docs: function(point, res) {
          return new OpenLayers.Pixel(
             Math.max(Math.floor((point.x - this.tileOrigin.lon) / (this.tileSize.w * res)),0),
             Math.max(Math.floor((this.tileOrigin.lat - point.y) / (this.tileSize.h * res)),0)
          );
      }