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: /** @type {olx.control.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();
});
