Cog Overviews

<!--
  示例：带概览层的影像 — OpenLayers cog-overviews
  核心逻辑：低分辨率概览层常驻底图，全分辨率层在缩放到位后接入，按分辨率区间切换
  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>带概览层的影像</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;flex-wrap:wrap}
  .bar label{display:flex;gap:5px;align-items:center;cursor:pointer}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3}
</style>
</head>
<body>
<div class="bar">
  <label><input type="checkbox" id="lock"> 锁定为概览层</label>
  <span id="info">—</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 extent = ol.proj.transformExtent(
    [116.20, 39.82, 116.56, 40.02], 'EPSG:4326', 'EPSG:3857'
  );

  function makeOverlay(size, cells) {
    var c = document.createElement('canvas');
    c.width = size;
    c.height = size;
    var ctx = c.getContext('2d');
    ctx.fillStyle = '#dfe7d8';
    ctx.fillRect(0, 0, size, size);

    for (var i = 0; i < cells; i++) {
      for (var j = 0; j < cells; j++) {
        var v = Math.abs(Math.sin(i / 4.3) * Math.cos(j / 5.1));
        ctx.fillStyle = 'rgba(' + Math.round(60 + 120 * v) + ',' +
          Math.round(110 + 90 * v) + ',' + Math.round(60 + 60 * v) + ',0.7)';
        ctx.fillRect(i * size / cells, j * size / cells,
          size / cells + 0.5, size / cells + 0.5);
      }
    }

    /* 街区格网与主干道，用于区分概览与全分辨率 */
    ctx.strokeStyle = 'rgba(255,255,255,0.55)';
    ctx.lineWidth = Math.max(1, size / 300);
    for (var k = 1; k < 6; k++) {
      ctx.beginPath();
      ctx.moveTo(0, size * k / 6);
      ctx.lineTo(size, size * k / 6);
      ctx.stroke();
      ctx.beginPath();
      ctx.moveTo(size * k / 6, 0);
      ctx.lineTo(size * k / 6, size);
      ctx.stroke();
    }

    ctx.fillStyle = 'rgba(15,15,15,0.75)';
    ctx.font = Math.round(size / 16) + 'px sans-serif';
    ctx.fillText(size + 'px 概览', size * 0.05, size * 0.12);
    return c.toDataURL();
  }

  var overview = new ol.layer.Image({
    source: new ol.source.ImageStatic({
      url: makeOverlay(256, 4),
      projection: 'EPSG:3857',
      imageExtent: extent,
      crossOrigin: 'anonymous'
    }),
    opacity: 0.9
  });

  var fullRes = new ol.layer.Image({
    source: new ol.source.ImageStatic({
      url: makeOverlay(1024, 20),
      projection: 'EPSG:3857',
      imageExtent: extent,
      crossOrigin: 'anonymous'
    }),
    /* 与概览层重叠显示，概览只在全分辨率缺席时露面 */
    minResolution: 0,
    maxResolution: 120
  });

  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: 0.5 }),
      overview,
      fullRes
    ],
    view: new ol.View({
      center: ol.proj.fromLonLat([116.38, 39.92]),
      zoom: 12,
      minZoom: 2
    }),
    controls: ol.control.defaults.defaults({ attribution: false })
  });

  function update() {
    var res = map.getView().getResolution();
    var useFull = res < 120;
    document.getElementById('info').textContent =
      '分辨率 ' + res.toFixed(0) + ' m/px · 显示 ' +
      (useFull ? '全分辨率（叠加概览）' : '仅概览');
  }

  document.getElementById('lock').addEventListener('change', function () {
    fullRes.setVisible(!this.checked);
    update();
  });

  map.on('moveend', update);
  update();
})();
</script>
</body>
</html>