d3 Integration

Example of using OpenLayers and d3 together.

The example loads TopoJSON geometries and uses d3 (d3.geo.path) to render these geometries to a canvas element that is then used as the image of an OpenLayers image layer.

<!DOCTYPE html>
<html>
  <head>
    <title>d3 Integration</title>
    <link rel="stylesheet" href="https://openlayers.org/en/v4.6.4/css/ol.css" type="text/css">
    <!-- The line below is only needed for old environments like Internet Explorer and Android 4.x -->
    <script src=""></script>
    <script src="https://openlayers.org/en/v4.6.4/build/ol.js"></script>
    <script src="https://d3js.org/d3.v3.min.js"></script>
    <script src="https://d3js.org/topojson.v1.min.js"></script>
  </head>
  <body>
    <div id="map" class="map"></div>
    <script>
      var map = new ol.Map({
        layers: [
          new ol.layer.Tile({
            source: new ol.source.Stamen({
              layer: 'watercolor'
            })
          })
        ],
        target: 'map',
        view: new ol.View({
          center: ol.proj.fromLonLat([-97, 38]),
          zoom: 4
        })
      });


      /**
       * Load the topojson data and create an ol.layer.Image for that data.
       */
      d3.json('https://openlayers.org/en/v4.6.4/examples/data/topojson/us.json', function(error, us) {
        var features = topojson.feature(us, us.objects.counties);

        /**
         * This function uses d3 to render the topojson features to a canvas.
         * @param {ol.Extent} extent Extent.
         * @param {number} resolution Resolution.
         * @param {number} pixelRatio Pixel ratio.
         * @param {ol.Size} size Size.
         * @param {ol.proj.Projection} projection Projection.
         * @return {HTMLCanvasElement} A canvas element.
         */
        var canvasFunction = function(extent, resolution, pixelRatio,
            size, projection) {
          var canvasWidth = size[0];
          var canvasHeight = size[1];

          var canvas = d3.select(document.createElement('canvas'));
          canvas.attr('width', canvasWidth).attr('height', canvasHeight);

          var context = canvas.node().getContext('2d');

          var d3Projection = d3.geo.mercator().scale(1).translate([0, 0]);
          var d3Path = d3.geo.path().projection(d3Projection);

          var pixelBounds = d3Path.bounds(features);
          var pixelBoundsWidth = pixelBounds[1][0] - pixelBounds[0][0];
          var pixelBoundsHeight = pixelBounds[1][1] - pixelBounds[0][1];

          var geoBounds = d3.geo.bounds(features);
          var geoBoundsLeftBottom = ol.proj.transform(
              geoBounds[0], 'EPSG:4326', projection);
          var geoBoundsRightTop = ol.proj.transform(
              geoBounds[1], 'EPSG:4326', projection);
          var geoBoundsWidth = geoBoundsRightTop[0] - geoBoundsLeftBottom[0];
          if (geoBoundsWidth < 0) {
            geoBoundsWidth += ol.extent.getWidth(projection.getExtent());
          }
          var geoBoundsHeight = geoBoundsRightTop[1] - geoBoundsLeftBottom[1];

          var widthResolution = geoBoundsWidth / pixelBoundsWidth;
          var heightResolution = geoBoundsHeight / pixelBoundsHeight;
          var r = Math.max(widthResolution, heightResolution);
          var scale = r / (resolution / pixelRatio);

          var center = ol.proj.transform(ol.extent.getCenter(extent),
              projection, 'EPSG:4326');
          d3Projection.scale(scale).center(center)
              .translate([canvasWidth / 2, canvasHeight / 2]);
          d3Path = d3Path.projection(d3Projection).context(context);
          d3Path(features);
          context.stroke();

          return canvas[0][0];
        };

        var layer = new ol.layer.Image({
          source: new ol.source.ImageCanvas({
            canvasFunction: canvasFunction,
            projection: 'EPSG:3857'
          })
        });
        map.addLayer(layer);
      });
    </script>
  </body>
</html>