Canvas/45_canvas_阴影圆环.html

<!DOCTYPE html>
<html>

<head>
  <meta charset="utf-8" />
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <title>Page Title</title>
  <meta name="viewport" content="width=device-width, initial-scale=1">
</head>

<body>
  <canvas id="myCanvas" style="border:1px solid red;">
    当前浏览器不支持canvas，请更换浏览器后再试
  </canvas>
  <script>
    window.onload = function () {
      var canvas = document.getElementById('myCanvas')

      canvas.width = 800
      canvas.height = 800

      var ctx = canvas.getContext('2d')

      // canvas的阴影永远只产生在绘制区域外侧
      // 所以如果想绘制一个内环带阴影的圆环就必须使用到非零环绕原则

      // 路径方向相反，则环内部根据非零环绕原则就视为外侧，不会填充，并在边缘产生阴影
      ctx.beginPath()
      ctx.arc(400, 400, 300, 0, Math.PI * 2, false)
      ctx.arc(400, 400, 150, 0, Math.PI * 2, true)
      ctx.closePath()

      ctx.fillStyle = 'blue'

      ctx.shadowColor = 'gray'
      ctx.shadowOffsetX = 20
      ctx.shadowOffsetY = 20
      ctx.shadowBlur = 10

      ctx.fill()

    }
  </script>
</body>

</html>