Layer Spy

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Layer Spy

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.

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index.html<!DOCTYPE html>
<html lang="en">
  <head>
    <title>Layer Spy</title>
    <!-- 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=fetch,requestAnimationFrame,Element.prototype.classList,URL"></script>
    <style>
      .map {
        width: 100%;
        height:400px;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <script src="index.js"></script>
  </body>
</html>

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index.jsimport 'ol/ol.css';
import Map from 'ol/Map';
import View from 'ol/View';
import TileLayer from 'ol/layer/Tile';
import {fromLonLat} from 'ol/proj';
import XYZ from 'ol/source/XYZ';
import {getRenderPixel} from 'ol/render';

var key = 'Get your own API key at https://www.maptiler.com/cloud/';
var 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>';

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

var imagery = new TileLayer({
  source: new XYZ({
    attributions: attributions,
    url: 'https://api.maptiler.com/tiles/satellite/{z}/{x}/{y}.jpg?key=' + key,
    maxZoom: 20
  })
});

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

var map = new Map({
  layers: [roads, imagery],
  target: container,
  view: new View({
    center: 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('prerender', function(event) {
  var ctx = event.context;
  ctx.save();
  ctx.beginPath();
  if (mousePosition) {
    // only show a circle around the mouse
    var pixel = getRenderPixel(event, mousePosition);
    var offset = getRenderPixel(event, [mousePosition[0] + radius, mousePosition[1]]);
    var 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) {
  var ctx = event.context;
  ctx.restore();
});

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package.json{
  "name": "layer-spy",
  "dependencies": {
    "ol": "6.3.1"
  },
  "devDependencies": {
    "parcel": "1.11.0"
  },
  "scripts": {
    "start": "parcel index.html",
    "build": "parcel build --experimental-scope-hoisting --public-url . index.html"
  }
}