canvas做出美爆了了下雨效果，本效果原创
雨点大小可自己调节，页面代码如下：
<body>
<canvas id="canvas" width="2000px" height="1200px"></canvas>
<script>
var canvas = document.getElementById("canvas");
var context = canvas.getContext("2d");
//构建构造函数 创建圆形
function Circle()
{
this.x = Math.random()*canvas.width;
this.y = -10;
this.width=Math.random()*2;
this.height = Math.random()*30;
//画一个圆
this.paint = function(){
context.beginPath();
context.fillStyle="rgba(255,255,255,0.3)";
context.fillRect(this.x,this.y,this.width,this.height);
context.fill();
}
//设置步数
this.step = function(){
this.y+=8;
}
}
//定义一个全局数组，盛放圆片数组，初始化为0
var circles = [];
//将一个圆片放入圆的数组中
function createCircles(){
var circle = new Circle();
circles[circles.length] = circle;
}
//将圆画出来
function paintCircls(){
for(var i=0;i<circles.length;i++){
circles[i].paint();
}
}
//将圆画出来
function stepCircles(){
for(var i=0;i<circles.length;i++){
circles[i].step();
}
}
var myimg = new Image();
myimg.src = "demo-2-bg.jpg";
var time = 0;
setInterval(
function(){
context.drawImage(myimg,0,0); //刷新背景图片
time++;
if(time%3==0){
createCircles(); //新生成一个圆
}
paintCircls(); //将当前页面所有圆绘出来
stepCircles(); //圆向下走
},10);
</script>
</body>
that's all! 换上不同图片就OK了