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";

import { bindSlider } from "./widgets/bindSlider";

const extent = new Extent(
  "EPSG:3857",
  -4553934 - 1000000,
  -4553934 + 1000000,
  -3910697 - 1000000,
  -3910697 + 1000000,
);

const instance = new Instance({
  target: "view",
  crs: extent.crs,
  backgroundColor: null,
});

instance.view.camera.position.set(-4553934, -3910697, 4600000);

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

const map = new 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: {
      content: 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("inspector", instance);

instance.notifyChange(map);

// GUI
bindSlider("map-opacity", (v) => {
  map.opacity = v;
  instance.notifyChange(map);
});
bindSlider("bg-opacity", (v) => {
  map.backgroundOpacity = v;
  instance.notifyChange(map);
});
bindSlider("blue-opacity", (v) => {
  blueTriangle.opacity = v;
  instance.notifyChange(map);
});
bindSlider("red-opacity", (v) => {
  redSquare.opacity = v;
  instance.notifyChange(map);
});
index.html 
<!doctype html>
<html lang="en">
  <head>
    <title>Types of opacity</title>
    <meta charset="UTF-8" />
    <meta name="name" content="transparent_map_bg" />
    <meta
      name="description"
      content="Illustrates various types of opacity (map, map background, and layer)"
    />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />

    <link rel="icon" href="https://giro3d.org/images/favicon.svg" />
    <link
      href="https://giro3d.org/assets/bootstrap-custom.css"
      rel="stylesheet"
    />
    <script src="https://giro3d.org/assets/bootstrap.bundle.min.js"></script>
    <link
      rel="stylesheet"
      type="text/css"
      href="https://giro3d.org/next/examples/css/example.css"
    />

    <style>
      #view 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="view" class="m-0 p-0 w-100 h-100"></div>
    <div
      id="inspector"
      class="position-absolute top-0 start-0 mh-100 overflow-auto"
    ></div>

    <div class="side-pane-with-status-bar">
      <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="form-range"
              id="map-opacity"
              autocomplete="off"
            />
          </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="form-range"
              id="bg-opacity"
              autocomplete="off"
            />
          </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="form-range"
              id="blue-opacity"
              autocomplete="off"
            />
          </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="form-range"
              id="red-opacity"
              autocomplete="off"
            />
          </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": "git+https://gitlab.com/giro3d/giro3d.git"
    },
    "devDependencies": {
        "vite": "^3.2.3"
    },
    "scripts": {
        "start": "vite",
        "build": "vite build"
    }
}