fireworks on a transparent background with canvas

HTML

<script src="http://www.a-jie.cn/proton/build/proton-1.0.0.min.js"></script>
<script src="http://www.a-jie.cn/proton/example/lib/stats.min.js"></script>
<canvas id="testCanvas" width="758" height="719"></canvas>

CSS

canvas{
    background:url(http://lorempixel.com/output/758/719/);
}

JavaScript

var lighten = (function () {
                var bgcan = document.createElement('canvas');
                return function (can) {
                    bgcan.width = can.width;
                    bgcan.height = can.height;
                    bgcan.getContext('2d').drawImage(can, 0, 0);
                    can.width = can.width;
                    // this is where you have to draw the image behind the fireworks again or erase the canvas that is positioned on top of the 
                    can.getContext('2d').globalAlpha = 0.9;
                    can.getContext('2d').drawImage(bgcan, 0, 0);
                    can.getContext('2d').globalAlpha = 1;
                };
            })();
            var canvas;
            var context;
            var proton;
            var renderer;
            var emitter;
            var stats;

            Main();

            function Main() {
                canvas = document.getElementById("testCanvas");
                canvas.width = window.innerWidth;
                canvas.height = window.innerHeight;
                context = canvas.getContext('2d');
                //context.globalCompositeOperation = "lighter";
                addStats();

                createProton();
                tick();
            }

            function addStats() {
                stats = new Stats();
                stats.setMode(2);
                stats.domElement.style.position = 'absolute';
                stats.domElement.style.left = '0px';
                stats.domElement.style.top = '0px';
                document.body.appendChild(stats.domElement);
            }

            function createProton(image) {
                proton = new Proton;
                emitter = new Proton.Emitter();
                emitter.rate = new Proton.Rate(new Proton.Span(1, 3), 1);
                emitter.addInitialize(new Proton.Mass(1));
                emitter.addInitialize(new Proton.Radius(2, 4));
                emitter.addInitialize(new...