canvas怎样做出黑色背景的青色烟花

<canvas></canvas><h1>201<span>8</span></h1>

html,body { padding: 0px; margin: 0px; background: #222; font-family: 'Karla', sans-serif; color: #FFF; height: 100%; overflow: hidden;
}h1 { z-index: 1000; position: fixed; top: 50%; left: 50%; transform: translateX(-50%) translateY(-100%); font-size: 58px; overflow: hidden;
}span { position: relative; display: inline-block; animation: drop 0.75s ease 0s;
}canvas { width: 100%; height: 100%;
}
@keyframes drop {
 0% { transform: translateY(-100px); opacity: 0;
 }
 90% { opacity: 1; transform: translateY(10px);
 }
 100% { transform: translateY(0px);
 }
}

var ctx = document.querySelector('canvas').getContext('2d')ctx.canvas.width = window.innerWidthctx.canvas.height = window.innerHeightvar sparks = []var fireworks = []var i = 20;while (i--) { fireworks.push( new Firework(Math.random() * window.innerWidth, window.innerHeight * Math.random()) )}render()function render() { setTimeout(render, 1000 / 60) ctx.fillStyle = 'rgba(0, 0, 0, 0.1)'; ctx.fillRect(0, 0, ctx.canvas.width, ctx.canvas.height) for (var firework of fireworks) { if (firework.dead) continue firework.move() firework.draw() } for (var spark of sparks) { if (spark.dead) continue spark.move() spark.draw() } if (Math.random() < 0.05) { fireworks.push(new Firework()) }}function Spark(x, y, color) { this.x = x this.y = y this.dir = Math.random() * (Math.PI * 2) this.dead = false this.color = color this.speed = Math.random() * 3 + 3; this.walker = new Walker({ radius: 20, speed: 0.25 }) this.gravity = 0.25 this.dur = this.speed / 0.1 this.move = function () { this.dur-- if (this.dur < 0) this.dead = true if (this.speed < 0) return if (this.speed > 0) this.speed -= 0.1 var walk = this.walker.step() this.x += Math.cos(this.dir + walk) * this.speed this.y += Math.sin(this.dir + walk) * this.speed this.y += this.gravity this.gravity += 0.05 } this.draw = function () { drawCircle(this.x, this.y, 3, this.color) }}function Firework(x, y) { this.xmove = new Walker({ radius: 10, speed: 0.5 }) this.x = x || Math.random() * ctx.canvas.width this.y = y || ctx.canvas.height this.height = Math.random() * ctx.canvas.height / 2 this.dead = false this.color = randomColor() this.move = function () { this.x += this.xmove.step() if (this.y > this.height) this.y -= 1; else this.burst() } this.draw = function () { drawCircle(this.x, this.y, 1, this.color) } this.burst = function () { this.dead = true var i = 100; while (i--) sparks.push(new Spark(this.x, this.y, this.color)) }}function drawCircle(x, y, radius, color) { color = color || '#FFF' ctx.fillStyle = color ctx.fillRect(x - radius / 2, y - radius / 2, radius, radius)}function randomColor() { return ['#6ae5ab', '#88e3b2', '#36b89b', '#7bd7ec', '#66cbe1'][Math.floor(Math.random() * 5)];}function Walker(options) { this.step = function () { this.direction = Math.sign(this.target) * this.speed this.value += this.direction this.target ? this.target -= this.direction : (this.value) ? (this.wander) ? this.target = this.newTarget() : this.target = -this.value : this.target = this.newTarget() return this.direction } this.newTarget = function () { return Math.round(Math.random() * (this.radius * 2) - this.radius) } this.start = 0 this.value = 0 this.radius = options.radius this.target = this.newTarget() this.direction = Math.sign(this.target) this.wander = options.wander this.speed = options.speed || 1}