Sea Level

<!--
  示例：海平面上升
  要点：高程影像按阈值重新着色，滑块改动阈值后 source.changed() 重绘
  依赖：OpenLayers 10.3.1 · jsDelivr CDN（底图 OSM，无 Key）
  替代：官方用真实 DEM 瓦片，这里用页面内生成的高程函数做同构演示
-->
<!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>海平面上升</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;color:#16181d}
  .bar{flex:0 0 auto;padding:8px 12px;background:#f6f7f9;border-bottom:1px solid #e3e6ea;
       display:flex;gap:14px;align-items:center;font-size:12px;color:#58606e;flex-wrap:wrap}
  .bar b{color:#16181d;font-weight:600}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3}
</style>
</head>
<body>
<div class="bar">
  <span>海平面 <b id="lv">0</b> 米</span>
  <input type="range" id="sl" min="0" max="600" value="0" step="10" style="width:200px">
  <span>淹没比例 <b id="pct">—</b></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 (!window.ol) {
    document.getElementById('map').textContent = 'OpenLayers 加载失败，请检查 cdn.jsdelivr.net 是否可访问';
    return;
  }

  var R = 6378137;
  var DEG = 180 / Math.PI;
  var water = 0;

  /* 高程函数带一个负海拔的浅海区，便于观察淹没 */
  function elev(lon, lat) {
    return 600 * Math.sin(lon * 1.3 + 0.4) * Math.cos(lat * 1.7)
         + 220 * Math.sin(lon * 4.1) * Math.sin(lat * 3.3)
         + 90 * Math.cos(lon * 9.7 + lat * 6.1) - 140;
  }

  var canvasFunction = function (extent, resolution, pixelRatio, size) {
    var w = Math.max(1, Math.round(size[0]));
    var h = Math.max(1, Math.round(size[1]));
    var canvas = document.createElement('canvas');
    canvas.width = w;
    canvas.height = h;
    var ctx = canvas.getContext('2d');
    var img = ctx.createImageData(w, h);
    var sx = (extent[2] - extent[0]) / w;
    var sy = (extent[3] - extent[1]) / h;
    var below = 0;

    for (var y = 0; y < h; y++) {
      var my = extent[3] - (y + 0.5) * sy;
      var lat = (2 * Math.atan(Math.exp(my / R)) - Math.PI / 2) * DEG;
      for (var x = 0; x < w; x++) {
        var lon = (extent[0] + (x + 0.5) * sx) / R * DEG;
        var e = elev(lon, lat);
        var i = (y * w + x) * 4;
        if (e < water) {
          below++;
          img.data[i] = 24;
          img.data[i + 1] = 92;
          img.data[i + 2] = 168;
          img.data[i + 3] = 235;
        } else {
          var t = Math.min(1, (e - water) / 900);
          img.data[i] = 90 + t * 150;
          img.data[i + 1] = 140 + t * 80;
          img.data[i + 2] = 90 + t * 60;
          img.data[i + 3] = 225;
        }
      }
    }
    ctx.putImageData(img, 0, 0);
    var ratio = below / (w * h) * 100;
    document.getElementById('pct').textContent = ratio.toFixed(1) + '%';
    return canvas;
  };

  var source = new ol.source.ImageCanvas({ canvasFunction: canvasFunction });

  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'}) }),
      new ol.layer.Image({ source: source, opacity: 0.88 })
    ],
    view: new ol.View({ center: ol.proj.fromLonLat([108.0, 18.0]), zoom: 5 }),
    controls: ol.control.defaults.defaults({ attribution: false, rotate: false })
  });

  document.getElementById('sl').addEventListener('input', function () {
    water = Number(this.value);
    document.getElementById('lv').textContent = water;
    source.changed();
  });
})();
</script>
</body>
</html>