为什么使用冲撞过滤器清除所有碰撞？

Question

我有4个身体：2个球和2个平台。

我希望球能够重叠（没有碰撞），但仍会与两个平台碰撞。我已经通读了官方Matter.js Documentation并与collisionFilter demo一起玩，但是仍然无法掌握实现自己所需的条件。

let defaultCollision = 0x0001;
let ballCollision = 0x0002;
let holderCollision = 0x0004;
let floorCollision = 0x0008;

ball1 = Bodies.circle(...
  collisionFiler: { mask: ballCollision }
  ...
});

ball2 = Bodies.circle(...
  collisionFiler: { mask: ballCollision }
  ...
});
// ^ these balls don't collide, which is good.

platform1 = Bodies.circle(...
  collisionFiler: { mask: holderCollision }
  ...
});

platform2 = Bodies.circle(...
  collisionFiler: { mask: floorCollision }
  ...
});
// I don't understand why the balls pass right through these platforms...

任何帮助将不胜感激。