  _computeWorldMatrix : function( skipParents ){

    var p = this._parent;

    if( p !== null ){
      if( ! skipParents && p._hasInvalidWorldMatrix( false ) ) {
        p.updateMatrix();
        p._computeWorldMatrix( false );
      }
      mat4.multiply( this._wmatrix, p._wmatrix, this._matrix );
    } else {
      mat4.copy( this._wmatrix, this._matrix );
    }

    this._invalidW = false;

  },