Synthetic Points

Synthetic points example.

<!DOCTYPE html>
<html>
  <head>
    <title>Synthetic Points</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="https://cdn.polyfill.io/v2/polyfill.min.js?features=requestAnimationFrame,Element.prototype.classList,URL"></script>
    <script src="https://openlayers.org/en/v4.6.4/build/ol.js"></script>
  </head>
  <body>
    <div id="map" class="map"></div>
    <script>
      var count = 20000;
      var features = new Array(count);
      var e = 18000000;
      for (var i = 0; i < count; ++i) {
        features[i] = new ol.Feature({
          'geometry': new ol.geom.Point(
              [2 * e * Math.random() - e, 2 * e * Math.random() - e]),
          'i': i,
          'size': i % 2 ? 10 : 20
        });
      }

      var styles = {
        '10': new ol.style.Style({
          image: new ol.style.Circle({
            radius: 5,
            fill: new ol.style.Fill({color: '#666666'}),
            stroke: new ol.style.Stroke({color: '#bada55', width: 1})
          })
        }),
        '20': new ol.style.Style({
          image: new ol.style.Circle({
            radius: 10,
            fill: new ol.style.Fill({color: '#666666'}),
            stroke: new ol.style.Stroke({color: '#bada55', width: 1})
          })
        })
      };

      var vectorSource = new ol.source.Vector({
        features: features,
        wrapX: false
      });
      var vector = new ol.layer.Vector({
        source: vectorSource,
        style: function(feature) {
          return styles[feature.get('size')];
        }
      });

      var map = new ol.Map({
        layers: [vector],
        target: document.getElementById('map'),
        view: new ol.View({
          center: [0, 0],
          zoom: 2
        })
      });

      var point = null;
      var line = null;
      var displaySnap = function(coordinate) {
        var closestFeature = vectorSource.getClosestFeatureToCoordinate(coordinate);
        if (closestFeature === null) {
          point = null;
          line = null;
        } else {
          var geometry = closestFeature.getGeometry();
          var closestPoint = geometry.getClosestPoint(coordinate);
          if (point === null) {
            point = new ol.geom.Point(closestPoint);
          } else {
            point.setCoordinates(closestPoint);
          }
          if (line === null) {
            line = new ol.geom.LineString([coordinate, closestPoint]);
          } else {
            line.setCoordinates([coordinate, closestPoint]);
          }
        }
        map.render();
      };

      map.on('pointermove', function(evt) {
        if (evt.dragging) {
          return;
        }
        var coordinate = map.getEventCoordinate(evt.originalEvent);
        displaySnap(coordinate);
      });

      map.on('click', function(evt) {
        displaySnap(evt.coordinate);
      });

      var stroke = new ol.style.Stroke({
        color: 'rgba(255,255,0,0.9)',
        width: 3
      });
      var style = new ol.style.Style({
        stroke: stroke,
        image: new ol.style.Circle({
          radius: 10,
          stroke: stroke
        })
      });

      map.on('postcompose', function(evt) {
        var vectorContext = evt.vectorContext;
        vectorContext.setStyle(style);
        if (point !== null) {
          vectorContext.drawGeometry(point);
        }
        if (line !== null) {
          vectorContext.drawGeometry(line);
        }
      });

      map.on('pointermove', function(evt) {
        if (evt.dragging) {
          return;
        }
        var pixel = map.getEventPixel(evt.originalEvent);
        var hit = map.hasFeatureAtPixel(pixel);
        if (hit) {
          map.getTarget().style.cursor = 'pointer';
        } else {
          map.getTarget().style.cursor = '';
        }
      });
    </script>
  </body>
</html>