将 Google Maps JS API ImageMapType 剪辑为多边形

function ClipMapType(polygon, map) {
  this.tileSize = new google.maps.Size(256, 256);
  this.polygon = polygon;
  this.map = map;
}

ClipMapType.prototype.getTile = function(coord, zoom, ownerDocument) {
  var map = this.map;
  var scale = Math.pow(2, zoom);
  if (coord.y < 0 || coord.y >= scale) return ownerDocument.createElement('div');
  var tileX = ((coord.x % scale) + scale) % scale;
  var tileY = coord.y;
  // Your url pattern below
  var url = "https://khms0.google.com/kh/v=694&x=" + tileX + "&y=" + tileY + "&z=" + zoom;
  var image = new Image();
  image.src = url;

  var canvas = ownerDocument.createElement('canvas');
  canvas.width = this.tileSize.width;
  canvas.height = this.tileSize.height;
  var context = canvas.getContext('2d');

  var xdif = coord.x * this.tileSize.width;
  var ydif = coord.y * this.tileSize.height;

  var ring = this.polygon.getArray()[0];
  var points = ring.getArray().map(function(x) {
    var worldPoint = map.getProjection().fromLatLngToPoint(x);
    return new google.maps.Point((worldPoint.x) * scale - xdif, (worldPoint.y) * scale - ydif);
  });

  image.onload = function() {
    context.beginPath();
    context.moveTo(points[0].x, points[0].y);
    var count = points.length;
    for (var i = 0; i < count; i++) {
      context.lineTo(points[i].x, points[i].y);
    }
    context.lineTo(points[count - 1].x, points[count - 1].y);

    context.clip();
    context.drawImage(image, 0, 0);
    context.closePath();
  };

  return canvas;
};

function initMap() {
  var map = new google.maps.Map(document.getElementById('map'), {
    zoom: 4,
    center: {
      lat: 15,
      lng: 15
    }
  });
  var polygon = new google.maps.Data.Polygon([
    [{
      lat: 0,
      lng: 0
    }, {
      lat: 30,
      lng: 30
    }, {
      lat: 0,
      lng: 30
    }]
  ]);
  var mapType = new ClipMapType(polygon, map);
  map.overlayMapTypes.insertAt(0, mapType);
}

html,
body {
  height: 100%;
  margin: 0;
  padding: 0;
}
#map {
  height: 100%;
}

<div id="map"></div>
<script async defer src="https://maps.googleapis.com/maps/api/js?callback=initMap">
</script>