code: function (xy, resolution) {
          var lonlat = null,
              size = this.getSize(),
              deltaX  = size.w/2 - xy.x,
              deltaY  = xy.y - size.h/2,
              zoomPoint = this.getLonLatFromPixel(xy);
          if (zoomPoint) {
              lonlat = new OpenLayers.LonLat(
                  zoomPoint.lon + deltaX * resolution,
                  zoomPoint.lat + deltaY * resolution
              );
          }
          return lonlat;
      }
docs: Method: getZoomTargetCenter
  
  Parameters:
  xy - {<OpenLayers.Pixel>} The zoom origin pixel location on the screen
  resolution - {Float} The resolution we want to get the center for
  
  Returns:
  {<OpenLayers.LonLat>} The location of the map center after the
  transformation described by the origin xy and the target resolution.