code: function(othis, scroll){
        
        //计算滚动所在容器的可视高度
        var height = notDocment ? (scroll||scrollElem).height() : $(window).height();
        var start = scrollElem.scrollTop(), end = start + height;
  
        that.lazyimg.elem = $(elem);
  
        if(othis){
          show(othis, height);
        } else {
          //计算未加载过的图片
          for(var i = 0; i < that.lazyimg.elem.length; i++){
            var item = that.lazyimg.elem.eq(i), elemTop = notDocment ? function(){
              return item.offset().top - scrollElem.offset().top + start;
            }() : item.offset().top;
            
            show(item, height);
            index = i;
            
            //如果图片的top坐标，超出了当前屏，则终止后续图片的遍历
            if(elemTop > end) break;
          }
        }
      }
docs: 显示图片