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<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Hello World</title>
    <style>
      body {
        background: #000;
        margin: 0;
        padding: 0;
        overflow: hidden;
        text-shadow: 0px 0px 80px;
      }
    </style>
  </head>
  <body>
    <canvas id="canvas"></canvas>

    <script>
      var canvas = document.getElementById('canvas')
      // 指定了二维绘图
      var ctx = canvas.getContext('2d')
      canvas.height = window.innerHeight
      canvas.width = window.innerWidth

      // 代码雨的文字
      var texts = 'Hello World'.split('')
      var fontSize = 16
      var columns = canvas.width / fontSize
      // 用于计算输出文字时坐标，所以长度即为列数
      var drops = []
      //初始值
      for (var x = 0; x < columns; x++) {
        drops[x] = 1
      }

      function draw() {
        //让背景逐渐由透明到不透明
        ctx.fillStyle = 'rgba(0, 0, 0, 0.05)'
        ctx.fillRect(0, 0, canvas.width, canvas.height)
        //文字颜色
        ctx.fillStyle = '#0F0'
        ctx.font = fontSize + 'px arial'
        //逐行输出文字
        for (var i = 0; i < drops.length; i++) {
          var text = texts[Math.floor(Math.random() * texts.length)]
          ctx.fillText(text, i * fontSize, drops[i] * fontSize)

          if (drops[i] * fontSize > canvas.height || Math.random() > 0.95) {
            drops[i] = 0
          }

          drops[i]++
        }
      }
      // 指定的周期（以毫秒计）来调用函数
      setInterval(draw, 33)
    </script>
  </body>
</html>