新旧两张图片效果的不同对比，实现效果通过拖拽按钮对比相同位置的差异。

HTML代码：

<div class="compare">
	<div class="pic before" style="background-image:url(before.jpg);"></div>
	<div class="pic after" style="background-image:url(after.jpg);"></div>
	<div class="drag">
		<div class="button"></div>
	</div>
</div>

CSS代码：

.compare {border:2px solid #888; background-color: #fff; position: relative;}
.compare:before {display: block; content: '';padding-bottom:40.625%; }
.compare .pic {background:#fff no-repeat center center; background-size: auto 100%; position: absolute; left:0; top:0;width: 100%; height: 100%;}
.compare .before {width: 50%; z-index: 2; background-color:#eee;}
.compare .drag {height: 100%; top:0; left:50%; width: 5%; z-index: 3; transform: translateX(-50%); position: absolute; display: flex; align-items: center; justify-content: center;}
.compare .drag:before {width: 3px; height: 100%; position: absolute; left:50%; top:0; transform: translateX(-50%); background-color: #888; display: block; content: '';}
.compare .button {width:100%; padding-bottom: 100%; border-radius: 50%; background: #888 url(../images/drag_arrow.svg) no-repeat center center; background-size: 62.5% auto; cursor: pointer;}

jquery代码：

var _compare = $('.compare');
var _this;
_compare.find('.button').mousedown(function(e){
	var _downX = e.pageX;
	_this = $(this).parents('.compare');            
	var _before = _this.find('.before').width();
	var _after = _this.find('.after').width();
	var _drag = _this.find('.drag').position().left;
	_this.mousemove(function(e){
		var _moveX = e.pageX;
		var _diffX = _downX - _moveX;                
		if(Math.floor(_before - _diffX) > 0 && Math.floor(_before - _diffX) < _after)  {
			_this.find('.before').width(Math.floor(_before - _diffX));
			_this.find('.drag').css('left', Math.floor(_before - _diffX));
		 }
	});
}).mouseup(function(){
	_this.off('mousemove');
}); 
_compare.mouseleave(function(){
	_this.off('mousemove');
});

代码思路：

点击指定元素获取鼠标横坐标位置，移动鼠标获取鼠标移动到的横坐标位置，两个横坐标的差值就是鼠标的移动多少的数值，再用元素原来的宽度减去这个数值。