code: function( pixel ) {
          var viewerCoordinates = pixel.minus(
                  OpenSeadragon.getElementPosition( this.viewer.element ));
          return this.viewerElementToImageCoordinates( viewerCoordinates );
      }
docs: Convert pixel coordinates relative to the window to image coordinates.
  @param {OpenSeadragon.Point} pixel
  @returns {OpenSeadragon.Point}