rainyday实现的雨滴效果

模拟雨滴落在玻璃表面的特效，不过在IE下只支持IE9以上的版本.
使用很简单
 <script src="rainyday.js"></script>
        <script>
            function run() {
                var image = document.getElementById('background');
                image.onload = function() {
                    var engine = new RainyDay({
                        image: this
                    });
                    engine.rain([ [1, 2, 8000] ]);
                    engine.rain([ [3, 3, 0.88], [5, 5, 0.9], [6, 2, 1] ], 100);
                };
                image.crossOrigin = 'anonymous';
                image.src = 'images/bridge.jpg';
            }
        </script>