JavaScript动画实例：烟花绽放迎新年
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>
View Code
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