20150620图片的透明度渐变

function fade(obj,tar) {
    obj.timer && clearInterval(obj.timer);
    obj.timer=setInterval(function () {
        var cur=parseInt(css(obj,'opacity')*100);
        var speed=(tar-cur)/7;
        speed=(speed>0)?Math.ceil(speed):Math.floor(speed);
        if(cur!=tar){
            obj.style.opacity=(cur+speed)/100;
            obj.style.filter='alpha(opacity='+(cur+speed)+')'
        }else{
            clearInterval(obj.timer);
            obj.timer=null;    
        }
    },40);
}
function css(obj, attr){
    if(obj.currentStyle){
        return obj.currentStyle[attr];
    } else {
        return getComputedStyle(obj, false)[attr];
    }
}
var box=document.getElementById('box');
fade(box,40)