为ModelEntity添加PhysicsBodyComponent后，12面骰子无法正常落地求助

问题描述 #

替换为12面骰子（d20）模型后，为其添加PhysicsBodyComponent后无法正确落在地面上，但此前使用6面骰子时功能正常。

相关代码 #

let floor = ModelEntity(mesh: .generatePlane(width: 50, depth: 50), materials: [OcclusionMaterial()])
floor.generateCollisionShapes(recursive: false)
floor.components[PhysicsBodyComponent.self] = .init(
    massProperties: .default,
    mode: .static
)

content.add(floor)

if let diceModel = try? await Entity(named: "d20"),
   let dice = diceModel.children.first?.children.first {
    dice.scale = [0.1, 0.1, 0.1]
    dice.position.y = 0.5
    dice.position.z = -1
    
    dice.generateCollisionShapes(recursive: true)
    dice.components.set(InputTargetComponent())
    
    dice.components.set(ImageBasedLightReceiverComponent(imageBasedLight: dice))
    dice.components.set(GroundingShadowComponent(castsShadow: true))
    
    dice.components[PhysicsBodyComponent.self] = .init(PhysicsBodyComponent(
        massProperties: .default,
        material: .generate(staticFriction: 0.8, dynamicFriction: 0.5, restitution: 0.1),
        mode: .dynamic
    ))
    
    dice.components[PhysicsBodyComponent]
    
    dice.components[PhysicsMotionComponent.self] = .init()
    
    content.add(dice)
}

问题原因与解决方法 #

1. 碰撞形状递归生成异常 #

generateCollisionShapes(recursive: true)会为所有子实体生成碰撞体，多层嵌套的d20模型可能产生多个重叠或偏移的碰撞形状，导致物理引擎计算错误。

• 解决：将recursive参数改为false，仅为当前骰子实体生成碰撞形状：

  dice.generateCollisionShapes(recursive: false)
  

2. 模型嵌套层级导致变换偏差 #

通过diceModel.children.first?.children.first获取实体，多层嵌套可能让缩放、位置等变换继承出现问题，导致碰撞体位置与视觉模型不匹配。

• 解决：直接使用顶层的diceModel作为骰子实体，避免多层嵌套：

  if let dice = try? await Entity(named: "d20") {
      // 后续操作直接使用dice
  }
  

3. 默认质量属性不匹配复杂形状 #

.default质量属性是针对基础形状设计的，12面骰子的不规则形状需要更贴合的质量参数，否则会导致物理运动异常。

• 解决：手动设置质量属性，比如：

  let massProps = MassProperties(mass: 0.1, centerOfMass: .zero, inertiaTensor: .init(diagonal: [0.001, 0.001, 0.001]))
  dice.components[PhysicsBodyComponent.self] = .init(
      massProperties: massProps,
      material: .generate(staticFriction: 0.8, dynamicFriction: 0.5, restitution: 0.1),
      mode: .dynamic
  )
  

4. 自动生成的碰撞形状精度不足 #

自动生成的碰撞形状可能无法精准贴合12面骰子的曲面，导致碰撞检测失效。

• 解决：手动为模型生成凸包碰撞形状：

  if let mesh = dice.model?.mesh {
      dice.collisionShape = ConvexHullCollisionShape(mesh: mesh)
  }
  

5. 清理冗余代码 #

代码中dice.components[PhysicsBodyComponent]这一行无实际作用，直接删除即可。

修改后的完整代码示例 #

let floor = ModelEntity(mesh: .generatePlane(width: 50, depth: 50), materials: [OcclusionMaterial()])
floor.generateCollisionShapes(recursive: false)
floor.components[PhysicsBodyComponent.self] = .init(
    massProperties: .default,
    mode: .static
)

content.add(floor)

if let dice = try? await Entity(named: "d20") {
    dice.scale = [0.1, 0.1, 0.1]
    dice.position.y = 0.5
    dice.position.z = -1
    
    dice.generateCollisionShapes(recursive: false)
    // 手动设置贴合的碰撞形状
    if let mesh = dice.model?.mesh {
        dice.collisionShape = ConvexHullCollisionShape(mesh: mesh)
    }
    
    dice.components.set(InputTargetComponent())
    dice.components.set(ImageBasedLightReceiverComponent(imageBasedLight: dice))
    dice.components.set(GroundingShadowComponent(castsShadow: true))
    
    let massProps = MassProperties(mass: 0.1, centerOfMass: .zero, inertiaTensor: .init(diagonal: [0.001, 0.001, 0.001]))
    dice.components[PhysicsBodyComponent.self] = .init(
        massProperties: massProps,
        material: .generate(staticFriction: 0.8, dynamicFriction: 0.5, restitution: 0.1),
        mode: .dynamic
    )
    
    dice.components.set(PhysicsMotionComponent())
    
    content.add(dice)
}

内容的提问来源于stack exchange，提问作者kunj Barvaliya