This example uses OpenLayers v10.9.0. The latest is v.
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#### Band Contrast Stretch

cog 15 webgl 18 style 27

#####

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

    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>
        <link rel="stylesheet" href="node_modules/ol/ol.css">
        <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 type="module" src="main.js"></script>
      </body>
    </html>

##### package.json

    {
      "name": "cog-stretch",
      "dependencies": {
        "ol": "10.9.0"
      },
      "devDependencies": {
        "vite": "^3.2.3"
      },
      "scripts": {
        "start": "vite",
        "build": "vite build"
      }
    }