code: function(viewPortPx) {
          var maxExtent = this.maxExtent;
          var resolution = this.getResolution();
          var tileMapWidth = resolution * this.tileSize.w;
          var tileMapHeight = resolution * this.tileSize.h;
          var mapPoint = this.getLonLatFromViewPortPx(viewPortPx);
          var tileLeft = maxExtent.left + (tileMapWidth *
                                           Math.floor((mapPoint.lon -
                                                       maxExtent.left) /
                                                      tileMapWidth));
          var tileBottom = maxExtent.bottom + (tileMapHeight *
                                               Math.floor((mapPoint.lat -
                                                           maxExtent.bottom) /
                                                          tileMapHeight));
          return new OpenLayers.Bounds(tileLeft, tileBottom,
                                       tileLeft + tileMapWidth,
                                       tileBottom + tileMapHeight);
      }
docs: APIMethod: getTileBounds
  Returns The tile bounds for a layer given a pixel location.
  
  Parameters:
  viewPortPx - {<OpenLayers.Pixel>} The location in the viewport.
  
  Returns:
  {<OpenLayers.Bounds>} Bounds of the tile at the given pixel location.