code: function() {
          var backBuffer;
          if(this.grid.length > 0) {
              backBuffer = document.createElement('div');
              backBuffer.id = this.div.id + '_bb';
              backBuffer.className = 'olBackBuffer';
              backBuffer.style.position = 'absolute';
              var map = this.map;
              backBuffer.style.zIndex = this.transitionEffect === 'resize' ?
                      this.getZIndex() - 1 :
                      // 'map-resize':
                      map.Z_INDEX_BASE.BaseLayer -
                              (map.getNumLayers() - map.getLayerIndex(this));
              for(var i=0, lenI=this.grid.length; i<lenI; i++) {
                  for(var j=0, lenJ=this.grid[i].length; j<lenJ; j++) {
                      var tile = this.grid[i][j],
                          markup = this.grid[i][j].createBackBuffer();
                      if (markup) {
                          markup._i = i;
                          markup._j = j;
                          markup._w = tile.size.w;
                          markup._h = tile.size.h;
                          markup.id = tile.id + '_bb';
                          backBuffer.appendChild(markup);
                      }
                  }
              }
          }
          return backBuffer;
      }
docs: Method: createBackBuffer
  Create a back buffer.
  
  Returns:
  {DOMElement} The DOM element for the back buffer, undefined if the
  grid isn't initialized yet.