js代码

<script>
var c = document.getElementById('canvas'), 
    $ = c.getContext("2d"),
	 w = c.width = window.innerWidth, 
    h = c.height = window.innerHeight;

function makeItSnow() {
  var snow,  
		arr = [],
		num = 1500,
		tsc = 1, 
		sp = 1;
		sc = 1.3, 
		t = 0, 
		mv = 20, 
		min = 1;
	for (var i = 0; i < num; ++i) {
      snow = new Flake();
      snow.y = Math.random() * (h + 50);
      snow.x = Math.random() * w;
      snow.t = Math.random() * (Math.PI * 2);
      snow.sz = (100 / (10 + (Math.random() * 100))) * sc;
      snow.sp = (Math.pow(snow.sz * .8, 2) * .15) * sp;
      snow.sp = snow.sp < min ? min : snow.sp;
      arr.push(snow);
    }
  	go();
  	function go(){
   	window.requestAnimationFrame(go);
      $.clearRect(0, 0, w, h);
      $.fillRect(0, 0, w, h);
      $.fill();
        for (var i = 0; i < arr.length; ++i) {
          f = arr[i];
          f.t += .05;
          f.t = f.t >= Math.PI * 2 ? 0 : f.t;
          f.y += f.sp;
          f.x += Math.sin(f.t * tsc) * (f.sz * .3);
          if (f.y > h + 50) f.y = -10 - Math.random() * mv;
          if (f.x > w + mv) f.x = - mv;
          if (f.x < - mv) f.x = w + mv;
          f.draw();
		  }
 	}
 	function Flake() {
 		this.draw = function() {
      	this.g = $.createRadialGradient(this.x, this.y, 0, this.x, this.y, this.sz);
			this.g.addColorStop(0, 'hsla(255,255%,255%,1)');
			this.g.addColorStop(1, 'hsla(255,255%,255%,0)');
			$.moveTo(this.x, this.y);
			$.fillStyle = this.g;
			$.beginPath();
			$.arc(this.x, this.y, this.sz, 0, Math.PI * 2, true);
			$.fill();
		}
	}
}


window.addEventListener('resize', function(){
  c.width = w = window.innerWidth;
  c.height = h = window.innerHeight;
}, false);

makeItSnow();</script>