蒙版

HTML：
        <div src="img/img.jpg" alt="pic">
            <div class="mb"> </div>
        </div>

    css:
        .box{
            width:500px;
            height:329px;
            border:1px solid;
            position:relation;
            overflow:hidden;
            }
        .mb{
            width:100%;
            height:100px;
            background-color:rgba(0,0,0,.5);
            position:absolute;
            bottom:0;
            left:0;
            transition:all linear .5s;
            }
        .box:hover .mb{
            bottom:-100px;
            S}