

检查createPhysicsVehicleWheel方法中的悬挂、抓地力参数，让车轮更好贴合地面：

func createPhysicsVehicleWheel(wheelNode: SCNNode, position: SCNVector3) -> SCNPhysicsVehicleWheel {
    let wheelShape = SCNPhysicsShape(geometry: SCNSphere(radius: 1.2), options: nil)
    let wheelBody = SCNPhysicsBody.dynamic()
    wheelBody.physicsShape = wheelShape
    wheelBody.mass = 0.5
    
    let wheel = SCNPhysicsVehicleWheel(node: wheelNode)
    wheel.connectionPosition = position
    wheel.suspensionStiffness = 12.0 // 提高悬挂刚度，减少下陷
    wheel.suspensionDamping = 3.0 // 调整阻尼，优化回弹效果
    wheel.frictionSlip = 1.2 // 提升抓地力，避免打滑
    return wheel
}

4. 开启物理形状可视化调试 #

开启SceneKit的物理形状显示，直观排查碰撞体位置/尺寸问题：

self.scene.debugOptions = [.showPhysicsShapes]

内容的提问来源于stack exchange，提问作者Damiano Miazzi