code: function calculateObscuringAlpha(elmIndex, elmStack, originalElm) {
  	var totalAlpha = 0;
  
  	if (elmIndex > 0) {
  		// there are elements above our element, check if they contribute to the background
  		for (var i = elmIndex - 1; i >= 0; i--) {
  			let bgElm = elmStack[i];
  			let bgElmStyle = window.getComputedStyle(bgElm);
  			let bgColor = color.getOwnBackgroundColor(bgElmStyle);
  			if (bgColor.alpha && contentOverlapping(originalElm, bgElm)) {
  				totalAlpha += bgColor.alpha;
  			} else {
  				// remove elements not contributing to the background
  				elmStack.splice(i, 1);
  			}
  		}
  	}
  	return totalAlpha;
  }
docs: Calculate alpha transparency of a background element obscuring the current node
  @private
  @param {Number} elmIndex
  @param {Array} elmStack
  @param {Element} originalElm
  @return {Number|undefined}