原文链接及内容

原文链接：https://openlayers.org/en/latest/examples/webgl-sea-level.html

运行界面

WebGL切片图层的style属性接受一个color表达式，该表达式可用于在渲染之前修改像素值，在这里，表示高程数据的RGB切片数据被加载和渲染，因此海平面以下的值是蓝色的，海平面以上的值是透明的。color表达式对归一化像素值进行操作，范围从0到1。band运算符用于从单个色带中选择归一化值。
将规范化的RGB值转换为高程后，使用case表达式来选择要赋予给定高程的颜色。var操作符允许您使用一个可以由应用程序修改的值，而不是使用常数数值作为颜色数组中的停止值。当用户拖动海平面滑块时，调用layer.updateStyleVariables()函数，用滑块中的值更新level样式变量。

main.js代码如下：

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import Map from 'ol/Map.js';
import TileLayer from 'ol/layer/WebGLTile.js';
import View from 'ol/View.js';
import XYZ from 'ol/source/XYZ.js';
import {fromLonLat} from 'ol/proj.js';

const key = 'Get your own API key at https://www.maptiler.com/cloud/';
const attributions =
  '<a href="https://www.maptiler.com/copyright/" target="_blank">&copy; MapTiler</a> ' +
  '<a href="https://www.openstreetmap.org/copyright" target="_blank">&copy; OpenStreetMap contributors</a>';

// band math operates on normalized values from 0-1
// so we scale by 255 to align with the elevation formula
// from https://cloud.maptiler.com/tiles/terrain-rgb/
const elevation = [
  '+',
  -10000,
  [
    '*',
    0.1 * 255,
    [
      '+',
      ['*', 256 * 256, ['band', 1]],
      ['+', ['*', 256, ['band', 2]], ['band', 3]],
    ],
  ],
];

const layer = new TileLayer({
  opacity: 0.6,
  source: new XYZ({
    url:
      'https://api.maptiler.com/tiles/terrain-rgb/{z}/{x}/{y}.png?key=' + key,
    tileSize: 512,
    maxZoom: 12,
  }),
  style: {
    variables: {
      level: 0,
    },
    color: [
      'case',
      // use the `level` style variable to determine the color
      ['<=', elevation, ['var', 'level']],
      [139, 212, 255, 1],
      [139, 212, 255, 0],
    ],
  },
});

const map = new Map({
  target: 'map',
  layers: [
    new TileLayer({
      source: new XYZ({
        url: 'https://api.maptiler.com/maps/streets/{z}/{x}/{y}.png?key=' + key,
        attributions: attributions,
        tileSize: 512,
        maxZoom: 22,
      }),
    }),
    layer,
  ],
  view: new View({
    center: fromLonLat([-122.3267, 37.8377]),
    zoom: 11,
  }),
});

const control = document.getElementById('level');
const output = document.getElementById('output');
control.addEventListener('input', function () {
  output.innerText = control.value;
  layer.updateStyleVariables({level: parseFloat(control.value)});
});
output.innerText = control.value;

const locations = document.getElementsByClassName('location');
for (let i = 0, ii = locations.length; i < ii; ++i) {
  locations[i].addEventListener('click', relocate);
}

function relocate(event) {
  const data = event.target.dataset;
  const view = map.getView();
  view.setCenter(fromLonLat(data.center.split(',').map(Number)));
  view.setZoom(Number(data.zoom));
}

界面布局文件index.html代码如下：

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<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Sea Level (with WebGL)</title>
    <link rel="stylesheet" href="node_modules/ol/ol.css">
    <style>
      .map {
        width: 100%;
        height: 400px;
      }
      #level {
        display: inline-block;
        width: 150px;
        vertical-align: text-bottom;
      }

      a.location {
        cursor: pointer;
      }

      #map {
        background: #8bd4ff;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <label>
      Sea level
      <input id="level" type="range" min="0" max="100" value="1"/>
      +<span id="output"></span> m
    </label>
    <br>
    Go to
    <a class="location" data-center="-122.3267,37.8377" data-zoom="11">San Francisco</a>,
    <a class="location" data-center="-73.9338,40.6861" data-zoom="11">New York</a>,
    <a class="location" data-center="72.9481,18.9929" data-zoom="11">Mumbai</a>, or
    <a class="location" data-center="120.831,31.160" data-zoom="9">Shanghai</a>
    <!-- Pointer events polyfill for old browsers, see https://caniuse.com/#feat=pointer -->
    <script src="https://cdn.jsdelivr.net/npm/elm-pep@1.0.6/dist/elm-pep.js"></script>
    <script type="module" src="main.js"></script>
  </body>
</html>