Canvas/30_canva_绘制星空_背景渐变.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 = 1200
      canvas.height = 800

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

      // 绘制背景
      var grd = ctx.createLinearGradient(0, 0, 0, 800)
      grd.addColorStop(0.0, '#000')
      grd.addColorStop(1.0, '#035')
      ctx.fillStyle = grd
      ctx.fillRect(0, 0, canvas.width, canvas.height)

      // 绘制星星
      for (var i = 0; i < 200; i++) {
        var r = Math.random() * 10 + 10
        var x = Math.random() * canvas.width
        var y = Math.random() * canvas.height * 0.55
        var rot = Math.random() * 360
        drawStar(ctx, x, y, r, r / 2, rot)
      }
    }

    function drawStar(ctx, x, y, R, r, rot) {

      ctx.save()

      ctx.translate(x, y)
      ctx.rotate(rot / 180 * Math.PI)
      ctx.scale(R, R)

      starPath(ctx)

      ctx.fillStyle = '#fb3'

      ctx.fill()

      ctx.restore()
    }

    function starPath(ctx) {
      ctx.beginPath()
      for (var i = 0; i < 5; i++) {
        ctx.lineTo(Math.cos((18 + i * 72) / 180 * Math.PI),
          -Math.sin((18 + i * 72) / 180 * Math.PI))
        ctx.lineTo(Math.cos((54 + i * 72) / 180 * Math.PI) * 0.5,
          -Math.sin((54 + i * 72) / 180 * Math.PI) * 0.5)
      }
      ctx.closePath()
    }
  </script>
</body>

</html>