Magnify

Show a magnified version of imager under the pointer

This example makes use of the postrender event listener to oversample imagery in a circle around the pointer location. Listeners for this event have access to the Canvas context and can manipulate image data.

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

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index.html<!DOCTYPE html>
<html lang="en">
  <head>
    <title>Magnify</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=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 BingMaps from 'ol/source/BingMaps';
import {getRenderPixel} from 'ol/render';

var key = 'Your Bing Maps Key from http://www.bingmapsportal.com/ here';

var imagery = new TileLayer({
  source: new BingMaps({key: key, imagerySet: 'Aerial'})
});

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

var map = new Map({
  layers: [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();
});

// after rendering the layer, show an oversampled version around the pointer
imagery.on('postrender', function(event) {
  if (mousePosition) {
    var pixel = getRenderPixel(event, mousePosition);
    var offset = getRenderPixel(event, [mousePosition[0] + radius, mousePosition[1]]);
    var half = Math.sqrt(Math.pow(offset[0] - pixel[0], 2) + Math.pow(offset[1] - pixel[1], 2));
    var context = event.context;
    var centerX = pixel[0];
    var centerY = pixel[1];
    var originX = centerX - half;
    var originY = centerY - half;
    var size = Math.round(2 * half + 1);
    var sourceData = context.getImageData(originX, originY, size, size).data;
    var dest = context.createImageData(size, size);
    var destData = dest.data;
    for (var j = 0; j < size; ++j) {
      for (var i = 0; i < size; ++i) {
        var dI = i - half;
        var dJ = j - half;
        var dist = Math.sqrt(dI * dI + dJ * dJ);
        var sourceI = i;
        var sourceJ = j;
        if (dist < half) {
          sourceI = Math.round(half + dI / 2);
          sourceJ = Math.round(half + dJ / 2);
        }
        var destOffset = (j * size + i) * 4;
        var sourceOffset = (sourceJ * size + sourceI) * 4;
        destData[destOffset] = sourceData[sourceOffset];
        destData[destOffset + 1] = sourceData[sourceOffset + 1];
        destData[destOffset + 2] = sourceData[sourceOffset + 2];
        destData[destOffset + 3] = sourceData[sourceOffset + 3];
      }
    }
    context.beginPath();
    context.arc(centerX, centerY, half, 0, 2 * Math.PI);
    context.lineWidth = 3 * half / radius;
    context.strokeStyle = 'rgba(255,255,255,0.5)';
    context.putImageData(dest, originX, originY);
    context.stroke();
    context.restore();
  }
});