巧用CSS3 border实现图片遮罩效果代码，需要的朋友可以参考下
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代码如下:<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<style type="text/css">
.trans {
-webkit-transition: 0.3s ease;
-moz-transition: 0.3s ease;
-ms-transition: 0.3s ease;
-o-transition: 0.3s ease;
transition: 0.3s ease;
}
.test_outer {
display: block;
width: 200px;
height: 200px;
margin: 1em auto;
position: relative;
overflow: hidden;
}
.test_cover {
width: 40px;
height: 40px;
border: 200px solid rgba(0, 0, 0, .35);
border-radius: 50%;
position: absolute;
left: -115px;
top: -165px;
}
.test_cover:hover {
width: 140px;
height: 140px;
left: -170px;
top: -165px;
}
.test_cover:hover:after {
content: "秋思-一叶知秋！";
text-align:center;
margin:55px 0 0 12px;
color: #fff;
font: bold 16px/1.2 '微软雅黑';
text-shadow: 1px 1px rgba(0, 0, 0, .35);
position: absolute;
}
</style>
</head>
<body>
<a href="#" class="test_outer">
<span class="test_cover trans"></span>
<img src="http://img.jb51.net/data/avatar/000/00/64/47_avatar_middle.jpg" width="200" height="200" />
</a>
</body>
</html>
其实现原理是利用个border偌大的半透明边框属性（demo中边框宽度200像素），边框颜色为rgba黑色半透明，然后50%圆角。再用用定位把它覆盖在图片上面。
主要核心代码:
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代码如下:.test_cover {
width: 40px;
height: 40px;
border: 200px solid rgba(0, 0, 0, .35);
border-radius: 50%;
position: absolute;
left: -115px;
top: -165px;
}
然后外面再用个父容器把它溢出隐藏掉就可以了
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代码如下:.test_outer {
display: block;
width: 200px;
height: 200px;
margin: 1em auto;
position: relative;
overflow: hidden;
}
就这么简单....