Layer Spy

This example uses OpenLayers v4.0.1. The latest is v.

#### Layer Spy

View a portion of one layer over another

Layer rendering can be manipulated in `precompose` and `postcompose` event listeners. These listeners get an event with a reference to the Canvas rendering context. In this example, the `precompose` 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.

Related API documentation:

- ol.Map
,
- ol.View
,
- ol.layer.Tile
,
- ol.proj
,
- ol.source.BingMaps

Copy Edit
Layer Spy View a portion of one layer over another var key = 'Your Bing Maps Key from http://www.bingmapsportal.com/ here'; var roads = new ol.layer.Tile({ source: new ol.source.BingMaps({key: key, imagerySet: 'Road'}) }); var imagery = new ol.layer.Tile({ source: new ol.source.BingMaps({key: key, imagerySet: 'Aerial'}) }); var container = document.getElementById('map'); var map = new ol.Map({ layers: [roads, imagery], target: container, view: new ol.View({ center: ol.proj.fromLonLat([-109, 46.5]), zoom: 6 }) }); var radius = 75; document.addEventListener('keydown', function(evt) { if (evt.which === 38) { radius = Math.min(radius + 5, 150); map.render(); evt.preventDefault(); } else if (evt.which === 40) { radius = Math.max(radius - 5, 25); map.render(); evt.preventDefault(); } }); // get the pixel position with every move var 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('precompose', function(event) { var ctx = event.context; var pixelRatio = event.frameState.pixelRatio; ctx.save(); ctx.beginPath(); if (mousePosition) { // only show a circle around the mouse ctx.arc(mousePosition[0] * pixelRatio, mousePosition[1] * pixelRatio, radius * pixelRatio, 0, 2 * Math.PI); ctx.lineWidth = 5 * pixelRatio; ctx.strokeStyle = 'rgba(0,0,0,0.5)'; ctx.stroke(); } ctx.clip(); }); // after rendering the layer, restore the canvas context imagery.on('postcompose', function(event) { var ctx = event.context; ctx.restore(); }); <div id="map" class="map"></div>

    <!DOCTYPE html>
    <html>
      <head>
        <title>Layer Spy</title>
        <link rel="stylesheet" href="https://openlayers.org/en/v4.0.1/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/v4.0.1/build/ol.js"></script>
      </head>
      <body>
        <div id="map" class="map"></div>
        <script>
          var key = 'Your Bing Maps Key from http://www.bingmapsportal.com/ here';

          var roads = new ol.layer.Tile({
            source: new ol.source.BingMaps({key: key, imagerySet: 'Road'})
          });

          var imagery = new ol.layer.Tile({
            source: new ol.source.BingMaps({key: key, imagerySet: 'Aerial'})
          });

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

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

          var radius = 75;
          document.addEventListener('keydown', function(evt) {
            if (evt.which === 38) {
              radius = Math.min(radius + 5, 150);
              map.render();
              evt.preventDefault();
            } else if (evt.which === 40) {
              radius = Math.max(radius - 5, 25);
              map.render();
              evt.preventDefault();
            }
          });

          // get the pixel position with every move
          var 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('precompose', function(event) {
            var ctx = event.context;
            var pixelRatio = event.frameState.pixelRatio;
            ctx.save();
            ctx.beginPath();
            if (mousePosition) {
              // only show a circle around the mouse
              ctx.arc(mousePosition[0] * pixelRatio, mousePosition[1] * pixelRatio,
                  radius * pixelRatio, 0, 2 * Math.PI);
              ctx.lineWidth = 5 * pixelRatio;
              ctx.strokeStyle = 'rgba(0,0,0,0.5)';
              ctx.stroke();
            }
            ctx.clip();
          });

          // after rendering the layer, restore the canvas context
          imagery.on('postcompose', function(event) {
            var ctx = event.context;
            ctx.restore();
          });
        </script>
      </body>
    </html>