code: function(capture) {
      // If compareTo{Page|Viewport} option is set, gets the filepath corresponding
      // to the capture index (page, capture, viewport):
      if (options.compareToViewport || options.compareToPage) {
        var pageReference = options.compareToPage || capture.page.name;
        var viewportReference = options.compareToViewport || capture.viewport.name;
        return createCaptureState(pageReference, capture.name, viewportReference).filePath;
      }
      else {
        return capture.filePath;
      }
    }
docs: Returns a filepath used as image reference.
  
  @param {Object} capture state
  @returns {String} base image filepath