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Icon Pixel Operations

vector71 style22 icon8 marker5 canvas6 select4

Canvas pixel operations on a point icon.

Example using an icon to symbolize a point. Click on the icon to select it, and it will be rendered using its negative image.

main.js
import Feature from 'ol/Feature';
import Map from 'ol/Map';
import Point from 'ol/geom/Point';
import Select from 'ol/interaction/Select';
import Stamen from 'ol/source/Stamen';
import VectorSource from 'ol/source/Vector';
import View from 'ol/View';
import {Icon, Style} from 'ol/style';
import {Tile as TileLayer, Vector as VectorLayer} from 'ol/layer';

function createStyle(src, img) {
  return new Style({
    image: new Icon({
      anchor: [0.5, 0.96],
      crossOrigin: 'anonymous',
      src: src,
      img: img,
      imgSize: img ? [img.width, img.height] : undefined,
    }),
  });
}

const iconFeature = new Feature(new Point([0, 0]));
iconFeature.set('style', createStyle('data/icon.png', undefined));

const map = new Map({
  layers: [
    new TileLayer({
      source: new Stamen({layer: 'watercolor'}),
    }),
    new VectorLayer({
      style: function (feature) {
        return feature.get('style');
      },
      source: new VectorSource({features: [iconFeature]}),
    }),
  ],
  target: document.getElementById('map'),
  view: new View({
    center: [0, 0],
    zoom: 3,
  }),
});

const selectStyle = {};
const select = new Select({
  style: function (feature) {
    const image = feature.get('style').getImage().getImage();
    if (!selectStyle[image.src]) {
      const canvas = document.createElement('canvas');
      const context = canvas.getContext('2d');
      canvas.width = image.width;
      canvas.height = image.height;
      context.drawImage(image, 0, 0, image.width, image.height);
      const imageData = context.getImageData(0, 0, canvas.width, canvas.height);
      const data = imageData.data;
      for (let i = 0, ii = data.length; i < ii; i = i + (i % 4 == 2 ? 2 : 1)) {
        data[i] = 255 - data[i];
      }
      context.putImageData(imageData, 0, 0);
      selectStyle[image.src] = createStyle(undefined, canvas);
    }
    return selectStyle[image.src];
  },
});
map.addInteraction(select);

map.on('pointermove', function (evt) {
  map.getTargetElement().style.cursor = map.hasFeatureAtPixel(evt.pixel)
    ? 'pointer'
    : '';
});
index.html
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Icon Pixel Operations</title>
    <link rel="stylesheet" href="node_modules/ol/ol.css">
    <style>
      .map {
        width: 100%;
        height: 400px;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <!-- 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": "icon-negative",
  "dependencies": {
    "ol": "7.1.0"
  },
  "devDependencies": {
    "vite": "^3.0.3",
    "@babel/core": "latest"
  },
  "scripts": {
    "start": "vite",
    "build": "vite build"
  }
}