code: function computeCentroid(geometry, face, v) {
      var a = geometry.vertices[face.a];
      var b = geometry.vertices[face.b];
      var c = geometry.vertices[face.c];
  
      v = v || new three.Vector3();
  
      v.x = (a.x + b.x + c.x) / 3;
      v.y = (a.y + b.y + c.y) / 3;
      v.z = (a.z + b.z + c.z) / 3;
  
      return v;
    }
docs: Compute the centroid (center) of a THREE.Face3.
  
  @param {THREE.Geometry} geometry Geometry instance the face is in.
  @param {THREE.Face3} face Face object from the THREE.Geometry.faces array
  @param {THREE.Vector3=} v Optional vector to store result in.
  @returns {THREE.Vector3}