Illustrates various types of opacity (map, map background, and layer)

100%

index.js 
import { Vector3 } from 'three';
import { MapControls } from 'three/examples/jsm/controls/MapControls.js';
import { Fill, Style } from 'ol/style.js';
import GeoJSON from 'ol/format/GeoJSON.js';

import VectorSource from '@giro3d/giro3d/sources/VectorSource.js';
import ColorLayer from '@giro3d/giro3d/core/layer/ColorLayer.js';
import Extent from '@giro3d/giro3d/core/geographic/Extent.js';
import Instance from '@giro3d/giro3d/core/Instance.js';
import Map from '@giro3d/giro3d/entities/Map.js';
import Inspector from '@giro3d/giro3d/gui/Inspector.js';

// Defines geographic extent: CRS, min/max X, min/max Y
const extent = new Extent(
    'EPSG:3857',
    -4553934 - 1000000,
    -4553934 + 1000000,
    -3910697 - 1000000,
    -3910697 + 1000000,
);

// `viewerDiv` will contain Giro3D' rendering area (the canvas element)
const viewerDiv = document.getElementById('viewerDiv');

// Creates a Giro3D instance
const instance = new Instance(viewerDiv, {
    crs: extent.crs(),
    renderer: {
        clearColor: false,
    },
});

// Instanciates camera
instance.camera.camera3D.position.set(-4553934, -3910697, 4600000);

// Instanciates controls
const controls = new MapControls(instance.camera.camera3D, instance.domElement);
controls.target = new Vector3(-4553934, -3910696, 0);
controls.enableDamping = true;
controls.dampingFactor = 0.25;

instance.useTHREEControls(controls);

const map = new Map('map', { extent, backgroundColor: 'green' });
instance.add(map);

const rectangle = {
    type: 'Feature',
    geometry: {
        type: 'Polygon',
        coordinates: [
            [
                [-46, -30],
                [-41, -30],
                [-41, -35],
                [-46, -35],
                [-46, -30],
            ],
        ],
    },
};

const triangle = {
    type: 'Feature',
    geometry: {
        type: 'Polygon',
        coordinates: [
            [
                [-45, -31],
                [-39, -31],
                [-39, -35],
                [-45, -31],
            ],
        ],
    },
};

function makeGeoJSONLayer(name, geojson, color) {
    const style = new Style({
        fill: new Fill({
            color,
        }),
    });
    const source = new VectorSource({
        data: geojson,
        format: new GeoJSON(),
        style,
        dataProjection: 'EPSG:4326',
    });
    const layer = new ColorLayer({
        name,
        extent,
        source,
    });
    return layer;
}

const redSquare = makeGeoJSONLayer('redSquare', rectangle, '#aa0000');
const blueTriangle = makeGeoJSONLayer('blueTriangle', triangle, '#0000aa');

map.addLayer(redSquare);
map.addLayer(blueTriangle);

Inspector.attach(document.getElementById('panelDiv'), instance);

instance.notifyChange(map);

// GUI
function bindSlider(id, action) {
    const slider = document.getElementById(id);
    slider.oninput = () => {
        action(slider.value);
        instance.notifyChange(map);
    };
}

bindSlider('map-opacity', v => {
    map.opacity = v;
});
bindSlider('bg-opacity', v => {
    map.materialOptions.backgroundOpacity = v;
});
bindSlider('blue-opacity', v => {
    blueTriangle.opacity = v;
});
bindSlider('red-opacity', v => {
    redSquare.opacity = v;
});
index.html 
<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <title>Giro3D - Types of opacity</title>
  <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.2/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-T3c6CoIi6uLrA9TneNEoa7RxnatzjcDSCmG1MXxSR1GAsXEV/Dwwykc2MPK8M2HN" crossorigin="anonymous">
  <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap-icons@1.11.3/font/bootstrap-icons.min.css">
  <script src="https://cdn.jsdelivr.net/npm/bootstrap@5.3.2/dist/js/bootstrap.bundle.min.js" integrity="sha384-C6RzsynM9kWDrMNeT87bh95OGNyZPhcTNXj1NW7RuBCsyN/o0jlpcV8Qyq46cDfL" crossorigin="anonymous"></script>
  <style>
    body {
      padding: 0;
      margin: 0;
      width: 100vw;
      height: 100vh;
    }

    #viewerDiv {
      width: 100%;
      height: 100%;
    }

    #panelDiv {
      position: absolute;
      top: 0;
      left: 0;
    }
    #viewerDiv canvas {
  background-image: linear-gradient(45deg, #aaaaaa 25%, transparent 25%),
    linear-gradient(-45deg, #aaaaaa 25%, transparent 25%),
    linear-gradient(45deg, transparent 75%, #aaaaaa 75%),
    linear-gradient(-45deg, transparent 75%, #aaaaaa 75%);
  background-size: 20px 20px;
  background-position:
    0 0,
    0 10px,
    10px -10px,
    -10px 0px;
}

  </style>
</head>

<body>
  <div id="viewerDiv"></div>
  <div id="panelDiv"></div>
  <div class="m-2 position-absolute top-0 end-0">
    <div class="card">
        <div class="card-body">
            <!-- Map global opacity -->
            <label for="map-opacity" class="form-label">Map opacity</label>
            <div class="input-group">
                <input
                    type="range"
                    min="0"
                    max="1"
                    value="1"
                    step="0.05"
                    class="slider"
                    id="map-opacity"
                />
            </div>

            <div class="my-2"></div>

            <!-- Map background opacity -->
            <label for="bg-opacity" class="form-label">Map background opacity</label>
            <div class="input-group">
                <input
                    type="range"
                    min="0"
                    max="1"
                    value="1"
                    step="0.05"
                    class="slider"
                    id="bg-opacity"
                />
            </div>

            <div class="my-2"></div>

            <!-- Blue layer opacity -->
            <label for="blue-opacity" class="form-label">Blue layer opacity</label>
            <div class="input-group">
                <input
                    type="range"
                    min="0"
                    max="1"
                    value="1"
                    step="0.05"
                    class="slider"
                    id="blue-opacity"
                />
            </div>

            <div class="my-2"></div>

            <!-- Red layer opacity -->
            <label for="red-opacity" class="form-label">Red layer opacity</label>
            <div class="input-group">
                <input
                    type="range"
                    min="0"
                    max="1"
                    value="1"
                    step="0.05"
                    class="slider"
                    id="red-opacity"
                />
            </div>
        </div>
    </div>
</div>

  <script type="module" src="index.js"></script>
  <script>
    /* activate popovers */
    const popoverTriggerList = [].slice.call(document.querySelectorAll('[data-bs-toggle="popover"]'));
    popoverTriggerList.map(
      // bootstrap is used as script in the template, disable warning about undef
      // eslint-disable-next-line no-undef
      popoverTriggerEl => new bootstrap.Popover(popoverTriggerEl, {
        trigger: 'hover',
        placement: 'left',
        content: document.getElementById(popoverTriggerEl.getAttribute('data-bs-content')).innerHTML,
        html: true,
      }),
    );
  </script>
</body>

</html>
package.json 
{
  "name": "transparent_map_bg",
  "dependencies": {
    "@giro3d/giro3d": "0.35.0"
  },
  "devDependencies": {
    "vite": "^3.2.3"
  },
  "scripts": {
    "start": "vite",
    "build": "vite build"
  }
}