Map Export

Example of exporting a map as a PNG image.

main.js

import GeoJSON from 'ol/format/GeoJSON';
import Map from 'ol/Map';
import VectorSource from 'ol/source/Vector';
import View from 'ol/View';
import {Fill, Style} from 'ol/style';
import {
  Heatmap as HeatmapLayer,
  Vector as VectorLayer,
} from 'ol/layer';
import {asArray} from 'ol/color';

const style = new Style({
  fill: new Fill({
    color: '#eeeeee',
  }),
});

const map = new Map({
  layers: [
    new VectorLayer({
      source: new VectorSource({
        url: 'https://openlayers.org/data/vector/ecoregions.json',
        format: new GeoJSON(),
      }),
      background: 'white',
      style: function (feature) {
        const color = asArray(feature.get('COLOR_NNH') || '#eeeeee');
        color[3] = 0.75;
        style.getFill().setColor(color);
        return style;
      },
    }),
    new HeatmapLayer({
      source: new VectorSource({
        url: 'data/geojson/world-cities.geojson',
        format: new GeoJSON(),
      }),
      weight: function (feature) {
        return feature.get('population') / 1e7;
      },
      radius: 15,
      blur: 15,
      opacity: 0.75,
    }),
  ],
  target: 'map',
  view: new View({
    center: [0, 0],
    zoom: 2,
  }),
});

document.getElementById('export-png').addEventListener('click', function () {
  map.once('rendercomplete', function () {
    const mapCanvas = document.createElement('canvas');
    const size = map.getSize();
    mapCanvas.width = size[0];
    mapCanvas.height = size[1];
    const mapContext = mapCanvas.getContext('2d');
    Array.prototype.forEach.call(
      map.getViewport().querySelectorAll('.ol-layer canvas, canvas.ol-layer'),
      function (canvas) {
        if (canvas.width > 0) {
          const opacity =
            canvas.parentNode.style.opacity || canvas.style.opacity;
          mapContext.globalAlpha = opacity === '' ? 1 : Number(opacity);
          let matrix;
          const transform = canvas.style.transform;
          if (transform) {
            // Get the transform parameters from the style's transform matrix
            matrix = transform
              .match(/^matrix\(([^\(]*)\)$/)[1]
              .split(',')
              .map(Number);
          } else {
            matrix = [
              parseFloat(canvas.style.width) / canvas.width,
              0,
              0,
              parseFloat(canvas.style.height) / canvas.height,
              0,
              0,
            ];
          }
          // Apply the transform to the export map context
          CanvasRenderingContext2D.prototype.setTransform.apply(
            mapContext,
            matrix
          );
          const backgroundColor = canvas.parentNode.style.backgroundColor;
          if (backgroundColor) {
            mapContext.fillStyle = backgroundColor;
            mapContext.fillRect(0, 0, canvas.width, canvas.height);
          }
          mapContext.drawImage(canvas, 0, 0);
        }
      }
    );
    mapContext.globalAlpha = 1;
    mapContext.setTransform(1, 0, 0, 1, 0, 0);
    const link = document.getElementById('image-download');
    link.href = mapCanvas.toDataURL();
    link.click();
  });
  map.renderSync();
});

index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Map Export</title>
    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/[email protected]/dist/css/bootstrap.min.css">
    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/@fortawesome/[email protected]/css/fontawesome.min.css">
    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/@fortawesome/[email protected]/css/solid.css">
    <link rel="stylesheet" href="node_modules/ol/ol.css">
    <style>
      .map {
        width: 100%;
        height: 400px;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <a id="export-png" class="btn btn-outline-dark" role="button"><i class="fa fa-download"></i> Download PNG</a>
    <a id="image-download" download="map.png"></a>
    <!-- Pointer events polyfill for old browsers, see https://caniuse.com/#feat=pointer -->
    <script src="https://unpkg.com/[email protected]/dist/elm-pep.js"></script>
    <script type="module" src="main.js"></script>
  </body>
</html>

package.json

{
  "name": "export-map",
  "dependencies": {
    "ol": "7.1.0"
  },
  "devDependencies": {
    "vite": "^3.0.3",
    "@babel/core": "latest"
  },
  "scripts": {
    "start": "vite",
    "build": "vite build"
  }
}