Layer Clipping

main.js

import Map from 'ol/Map.js';
import OSM from 'ol/source/OSM.js';
import TileLayer from 'ol/layer/Tile.js';
import View from 'ol/View.js';

const osm = new TileLayer({
  source: new OSM(),
});

const map = new Map({
  layers: [osm],
  target: 'map',
  view: new View({
    center: [0, 0],
    zoom: 2,
  }),
});

osm.on('prerender', function (event) {
  const ctx = event.context;

  // calculate the pixel ratio and rotation of the canvas
  const matrix = event.inversePixelTransform;
  const canvasPixelRatio = Math.sqrt(
    matrix[0] * matrix[0] + matrix[1] * matrix[1],
  );
  const canvasRotation = -Math.atan2(matrix[1], matrix[0]);
  ctx.save();
  // center the canvas and remove rotation to position clipping
  ctx.translate(ctx.canvas.width / 2, ctx.canvas.height / 2);
  ctx.rotate(-canvasRotation);

  ctx.scale(3 * canvasPixelRatio, 3 * canvasPixelRatio);
  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 / canvasPixelRatio, 1 / 3 / canvasPixelRatio);

  // reapply canvas rotation and position
  ctx.rotate(canvasRotation);
  ctx.translate(-ctx.canvas.width / 2, -ctx.canvas.height / 2);
});

osm.on('postrender', function (event) {
  const ctx = event.context;
  ctx.restore();
});

index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Layer Clipping</title>
    <link rel="stylesheet" href="node_modules/ol/ol.css">
    <style>
      .map {
        width: 100%;
        height: 400px;
      }
      #map {
        background: transparent;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>

    <script type="module" src="main.js"></script>
  </body>
</html>

package.json

{
  "name": "layer-clipping",
  "dependencies": {
    "ol": "9.1.0"
  },
  "devDependencies": {
    "vite": "^3.2.3"
  },
  "scripts": {
    "start": "vite",
    "build": "vite build"
  }
}