Synthetic Points

main.js

import 'ol/ol.css';
import Feature from 'ol/Feature';
import Map from 'ol/Map';
import VectorLayer from 'ol/layer/Vector';
import VectorSource from 'ol/source/Vector';
import View from 'ol/View';
import {Circle as CircleStyle, Fill, Stroke, Style} from 'ol/style';
import {LineString, Point} from 'ol/geom';
import {getVectorContext} from 'ol/render';

const count = 20000;
const features = new Array(count);
const e = 18000000;
for (let i = 0; i < count; ++i) {
  features[i] = new Feature({
    'geometry': new Point([
      2 * e * Math.random() - e,
      2 * e * Math.random() - e,
    ]),
    'i': i,
    'size': i % 2 ? 10 : 20,
  });
}

const styles = {
  '10': new Style({
    image: new CircleStyle({
      radius: 5,
      fill: new Fill({color: '#666666'}),
      stroke: new Stroke({color: '#bada55', width: 1}),
    }),
  }),
  '20': new Style({
    image: new CircleStyle({
      radius: 10,
      fill: new Fill({color: '#666666'}),
      stroke: new Stroke({color: '#bada55', width: 1}),
    }),
  }),
};

const vectorSource = new VectorSource({
  features: features,
  wrapX: false,
});
const vector = new VectorLayer({
  source: vectorSource,
  style: function (feature) {
    return styles[feature.get('size')];
  },
});

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

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

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

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

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

vector.on('postrender', function (evt) {
  const vectorContext = getVectorContext(evt);
  vectorContext.setStyle(style);
  if (point !== null) {
    vectorContext.drawGeometry(point);
  }
  if (line !== null) {
    vectorContext.drawGeometry(line);
  }
});

map.on('pointermove', function (evt) {
  if (evt.dragging) {
    return;
  }
  const pixel = map.getEventPixel(evt.originalEvent);
  const hit = map.hasFeatureAtPixel(pixel);
  if (hit) {
    map.getTarget().style.cursor = 'pointer';
  } else {
    map.getTarget().style.cursor = '';
  }
});

index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Synthetic Points</title>
    <!-- Pointer events polyfill for old browsers, see https://caniuse.com/#feat=pointer -->
    <script src="https://unpkg.com/elm-pep@1.0.6/dist/elm-pep.js"></script>
    <!-- The lines below are only needed for old environments like Internet Explorer and Android 4.x -->
    <script src="https://cdn.polyfill.io/v3/polyfill.min.js?features=fetch,requestAnimationFrame,Element.prototype.classList,TextDecoder"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/core-js/3.18.3/minified.js"></script>
    <style>
      .map {
        width: 100%;
        height:400px;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <script src="main.js"></script>
  </body>
</html>

Synthetic Points

main.js

index.html

package.json