canvas怎样做出黑色背景的红色烟花

<canvas id="canvas"></canvas>

body {    background: #000;    margin: 0;
}canvas {    cursor: crosshair;    display: block;}

// when animating on canvas, it is best to use requestAnimationFrame instead of setTimeout or setInterval// not supported in all browsers though and sometimes needs a prefix, so we need a shimwindow.requestAnimFrame = (function () {    return window.requestAnimationFrame ||        window.webkitRequestAnimationFrame ||        window.mozRequestAnimationFrame ||        function (callback) {            window.setTimeout(callback, 1000 / 60);
        };
})();// now we will setup our basic variables for the demovar canvas = document.getElementById('canvas'),
    ctx = canvas.getContext('2d'),    // full screen dimensions
    cw = window.innerWidth,
    ch = window.innerHeight,    // firework collection
    fireworks = [],    // particle collection
    particles = [],    // starting hue
    hue = 120,    // when launching fireworks with a click, too many get launched at once without a limiter, one launch per 5 loop ticks
    limiterTotal = 5,
    limiterTick = 0,    // this will time the auto launches of fireworks, one launch per 80 loop ticks
    timerTotal = 80,
    timerTick = 0,
    mousedown = false,    // mouse x coordinate,
    mx,    // mouse y coordinate
    my;// set canvas dimensionscanvas.width = cw;
canvas.height = ch;// now we are going to setup our function placeholders for the entire demo// get a random number within a rangefunction random(min, max) {    return Math.random() * (max - min) + min;
}// calculate the distance between two pointsfunction calculateDistance(p1x, p1y, p2x, p2y) {    var xDistance = p1x - p2x,
        yDistance = p1y - p2y;    return Math.sqrt(Math.pow(xDistance, 2) + Math.pow(yDistance, 2));
}// create fireworkfunction Firework(sx, sy, tx, ty) {    // actual coordinates
    this.x = sx;    this.y = sy;    // starting coordinates
    this.sx = sx;    this.sy = sy;    // target coordinates
    this.tx = tx;    this.ty = ty;    // distance from starting point to target
    this.distanceToTarget = calculateDistance(sx, sy, tx, ty);    this.distanceTraveled = 0;    // track the past coordinates of each firework to create a trail effect, increase the coordinate count to create more prominent trails
    this.coordinates = [];    this.coordinateCount = 3;    // populate initial coordinate collection with the current coordinates
    while (this.coordinateCount--) {        this.coordinates.push([this.x, this.y]);
    }    this.angle = Math.atan2(ty - sy, tx - sx);    this.speed = 2;    this.acceleration = 1.05;    this.brightness = random(50, 70);    // circle target indicator radius
    this.targetRadius = 1;
}// update fireworkFirework.prototype.update = function (index) {    // remove last item in coordinates array
    this.coordinates.pop();    // add current coordinates to the start of the array
    this.coordinates.unshift([this.x, this.y]);    // cycle the circle target indicator radius
    if (this.targetRadius < 8) {        this.targetRadius += 0.3;
    } else {        this.targetRadius = 1;
    }    // speed up the firework
    this.speed *= this.acceleration;    // get the current velocities based on angle and speed
    var vx = Math.cos(this.angle) * this.speed,
        vy = Math.sin(this.angle) * this.speed;    // how far will the firework have traveled with velocities applied?
    this.distanceTraveled = calculateDistance(this.sx, this.sy, this.x + vx, this.y + vy);    // if the distance traveled, including velocities, is greater than the initial distance to the target, then the target has been reached
    if (this.distanceTraveled >= this.distanceToTarget) {
        createParticles(this.tx, this.ty);        // remove the firework, use the index passed into the update function to determine which to remove
        fireworks.splice(index, 1);
    } else {        // target not reached, keep traveling
        this.x += vx;        this.y += vy;
    }
}// draw fireworkFirework.prototype.draw = function () {
    ctx.beginPath();    // move to the last tracked coordinate in the set, then draw a line to the current x and y
    ctx.moveTo(this.coordinates[this.coordinates.length - 1][0], this.coordinates[this.coordinates.length - 1][        1
    ]);
    ctx.lineTo(this.x, this.y);
    ctx.strokeStyle = &#39;hsl(&#39; + hue + &#39;, 100%, &#39; + this.brightness + &#39;%)&#39;;
    ctx.stroke();
    ctx.beginPath();    // draw the target for this firework with a pulsing circle
    ctx.arc(this.tx, this.ty, this.targetRadius, 0, Math.PI * 2);
    ctx.stroke();
}// create particlefunction Particle(x, y) {    this.x = x;    this.y = y;    // track the past coordinates of each particle to create a trail effect, increase the coordinate count to create more prominent trails
    this.coordinates = [];    this.coordinateCount = 5;    while (this.coordinateCount--) {        this.coordinates.push([this.x, this.y]);
    }    // set a random angle in all possible directions, in radians
