Webgl Sea Level

<!--
  示例：OpenLayers WebGL 海平面淹没
  要点：DataTile 生成高程瓦片，WebGLTile 的 band 表达式按水位阈值筛选像素
  依赖：OpenLayers 10.3.1 · jsDelivr CDN
-->
<!DOCTYPE html>
<html lang="zh-CN">
<head>
<meta name="robots" content="noindex,follow">
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>WebGL 海平面淹没</title>
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/ol@10.3.1/ol.css">
<style>
  html,body{height:100%;margin:0}
  body{display:flex;flex-direction:column;overflow:hidden;
       font:13px/1.6 -apple-system,"Segoe UI","PingFang SC",sans-serif}
  .bar{flex:0 0 auto;padding:8px 12px;background:#f6f7f9;border-bottom:1px solid #e3e6ea;
       display:flex;gap:10px;align-items:center;flex-wrap:wrap;font-size:12px;color:#58606e}
  .bar label{display:flex;gap:5px;align-items:center;cursor:pointer}
  .bar select,.bar input[type=range]{font-size:12px}
  .bar .val{font-family:ui-monospace,Consolas,monospace;color:#1f5fd0;min-width:44px}
  .bar .sep{width:1px;height:16px;background:#d8dce2}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3}
</style>
</head>
<body>
<div class="bar">
  <label>水位 <input type="range" id="level" min="0" max="200" value="30"><span class="val" id="levelOut">30 m</span></label>
  <span class="sep"></span>
  <span>蓝色区域表示高程低于当前水位</span>
</div>
<div id="map"></div>
<script src="https://cdn.jsdelivr.net/npm/ol@10.3.1/dist/ol.js"></script>
<script>
(function () {
  'use strict';

  if (typeof ol === 'undefined') {
    document.getElementById('map').textContent = 'OpenLayers 加载失败，请检查能否访问 cdn.jsdelivr.net';
    return;
  }


  var demGrid = ol.tilegrid.createXYZ({tileSize: 256, maxZoom: 12});
  var size = 256;

  function elevationAt(lon, lat) {
    var west = Math.max(0, 116.9 - lon) * 260;
    var hills = 170 * Math.exp(-Math.pow((lon - 115.7) / 0.45, 2)) *
      Math.cos((lat - 40.2) * 2.2);
    var plain = 16 * Math.sin(lon * 19) * Math.cos(lat * 17);
    var bay = Math.max(0, 1 - Math.hypot((lon - 117.9) / 0.9, (lat - 38.9) / 0.55)) * 260;
    return 8 + west + hills + plain - bay;
  }

  var terrain = new ol.source.DataTile({
    tileGrid: demGrid,
    projection: 'EPSG:3857',
    transition: 0,
    wrapX: true,
    loader: function (z, x, y) {
      var extent = demGrid.getTileCoordExtent([z, x, y]);
      var resolution = demGrid.getResolution(z);
      var data = new Uint8Array(size * size * 3);
      var offset = 0;
      for (var row = 0; row < size; ++row) {
        var mapY = extent[3] - (row + 0.5) * resolution;
        for (var col = 0; col < size; ++col) {
          var mapX = extent[0] + (col + 0.5) * resolution;
          var lonlat = ol.proj.toLonLat([mapX, mapY]);
          var height = elevationAt(lonlat[0], lonlat[1]);
          var value = Math.max(0, Math.round((height + 10000) / 0.1));
          data[offset] = (value >> 16) & 255;
          data[offset + 1] = (value >> 8) & 255;
          data[offset + 2] = value & 255;
          offset += 3;
        }
      }
      return data;
    }
  });

  var elevation = [
    '+', -10000,
    ['*', 0.1 * 255 * 256 * 256, ['band', 1]],
    ['*', 0.1 * 255 * 256, ['band', 2]],
    ['*', 0.1 * 255, ['band', 3]]
  ];

  var layer = new ol.layer.WebGLTile({
    opacity: 0.65,
    source: terrain,
    style: {
      variables: {level: 30},
      color: [
        'case',
        ['<=', elevation, ['var', 'level']],
        [46, 116, 196, 0.85],
        [46, 116, 196, 0]
      ]
    }
  });

  var map = new ol.Map({
    target: 'map',
    layers: [
      new ol.layer.Tile({source: new ol.source.XYZ({url: 'https://server.arcgisonline.com/ArcGIS/rest/services/World_Street_Map/MapServer/tile/{z}/{y}/{x}', attributions: 'Tiles © Esri'}), opacity: 0.75}),
      layer
    ],
    view: new ol.View({
      center: ol.proj.fromLonLat([117.35, 39.05]),
      zoom: 9
    })
  });

  var control = document.getElementById('level');
  var output = document.getElementById('levelOut');

  control.addEventListener('input', function () {
    var value = parseFloat(control.value);
    output.textContent = value + ' m';
    layer.updateStyleVariables({level: value});
  });

})();
</script>
</body>
</html>