How to build a physics compound shape for an entity with child entities with aframe-physics-system?

Question

I have a wrapper entity with child entities with meshes:

<a-entity id="parent" dynamic-body="shape: none">
  <a-entity id="box" geometry="primitive: box" position="1 2 3"></a-entity>
  <a-entity geometry="primitive: sphere" position="3 4 5"></a-entity>
</a-entity>

I am adding the shapes with three-to-cannon and CANNON. For example with a box.

parentEl = document.querySelector('#parent');
childEl = parentEl.querySelector('#box');
parentEl.body.addShape(
    mesh2shape(childEl.getObject3D('mesh'), {type: mesh2shape.Type.BOX}),
    new CANNON.Vec3().copy(childEl.object3D.position),
    new CANNON.Quaternion().copy(childEl.object3D.quaternion));

Since the children are offset, but the dynamic-body/shape are being added to the parent, does the above code look right to apply that? I currently have code to grab and throw objects, but the objects act very erratically (goes too fast, rolls weirdly on the ground, shakes around in the hand)

ngokevin · Accepted Answer · 2017-09-09 07:36:32Z

I got this working by changing the center of mass (e.g., the position of the parent entity) to the average positions of the children:

  // Move center of mass, currently just the average of the positions of the shapes.
  centerOfMass = new THREE.Vector3();
  for (i = 0; i < shapeEls.length; i++) {
    centerOfMass.add(shapeEls[i].object3D.position);
  }
  centerOfMass.divideScalar(shapeEls.length);
  body.position.copy(centerOfMass);
  this.el.setAttribute('position', body.position);

And then apply the center of mass to the offsets of the shapes, and also sync to A-Frame:

     for (i = 0; i < shapeEls.length; i++) {
       shapeEl = shapeEls[i];
       shapeOffset = new CANNON.Vec3().copy(shapeEl.object3D.position).vsub(centerOfMass);
        body.addShape(
         this.getShape(shapeEl.getObject3D('mesh'), shapeEl.getAttribute('data-shape')),
           shapeOffset,
           new CANNON.Quaternion().copy(shapeEl.object3D.quaternion));
      shapeEl.setAttribute('position', shapeOffset);
  }

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