docs: function (size, base) {
              var viewportSize = this._getViewportSize();
              var documentScroll = this._getDocumentScroll(base);
              return {
                  left : parseInt(documentScroll.scrollLeft + (viewportSize.width - size.width) / 2, 10),
                  top : parseInt(documentScroll.scrollTop + (viewportSize.height - size.height) / 2, 10)
              };
          }