初学canvas,模仿google(let it snow)遇到问题

genius_45

先贴出代码

function Snow(x, y, s){this.x = x;this.y = y;this.s = s;    }    Snow.prototype.draw = function(ctx, canvas){ctx.save();    var imageObj = new Image();    var x = this.x;    var y = this.y;    var s = this.s;    imageObj.onload = function(){ctx.drawImage(imageObj, x, y, s, s);    };    imageObj.src = "snow.png";ctx.restore();if(this.y < 600){    this.y += 6;    if(Math.random() < 0.3){if(Math.random() > 0.3){    this.x += Math.random()*2;}else{    this.x -= Math.random()*2;}    }    }else{    snowCanvas.remove(this);}    }    function snowDown(){snowCanvas.add(new Snow(Math.random()*document.body.clientWidth-100, 0, Math.random() * 30)).play(150);    }    setInterval(snowDown,500);


使用得是  gury这个框架.
能展示出雪花从天降落得效果,但是总是闪烁,显得不连贯...请问各位高人..怎么解决这种问题