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

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