<canvas id="snow" class="snow" width="600px" height="600px">

您的浏览器不支持canvas！

</canvas>

//雪花绘制

var snow = document.getElementById("snow");

var snowCtx = snow.getContext("2d");

var posx = 20;

var posy = 20;

setInterval(function(){

//清除矩形区域

snowCtx.clearRect(0,0,snow.width,snow.height);

snowCtx.fillStyle="rgba(255,255,255,0.5)";

snowCtx.arc(posx,posy,20,0,2*Math.PI,true);

snowCtx.fill();

posy += 10;

if(posy>= snow.height){

posy=20;

}

},100);