This example uses OpenLayers v10.9.0. The latest is v.
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#### Layer Spy

spy 1 imagemanipulation 3 maptiler 26

#####

View a portion of one layer over another

Layer rendering can be manipulated in `prerender` and `postrender` event listeners. These listeners get an event with a reference to the Canvas rendering context. In this example, the `prerender` listener sets a clipping mask around the most recent mouse position, giving you a spyglass effect for viewing one layer over another.

Move around the map to see the effect. Use the ↑ up and ↓ down arrow keys to adjust the spyglass size.

##### main.js

    import Map from 'ol/Map.js';
    import View from 'ol/View.js';
    import TileLayer from 'ol/layer/Tile.js';
    import {fromLonLat} from 'ol/proj.js';
    import {getRenderPixel} from 'ol/render.js';
    import ImageTile from 'ol/source/ImageTile.js';

    const key = 'Get your own API key at https://www.maptiler.com/cloud/';
    const attributions =
      '<a href="https://www.maptiler.com/copyright/" target="_blank">&copy; MapTiler</a> ' +
      '<a href="https://www.openstreetmap.org/copyright" target="_blank">&copy; OpenStreetMap contributors</a>';

    const roads = new TileLayer({
      source: new ImageTile({
        attributions: attributions,
        url: 'https://api.maptiler.com/maps/streets-v2/{z}/{x}/{y}.png?key=' + key,
        tileSize: 512,
        maxZoom: 22,
      }),
    });

    const imagery = new TileLayer({
      source: new ImageTile({
        attributions: attributions,
        url: 'https://api.maptiler.com/maps/satellite/{z}/{x}/{y}.jpg?key=' + key,
        tileSize: 512,
        maxZoom: 20,
      }),
    });

    const container = document.getElementById('map');

    const map = new Map({
      layers: [roads, imagery],
      target: container,
      view: new View({
        center: fromLonLat([-109, 46.5]),
        zoom: 6,
      }),
    });

    let radius = 75;
    document.addEventListener('keydown', function (evt) {
      if (evt.key === 'ArrowUp') {
        radius = Math.min(radius + 5, 150);
        map.render();
        evt.preventDefault();
      } else if (evt.key === 'ArrowDown') {
        radius = Math.max(radius - 5, 25);
        map.render();
        evt.preventDefault();
      }
    });

    // get the pixel position with every move
    let mousePosition = null;

    container.addEventListener('mousemove', function (event) {
      mousePosition = map.getEventPixel(event);
      map.render();
    });

    container.addEventListener('mouseout', function () {
      mousePosition = null;
      map.render();
    });

    // before rendering the layer, do some clipping
    imagery.on('prerender', function (event) {
      const ctx = event.context;
      ctx.save();
      ctx.beginPath();
      if (mousePosition) {
        // only show a circle around the mouse
        const pixel = getRenderPixel(event, mousePosition);
        const offset = getRenderPixel(event, [
          mousePosition[0] + radius,
          mousePosition[1],
        ]);
        const canvasRadius = Math.sqrt(
          Math.pow(offset[0] - pixel[0], 2) + Math.pow(offset[1] - pixel[1], 2),
        );
        ctx.arc(pixel[0], pixel[1], canvasRadius, 0, 2 * Math.PI);
        ctx.lineWidth = (5 * canvasRadius) / radius;
        ctx.strokeStyle = 'rgba(0,0,0,0.5)';
        ctx.stroke();
      }
      ctx.clip();
    });

    // after rendering the layer, restore the canvas context
    imagery.on('postrender', function (event) {
      const ctx = event.context;
      ctx.restore();
    });

##### index.html

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

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

##### package.json

    {
      "name": "layer-spy",
      "dependencies": {
        "ol": "10.9.0"
      },
      "devDependencies": {
        "vite": "^3.2.3"
      },
      "scripts": {
        "start": "vite",
        "build": "vite build"
      }
    }