纯JS实现Canvas遮罩画树效果

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>My JS tree</title>
<style>
body {
  width: 1000px;
  height: 570px;
  background-image: url(image/tree.png);
  background-size: 1000px, 570px;
  background-repeat: no-repeat;
  margin-top: 0px;
  margin-bottom: 0px;
}
</style>
</head>

<body>
  <canvas id="mycanvas" width="1000px" height="570px"></canvas>

  <script>
    var c = document.getElementById("mycanvas");
    var ctx = c.getContext("2d");

    ctx.fillStyle = "#ffffff";
    ctx.fillRect(0, 0, c.width, c.height);//矩形遮住背景图像

    var y = c.height;

    window.setInterval(function() {
      if (y > 2) {
        ctx.clearRect(0, y - 1, c.width, y);
        y = y - 1;
      } else {
        window.clearInterval(this);//清除定时
        ctx.clearRect(0, 0, c.width, c.height);
      }
    }, 10);//每隔10ms清除1px的遮照
  </script>
</body>
</html>