Layer Spy

<!--
  说明：图层放大镜
  核心逻辑：上层用反色 Raster 源，prerender 按指针像素裁剪圆形窗并跟随移动
  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>图层放大镜</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 input[type=range]{width:150px}
  #info{margin-left:auto}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3;cursor:crosshair}
</style>
</head>
<body>
<div class="bar">
  <label>半径 <input type="range" id="r" min="40" max="200" step="10" value="100"></label>
  <span id="rv">100 px</span>
  <span>移动指针查看反色影像</span>
  <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 fromLonLat = ol.proj.fromLonLat;

  var spySource = new ol.source.Raster({
    sources: [new ol.source.XYZ({url: 'https://server.arcgisonline.com/ArcGIS/rest/services/World_Street_Map/MapServer/tile/{z}/{y}/{x}', attributions: 'Tiles © Esri'})],
    operation: function (pixels) {
      var band = pixels[0];
      for (var i = 0; i < band.length; i += 4) {
        band[i] = 255 - band[i];
        band[i + 1] = 255 - band[i + 1];
        band[i + 2] = 255 - band[i + 2];
      }
      return pixels;
    }
  });

  var spyLayer = new ol.layer.Image({ source: spySource });

  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'}) }), spyLayer],
    view: new ol.View({ center: fromLonLat([116.391, 39.907]), zoom: 10, minZoom: 2, maxZoom: 19 }),
    controls: ol.control.defaults.defaults({ attribution: false, rotate: false })
  });

  var pointer = null;
  var radius = 100;
  var info = document.getElementById('info');

  spyLayer.on('prerender', function (evt) {
    var ctx = evt.context;
    var size = map.getSize();
    ctx.save();
    ctx.beginPath();
    if (pointer) {
      ctx.arc(pointer[0], pointer[1], radius, 0, Math.PI * 2);
    } else {
      ctx.rect(0, 0, size[0], size[1]);
    }
    ctx.clip();
  });

  spyLayer.on('postrender', function (evt) {
    evt.context.restore();
  });

  map.on('pointermove', function (evt) {
    if (evt.dragging) return;
    pointer = evt.pixel;
    map.render();
    info.textContent = '放大镜位置 ' + Math.round(pointer[0]) + ', ' + Math.round(pointer[1]);
  });

  map.getTargetElement().addEventListener('pointerleave', function () {
    pointer = null;
    map.render();
    info.textContent = '指针已离开地图';
  });

  document.getElementById('r').addEventListener('input', function () {
    radius = Number(this.value);
    document.getElementById('rv').textContent = radius + ' px';
    map.render();
  });

  info.textContent = '移动指针查看';
})();
</script>
</body>
</html>