Layer Clipping

Layer clipping example

Layer clipping example

<!DOCTYPE html>
<html>
  <head>
    <title>Layer Clipping</title>
    <link rel="stylesheet" href="https://openlayers.org/en/v4.6.4/css/ol.css" type="text/css">
    <!-- The line below is only needed for old environments like Internet Explorer and Android 4.x -->
    <script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=requestAnimationFrame,Element.prototype.classList,URL"></script>
    <script src="https://openlayers.org/en/v4.6.4/build/ol.js"></script>
    <style>
      #map {
        background: transparent;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <script>
      var osm = new ol.layer.Tile({
        source: new ol.source.OSM()
      });

      var map = new ol.Map({
        layers: [osm],
        target: 'map',
        controls: ol.control.defaults({
          attributionOptions: {
            collapsible: false
          }
        }),
        view: new ol.View({
          center: [0, 0],
          zoom: 2
        })
      });

      osm.on('precompose', function(event) {
        var ctx = event.context;
        ctx.save();
        var pixelRatio = event.frameState.pixelRatio;
        var size = map.getSize();
        ctx.translate(size[0] / 2 * pixelRatio, size[1] / 2 * pixelRatio);
        ctx.scale(3 * pixelRatio, 3 * pixelRatio);
        ctx.translate(-75, -80);
        ctx.beginPath();
        ctx.moveTo(75, 40);
        ctx.bezierCurveTo(75, 37, 70, 25, 50, 25);
        ctx.bezierCurveTo(20, 25, 20, 62.5, 20, 62.5);
        ctx.bezierCurveTo(20, 80, 40, 102, 75, 120);
        ctx.bezierCurveTo(110, 102, 130, 80, 130, 62.5);
        ctx.bezierCurveTo(130, 62.5, 130, 25, 100, 25);
        ctx.bezierCurveTo(85, 25, 75, 37, 75, 40);
        ctx.clip();
        ctx.translate(75, 80);
        ctx.scale(1 / 3 / pixelRatio, 1 / 3 / pixelRatio);
        ctx.translate(-size[0] / 2 * pixelRatio, -size[1] / 2 * pixelRatio);
      });

      osm.on('postcompose', function(event) {
        var ctx = event.context;
        ctx.restore();
      });
    </script>
  </body>
</html>