Canvas/34_canvas_绘制月亮.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')

      ctx.arc(400, 400, 300, 0.5 * Math.PI, 1.5 * Math.PI, true)

      ctx.moveTo(400, 100)
      ctx.arcTo(1200, 400, 400, 700, (400 - 100) * dis(400, 100, 1200, 400) / (1200 - 400))

      ctx.stroke()

    }
    function dis(x1, y1, x2, y2) {
      return Math.sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2))

    }
  </script>
</body>

</html>