Heatmap Gradient

<!--
  示例：热力渐变色
  要点：gradient 数组定义色带，用两个热力层切换而不重建数据源
  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>热力渐变色</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:12px;align-items:center;font-size:12px;color:#58606e}
  .bar select{padding:3px 6px;border:1px solid #cfd4dc;border-radius:4px;font-size:12px}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3}
</style>
</head>
<body>
<div class="bar">
  <label>色带
    <select id="scheme">
      <option value="cool">冷色带（蓝绿黄红）</option>
      <option value="warm">暖色带（紫橙）</option>
    </select>
  </label>
  <span>同一份点位，只切换渐变</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').innerHTML =
      '<div style="padding:20px;color:#b42318">OpenLayers 加载失败，请检查能否访问 cdn.jsdelivr.net</div>';
    return;
  }

  var seed = 4402;
  function rand() {
    seed = (seed * 1664525 + 1013904223) % 4294967296;
    return seed / 4294967296;
  }

  var features = [];
  for (var i = 0; i < 90; i++) {
    /* 两个热力中心 + 均匀噪声 */
    var cx = i % 2 === 0 ? 116.395 : 116.425;
    var lon = cx + (rand() - 0.5) * 0.05;
    var lat = 39.905 + (rand() - 0.5) * 0.04;
    features.push(new ol.Feature({
      geometry: new ol.geom.Point(ol.proj.fromLonLat([lon, lat])),
      value: 0.3 + rand() * 0.7
    }));
  }

  var source = new ol.source.Vector({ features: features });

  function weight(feature) {
    return feature.get('value');
  }

  var coolLayer = new ol.layer.Heatmap({
    source: source,
    radius: 12,
    blur: 16,
    weight: weight,
    gradient: ['#0b3d91', '#1f8a4c', '#f2c94c', '#e8590c', '#c0392b']
  });

  var warmLayer = new ol.layer.Heatmap({
    source: source,
    radius: 12,
    blur: 16,
    weight: weight,
    gradient: ['#2b1055', '#7b2d8e', '#e8590c', '#ffb703'],
    visible: false
  });

  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'}) }),
      coolLayer,
      warmLayer
    ],
    view: new ol.View({
      center: ol.proj.fromLonLat([116.410, 39.905]),
      zoom: 12
    }),
    controls: ol.control.defaults.defaults({ attribution: false })
  });

  document.getElementById('scheme').addEventListener('change', function () {
    var warm = this.value === 'warm';
    coolLayer.setVisible(!warm);
    warmLayer.setVisible(warm);
  });
})();
</script>
</body>
</html>