code: function() {
          var origin = this.tileOrigin;
          if (!origin) {
              var extent = this.getMaxExtent();
              var edges = ({
                  "tl": ["left", "top"],
                  "tr": ["right", "top"],
                  "bl": ["left", "bottom"],
                  "br": ["right", "bottom"]
              })[this.tileOriginCorner];
              origin = new OpenLayers.LonLat(extent[edges[0]], extent[edges[1]]);
          }
          return origin;
      }
docs: Method: getTileOrigin
  Determine the origin for aligning the grid of tiles.  If a <tileOrigin>
  property is supplied, that will be returned.  Otherwise, the origin
  will be derived from the layer's <maxExtent> property.  In this case,
  the tile origin will be the corner of the <maxExtent> given by the
  <tileOriginCorner> property.
  
  Returns:
  {<OpenLayers.LonLat>} The tile origin.