Band Contrast Stretch

Red channel max Green channel max Blue channel max

Choosing bands and applying contrast stretch

This example uses the layer.updateStyleVariables() method to update the rendering of a GeoTIFF based on user selected bands and contrast stretch parameters.

main.js

import 'ol/ol.css';
import GeoTIFF from 'ol/source/GeoTIFF';
import Map from 'ol/Map';
import TileLayer from 'ol/layer/WebGLTile';
import View from 'ol/View';

const channels = ['red', 'green', 'blue'];
for (const channel of channels) {
  const selector = document.getElementById(channel);
  selector.addEventListener('change', update);

  const input = document.getElementById(`${channel}Max`);
  input.addEventListener('input', update);
}

function getVariables() {
  const variables = {};
  for (const channel of channels) {
    const selector = document.getElementById(channel);
    variables[channel] = parseInt(selector.value, 10);

    const inputId = `${channel}Max`;
    const input = document.getElementById(inputId);
    variables[inputId] = parseInt(input.value, 10);
  }
  return variables;
}

const layer = new TileLayer({
  style: {
    variables: getVariables(),
    color: [
      'array',
      ['/', ['band', ['var', 'red']], ['var', 'redMax']],
      ['/', ['band', ['var', 'green']], ['var', 'greenMax']],
      ['/', ['band', ['var', 'blue']], ['var', 'blueMax']],
      1,
    ],
  },
  source: new GeoTIFF({
    normalize: false,
    sources: [
      {
        url: 'https://s2downloads.eox.at/demo/EOxCloudless/2020/rgbnir/s2cloudless2020-16bits_sinlge-file_z0-4.tif',
      },
    ],
  }),
});

function update() {
  layer.updateStyleVariables(getVariables());
}

const map = new Map({
  target: 'map',
  layers: [layer],
  view: new View({
    projection: 'EPSG:4326',
    center: [0, 0],
    zoom: 2,
    maxZoom: 6,
  }),
});

index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Band Contrast Stretch</title>
    <!-- Pointer events polyfill for old browsers, see https://caniuse.com/#feat=pointer -->
    <script src="https://unpkg.com/elm-pep"></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;
      }
      .controls {
        display: grid;
        grid-template-columns: auto auto 1fr;
        align-items: baseline;
        gap: 0 1em;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <div class="controls">
      <label for="red">Red channel</label>
      <select id="red">
        <option value="1" selected>visible red</option>
        <option value="2">visible green</option>
        <option value="3">visible blue</option>
        <option value="4">near infrared</option>
      </select>
      <label>max
        <input type="range" id="redMax" value="3000" min="2000" max="5000">
      </label>

      <label for="green">Green channel</label>
      <select id="green">
        <option value="1">visible red</option>
        <option value="2" selected>visible green</option>
        <option value="3">visible blue</option>
        <option value="4">near infrared</option>
      </select>
      <label>max
        <input type="range" id="greenMax" value="3000" min="2000" max="5000">
      </label>

      <label for="blue">Blue channel</label>
      <select id="blue">
        <option value="1">visible red</option>
        <option value="2">visible green</option>
        <option value="3" selected>visible blue</option>
        <option value="4">near infrared</option>
      </select>
      <label>max
        <input type="range" id="blueMax" value="3000" min="2000" max="5000">
      </label>
    </div>
    <script src="main.js"></script>
  </body>
</html>

package.json

{
  "name": "cog-stretch",
  "dependencies": {
    "ol": "6.12.0"
  },
  "devDependencies": {
    "parcel": "^2.0.0"
  },
  "scripts": {
    "start": "parcel index.html",
    "build": "parcel build --public-url . index.html"
  }
}