    this.angle = random(0, Math.PI * 2);    this.speed = random(1, 10);    // friction will slow the particle down
    this.friction = 0.95;    // gravity will be applied and pull the particle down
    this.gravity = 1;    // set the hue to a random number +-20 of the overall hue variable
    this.hue = random(hue - 20, hue + 20);    this.brightness = random(50, 80);    this.alpha = 1;    // set how fast the particle fades out
    this.decay = random(0.015, 0.03);
}// update particleParticle.prototype.update = function (index) {    // remove last item in coordinates array
    this.coordinates.pop();    // add current coordinates to the start of the array
    this.coordinates.unshift([this.x, this.y]);    // slow down the particle
    this.speed *= this.friction;    // apply velocity
    this.x += Math.cos(this.angle) * this.speed;    this.y += Math.sin(this.angle) * this.speed + this.gravity;    // fade out the particle
    this.alpha -= this.decay;    // remove the particle once the alpha is low enough, based on the passed in index
    if (this.alpha <= this.decay) {
        particles.splice(index, 1);
    }
}// draw particleParticle.prototype.draw = function () {
    ctx.beginPath();    // move to the last tracked coordinates in the set, then draw a line to the current x and y
    ctx.moveTo(this.coordinates[this.coordinates.length - 1][0], this.coordinates[this.coordinates.length - 1][        1
    ]);
    ctx.lineTo(this.x, this.y);
    ctx.strokeStyle = &#39;hsla(&#39; + this.hue + &#39;, 100%, &#39; + this.brightness + &#39;%, &#39; + this.alpha + &#39;)&#39;;
    ctx.stroke();
}// create particle group/explosionfunction createParticles(x, y) {    // increase the particle count for a bigger explosion, beware of the canvas performance hit with the increased particles though
    var particleCount = 30;    while (particleCount--) {
        particles.push(new Particle(x, y));
    }
}// main demo loopfunction loop() {    // this function will run endlessly with requestAnimationFrame
    requestAnimFrame(loop);    // increase the hue to get different colored fireworks over time
    hue += 0.5;    // normally, clearRect() would be used to clear the canvas
    // we want to create a trailing effect though
    // setting the composite operation to destination-out will allow us to clear the canvas at a specific opacity, rather than wiping it entirely
    ctx.globalCompositeOperation = &#39;destination-out&#39;;    // decrease the alpha property to create more prominent trails
    ctx.fillStyle = &#39;rgba(0, 0, 0, 0.5)&#39;;
    ctx.fillRect(0, 0, cw, ch);    // change the composite operation back to our main mode
    // lighter creates bright highlight points as the fireworks and particles overlap each other
    ctx.globalCompositeOperation = &#39;lighter&#39;;    var text = "HAPPY NEW YEAR !";
    ctx.font = "50px sans-serif";    var textData = ctx.measureText(text);
    ctx.fillStyle = "rgba(" + parseInt(random(0, 255)) + "," + parseInt(random(0, 255)) + "," + parseInt(random(0,        255)) + ",0.3)";
    ctx.fillText(text, cw / 2 - textData.width / 2, ch / 2);    // loop over each firework, draw it, update it
    var i = fireworks.length;    while (i--) {
        fireworks[i].draw();
        fireworks[i].update(i);
    }    // loop over each particle, draw it, update it
    var i = particles.length;    while (i--) {
        particles[i].draw();
        particles[i].update(i);
    }    // launch fireworks automatically to random coordinates, when the mouse isn&#39;t down
    if (timerTick >= timerTotal) {        if (!mousedown) {            // start the firework at the bottom middle of the screen, then set the random target coordinates, the random y coordinates will be set within the range of the top half of the screen
            for (var h = 0; h < 50; h++) {
                fireworks.push(new Firework(cw / 2, ch / 2, random(0, cw), random(0, ch)));
            }
            timerTick = 0;
        }
    } else {
        timerTick++;
    }    // limit the rate at which fireworks get launched when mouse is down
    if (limiterTick >= limiterTotal) {        if (mousedown) {            // start the firework at the bottom middle of the screen, then set the current mouse coordinates as the target
            fireworks.push(new Firework(cw / 2, ch / 2, mx, my));
            limiterTick = 0;
        }
    } else {
        limiterTick++;
    }
}// mouse event bindings// update the mouse coordinates on mousemovecanvas.addEventListener(&#39;mousemove&#39;, function (e) {
    mx = e.pageX - canvas.offsetLeft;
    my = e.pageY - canvas.offsetTop;
});// toggle mousedown state and prevent canvas from being selectedcanvas.addEventListener(&#39;mousedown&#39;, function (e) {
    e.preventDefault();
    mousedown = true;
});
canvas.addEventListener(&#39;mouseup&#39;, function (e) {
    e.preventDefault();
    mousedown = false;
});// once the window loads, we are ready for some fireworks!window.onload = loop;