function play() {
	var ps = document.documentElement.scrollTop < marginTop ? marginTop : document.documentElement.scrollTop;
	if(objY > ps) {
		objY -= moveSpeed;
		if(objY < ps) objY = ps;
	}
	else {
		objY += moveSpeed;
		if(objY > ps) objY = ps;
	}
	if(objL) objL.style.top = objY + "px";
	if(objR) objR.style.top = objY + "px";
}
var marginTop = 0; //顶部留白距离 单位px
var marginSide = 0; //两侧留白距离 单位px
var moveSpeed = 25; //浮动对象单位时间移动速度 单位px，一般不用修改
var objY = marginTop;
var objL = document.getElementById("dl_left");
var objR = document.getElementById("dl_right");
var isUse = false;
if(objL) {
	objL.style.position = "absolute";
	objL.style.left = marginSide + "px";
	isUse = true;
}
if(objR) {
	objR.style.position = "absolute";
	objR.style.right = 0;
	objR.style.marginRight = marginSide + "px";
	isUse = true;
}
if(isUse) setInterval(this.play,30);