先编写一个烟花绽放的动画效果。
放烟花时，一个烟花可分为两个阶段：（1）烟花上升到空中；（2）烟花炸开成碎片，炸开的碎片慢慢消散。
为此抽象出两个对象类：Firework和Particle。其中，Firework用于表示一个烟花对象，Particle用于表示一个烟花炸开后的各碎片。
Firework对象类定义6个属性：表示烟花上升轨迹中各点的坐标（x,y）、烟花弧状轨迹的偏转角度angle、上升阶段水平和垂直方向的位移改变量xSpeed和ySpeed、烟花的色彩色相hue。
坐标属性值y的初始值取画布的高度，表示烟花从地面上升到空中，其余各属性的初始值采用随机数确定。具体定义如下：
function Firework()
{
this.x = canvas.width/4*(1+3*Math.random());
this.y = canvas.height - 15;
this.angle = Math.random() * Math.PI / 4 - Math.PI / 6;
this.xSpeed = Math.sin(this.angle) *(6+Math.random()*7);
this.ySpeed = -Math.cos(this.angle) *(6+Math.random()*7);
this.hue = Math.floor(Math.random() * 360);
}
Firework对象类定义3个方法：绘制烟花上升轨迹的方法draw()、烟花上升时坐标改变方法update()和烟花炸开方法explode()。绘制烟花轨迹时，在各点（x,y）处绘制一个宽度为5、高度为15的填充小矩形表示一个轨迹点。烟花上升时，垂直方向速度ySpeed初始值为负的，每次上升时，ySpeed加上一个正值，表示上升在减速，当ySpeed的值大于0时，烟花上升到顶了（不能再上升），就炸开为70个碎片。具体方法的实现见后面的HTML文件内容。
Particle对象类定义8个属性：表示碎片散开轨迹中各点的坐标（x,y）、碎片弧状轨迹的偏转角度angle、散开时水平和垂直方向的位移改变量xSpeed和ySpeed、碎片的色彩色相hue、表示碎片小圆的半径size、碎片的亮度lightness。
function Particle(x,y,hue)
{
this.x = x;
this.y = y;
this.hue = hue;
this.lightness = 50;
this.size = 15 + Math.random() * 10;
this.angle = Math.random() * 2 * Math.PI;
this.xSpeed = Math.cos(this.angle) *(1+Math.random() * 6);
this.ySpeed = Math.sin(this.angle) *(1+Math.random() * 6);
}
Particle对象类定义2个方法：绘制碎片散开轨迹的方法draw()、碎片散开时坐标改变方法update()。碎片散开时逐渐变小（属性size值减量），当size值小于1时，从碎片数组中删除该碎片，表示碎片已消亡。
定义两个数组var fireworks=[];和var particles=[];分别存储烟花对象和炸开的碎片对象。
模拟动画的函数loop中，每隔一段时间（用count计数来实现）向fireworks数组中添加一个烟花对象，烟花对象上升到顶炸开后，从fireworks数组中删除该对象元素，然后向particles数组中添加70个碎片对象。
遍历两个数组的各对象，分别调用它们的draw()和update()方法。
编写的完整HTML文件内容如下。
<html> <head> <title>烟花绽放</title> </head> <body> <canvas id="myCanvas" width="800" height="600" style="border:3px double #996633;background:black;"> </canvas> <script type="text/javascript"> var canvas=document.getElementById('myCanvas'); ctx= canvas.getContext('2d'); var fireworks=[]; var particles=[]; var counter = 0; function Firework() { this.x = canvas.width/4*(1+3*Math.random()); this.y = canvas.height - 15; this.angle = Math.random() * Math.PI / 4 - Math.PI / 6; this.xSpeed = Math.sin(this.angle) *(6+Math.random()*7); this.ySpeed = -Math.cos(this.angle) *(6+Math.random()*7); this.hue = Math.floor(Math.random() * 360); } Firework.prototype.draw= function() { ctx.save(); ctx.translate(this.x, this.y); ctx.rotate(Math.atan2(this.ySpeed, this.xSpeed) + Math.PI / 2); ctx.fillStyle =`hsl(${this.hue}, 100%, 50%)`; ctx.fillRect(0, 0, 5, 15); ctx.restore(); } Firework.prototype.update= function() { this.x = this.x + this.xSpeed; this.y = this.y + this.ySpeed; this.ySpeed += 0.1; } Firework.prototype.explode= function() { for (var i = 0; i < 70; i++) { particles.push(new Particle(this.x, this.y, this.hue)); } } function Particle(x,y,hue) { this.x = x; this.y = y; this.hue = hue; this.lightness = 50; this.size = 15 + Math.random() * 10; this.angle = Math.random() * 2 * Math.PI; this.xSpeed = Math.cos(this.angle) *(1+Math.random() * 6); this.ySpeed = Math.sin(this.angle) *(1+Math.random() * 