code: function(px) {
          var lonlat = this.map.getLonLatFromLayerPx(px);        
          
          var extent = this.map.getExtent();
          var quadrant = extent.determineQuadrant(lonlat);
          
          return OpenLayers.Bounds.oppositeQuadrant(quadrant);
      }
docs: Method: calculateRelativePosition
  
  Parameters:
  px - {<OpenLayers.Pixel>}
  
  Returns:
  {String} The relative position ("br" "tr" "tl" "bl") at which the popup
  should be placed.