花火

README

http://thecodeplayer.com/walkthrough/canvas-fireworks-tutorial

https://developer.mozilla.org/samples/canvas-tutorial/6_1_canvas_composite.html

jsdoit.css

canvas {
    display: block;
}

body {
    background: #000;
    margin: 0;
}

jsdoit.html

<canvas id="canvas">Canvas is not supported in this browser.</canvas>

jsdoit.js

(function() {
    var hue = 120;
    var particles = [];
    var canvas = document.getElementById('canvas'),
    ctx = canvas.getContext('2d'),
    cw = window.innerWidth,
    ch = window.innerHeight;

    canvas.width = cw;
    canvas.height = ch;
    
    function random(min, max) {
        return Math.random() * (max - min) + min;
    }
    
    function Particle(x, y) {
        this.x = x; this.y = y;
        this.coordinates = [];
        this.coordinateCount = 3;
        while (this.coordinateCount--) {
            this.coordinates.push([this.x, this.y]);
        }
        this.angle = random(0, Math.PI * 2);
        this.speed = random(1, 10);
        this.friction = 0.95;
        this.gravity = 1;
        this.hue = random(hue - 20, hue + 20);
        this.brightness = random(50, 80);
        this.alpha = 1;
        //残像を残したければ小さくする
        this.decay = random(0.01, 0.02);
    }
    
    Particle.prototype.update = function(index) {
        this.coordinates.pop();
        this.coordinates.unshift([this.x, this.y]);
        this.speed *= this.friction;
        this.x += Math.cos(this.angle) * this.speed;
        this.y += Math.sin(this.angle) * this.speed + this.gravity;
        this.alpha -= this.decay;
        if (this.alpha <= this.decay) {
            particles.splice(index, 1);
        }
    };
    
    Particle.prototype.draw = function() {
        ctx.beginPath();
        ctx.moveTo(this.coordinates[this.coordinates.length - 1][0], this.coordinates[this.coordinates.length - 1][1]);
        ctx.lineTo(this.x, this.y);
        ctx.strokeStyle = 'hsla(' + this.hue + ',100%,' + this.brightness + '%,' + this.alpha + ')';
        ctx.stroke();
    };
    
    function createParticles(x, y) {
        var particlesCount = parseInt(random(50, 200));
        while (particlesCount--) {
            particles.push(new Particle(x, y));
        }
    }
    
    window.requestAnimFrame = (function() {
        return window.requestAnimationFrame ||
            window.webkitRequestAnimationFrame ||
            window.mozRequestAnimationFrame ||
                function (callback) {
                    window.setTimeout(callback, 1000 / 60);
                };
    })();
    
    function createRandomParticles() {
        var count = parseInt(random(1, 8));
        while(count--) {
            setTimeout(function() {
                hue += 30;
                createParticles(random(0, cw), random(0, ch));
            }, parseInt(random(50, 300)));
        }
    }
    
    var tick = 0, fireTick = 100;
    
    function loop() {
        requestAnimFrame(loop);
        //軌跡はそのまま、透明度0.5で背景を黒で塗る
        ctx.globalCompositeOperation = 'destination-out';
        ctx.fillStyle = 'rgba(0, 0, 0, 0.5)';
        ctx.fillRect(0, 0, cw, ch);
        //光が重なったら明るくする
        ctx.globalCompositeOperation = 'lighter';
        var i = particles.length;
        while(i--) {
            particles[i].draw();
            particles[i].update(i);
        }
        tick++;
        if (tick > fireTick) {
            createRandomParticles();
            tick = parseInt(random(0, 50));
        }
        hue += 50;
    }

    canvas.addEventListener('mousedown', function(e) {
        var x = e.pageX - canvas.offsetLeft;
        var y = e.pageY - canvas.offsetTop;
        createParticles(x, y);
    });
    
    window.onload = loop;
}());