6); } Particle.prototype.draw= function() { ctx.fillStyle = `hsl(${this.hue}, 100%, ${this.lightness}%)`; ctx.beginPath(); ctx.arc(this.x, this.y, this.size, 0, 2 * Math.PI); ctx.closePath(); ctx.fill(); } Particle.prototype.update= function(index) { this.ySpeed += 0.05; this.size = this.size*0.95; this.x = this.x + this.xSpeed; this.y = this.y + this.ySpeed; if (this.size<1) { particles.splice(index,1); } } function loop() { ctx.fillStyle = "rgba(0, 0, 0, 0.1)"; ctx.fillRect(0,0,canvas.width,canvas.height); counter++; if (counter==15) { fireworks.push(new Firework()); counter=0; } var i=fireworks.length; while (i--) { fireworks[i].draw(); fireworks[i].update(); if (fireworks[i].ySpeed > 0) { fireworks[i].explode(); fireworks.splice(i, 1); } } var i=particles.length; while (i--) { particles[i].draw(); particles[i].update(i); } requestAnimationFrame(loop); } loop(); </script> </body> </html>
在浏览器中打开包含这段HTML代码的html文件，可以看到在浏览器窗口中呈现出如图1所示的烟花绽放动画效果。
图1 烟花绽放
实现了烟花绽放的效果，我们还可以继续让一定区域内的绽放的烟花碎片拼成“Happy New Year”粒子文本。粒子文本的实现方法参见文章“JavaScript动画实例：粒子文本”（https://www.cnblogs.com/cs-whut/p/13334882.html）。
编写如下的HTML代码。
<html> <head> <title>迎新年烟花绽放</title> <style> body { margin: 0; background: black; } canvas { position: absolute; } </style> </head> <body> <canvas id="myCanvas1"></canvas> <canvas id="myCanvas2"></canvas> <canvas id="myCanvas3"></canvas> <script type="text/javascript"> function Particle(x, y, hue) { this.x = x; this.y = y; this.hue = hue; this.lightness = 50; this.size = 15 + Math.random() * 10; this.angle = Math.random() * 2 * Math.PI; this.xSpeed = Math.cos(this.angle) * (1 + Math.random() * 6); this.ySpeed = Math.sin(this.angle) * (1 + Math.random() * 6); this.target = getTarget(); this.timer = 0; } Particle.prototype.draw= function() { ctx2.fillStyle =`hsl(${this.hue}, 100%, ${this.lightness}%)`; ctx2.beginPath(); ctx2.arc(this.x, this.y, this.size, 0, 2 * Math.PI); ctx2.closePath(); ctx2.fill(); } Particle.prototype.update= function(idx) { if (this.target) { var dx = this.target.x - this.x; var dy = this.target.y - this.y; var dist = Math.sqrt(dx * dx + dy * dy); var a = Math.atan2(dy, dx); var tx = Math.cos(a) * 5; var ty = Math.sin(a) * 5; this.size = lerp(this.size, 1.5, 0.05); if (dist < 5) { this.lightness = lerp(this.lightness, 100, 0.01); this.xSpeed = this.ySpeed = 0; this.x = lerp(this.x, this.target.x + fidelity / 2, 0.05); this.y = lerp(this.y, this.target.y + fidelity / 2, 0.05); this.timer += 1; } else if (dist < 10) { this.lightness = lerp(this.lightness, 100, 0.01); this.xSpeed = lerp(this.xSpeed, tx, 0.1); this.ySpeed = lerp(this.ySpeed, ty, 0.1); this.timer += 1; } else { this.xSpeed = lerp(this.xSpeed, tx, 0.02); this.ySpeed = lerp(this.ySpeed, ty, 0.02); } } else { this.ySpeed += 0.05; this.size = this.size*0.95; if (this.size<1) { particles.splice(idx,1); } } this.x = this.x + this.xSpeed; this.y = this.y + this.ySpeed; } function Firework() { this.x = canvas2.width*(1+ 3*Math.random())/4; this.y = canvas2.height - 15; this.angle = Math.random() * Math.PI / 4 - Math.PI / 6; this.xSpeed = Math.sin(this.angle) * (6 + Math.random() * 7); this.ySpeed = -Math.cos(this.angle) * (6 + Math.random() * 7); this.hue = Math.floor(Math.random() * 360); } Firework.prototype.draw= function() { ctx2.save(); ctx2.translate(this.x, this.y); ctx2.rotate(Math.atan2(this.ySpeed, this.xSpeed) + Math.PI / 2); ctx2.fillStyle = `hsl(${this.hue}, 100%, 50%)`; ctx2.fillRect(0, 0, 5, 15); ctx2.restore(); } Firework.prototype.update= function() { this.x = this.x + this.xSpeed; this.y = this.y + this.ySpeed; this.ySpeed += 0.1; } Firework.prototype.explode= function() { for (var i = 0; i < 70; i++) { particles.push(new Particle(this.x, this.y, this.hue)); } } function lerp(a, b, t) { return Math.abs(b - a)> 0.1 ? a + t * (b - a) : b; } function getTarget() { if (targets.length > 0) { var idx = Math.floor(Math.random() * targets.length); var { x, y } = targets[idx]; targets.splice(idx, 1); x += canvas2.width / 2 - textWidth / 2; y += canvas2.height / 2 - fontSize / 2; return { x, y }; } } var canvas1=document.getElementById('myCanvas1'); ctx1= canvas1.getContext('2d'); var canvas2=document.getElementById('myCanvas2'); ctx2= canvas2.getContext('2d'); var canvas3=document.getElementById('myCanvas3'); ctx3= canvas3.getContext('2d'); var fontSize = 200; var fireworks = []; var particles = []; var targets = []; var fidelity = 3; var counter = 0; canvas2.width = canvas3.width = window.innerWidth; canvas2.height = canvas3.height = window.innerHeight; ctx1.fillStyle = '#000'; var text = 'Happy New Year'; var textWidth = 999999; while (textWidth > window.innerWidth) { ctx1.font = `900 ${fontSize--}px Arial`; textWidth = ctx1.measureText(text).width; } canvas1.width = textWidth; canvas1.height = fontSize * 1.5; ctx1.font = `900 ${fontSize}px Arial`; ctx1.fillText(text, 0, fontSize); var imgData = ctx1.getImageData(0, 0, canvas1.width, canvas1.height); for (var i = 0, max = imgData.data.length; i < max; i += 4) { var alpha = imgData.data[i + 3]; var x = Math.floor(i / 4) % imgData.width; var y = Math.floor(i / 4 / imgData.width); if (alpha && x % fidelity === 0 && y % fidelity === 0) { targets.push({ x, y }); } } ctx3.fillStyle = '#FFF'; ctx3.shadowColor = '#FFF'; ctx3.shadowBlur = 25; function loop() { ctx2.fillStyle = "rgba(0, 0, 0, .1)"; ctx2.fillRect(0, 0, canvas2.width, canvas2.height); counter += 1; if (counter==15) { fireworks.push(new Firework()); counter=0; } var i=fireworks.length; while (i--) { fireworks[i].draw(); fireworks[i].update(); if (fireworks[i].ySpeed > 0) { fireworks[i].explode(); fireworks.splice(i, 1); } } var i=particles.length; while (i--) { particles[i].draw(); particles[i].update(i); if (particles[i].timer >= 100 || particles[i].lightness >= 99) { ctx3.fillRect(particles[i].target.x, particles[i].target.y, fidelity + 1, fidelity + 1); particles.splice(i, 1); } } requestAnimationFrame(loop); } loop(); </script> </body> </html>
在浏览器中打开包含这段HTML代码的html文件，可以看到在浏览器窗口中呈现出如图2所示的烟花绽放迎新年动画效果。图2中为了控制图片的大小，删除了大量的中间帧，因此和实际运行的效果有所不同。
图2 烟花绽放迎新年