Jsts

<!--
  示例：几何缓冲 — OpenLayers jsts
  核心逻辑：绘制多边形后对每个顶点采样并求凸包，得到向外扩张的缓冲区几何
  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>几何缓冲</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:14px;align-items:center;font-size:12px;color:#58606e;flex-wrap:wrap}
  .bar label{display:flex;gap:6px;align-items:center}
  .bar button{font:inherit;padding:3px 9px;border:1px solid #ccd2da;background:#fff;
       border-radius:3px;cursor:pointer;color:#2b3140}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3}
</style>
</head>
<body>
<div class="bar">
  <label>缓冲距离 <input type="range" id="dist" min="100" max="2000" step="50" value="600"><b id="distVal">600 m</b></label>
  <button id="clear">清空</button>
  <span id="info">在地图上画一个多边形</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 source = new ol.source.Vector();
  var bufferSource = new ol.source.Vector();

  var polyStyle = new ol.style.Style({
    fill: new ol.style.Fill({ color: 'rgba(31,95,208,0.20)' }),
    stroke: new ol.style.Stroke({ color: '#1f5fd0', width: 2 })
  });

  var bufferStyle = new ol.style.Style({
    fill: new ol.style.Fill({ color: 'rgba(232,89,12,0.14)' }),
    stroke: new ol.style.Stroke({ color: '#e8590c', width: 2, lineDash: [8, 5] })
  });

  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: 0.7 }),
      new ol.layer.Vector({ source: bufferSource, style: bufferStyle }),
      new ol.layer.Vector({ source: source, style: polyStyle })
    ],
    view: new ol.View({
      center: ol.proj.fromLonLat([116.391, 39.907]),
      zoom: 13,
      minZoom: 2
    }),
    controls: ol.control.defaults.defaults({ attribution: false })
  });

  /* Andrew 单调链凸包，输入 [[x,y], ...] */
  function convexHull(points) {
    var pts = points.slice().sort(function (a, b) {
      return a[0] === b[0] ? a[1] - b[1] : a[0] - b[0];
    });
    if (pts.length < 3) return pts;

    function cross(o, a, b) {
      return (a[0] - o[0]) * (b[1] - o[1]) - (a[1] - o[1]) * (b[0] - o[0]);
    }

    var lower = [];
    pts.forEach(function (p) {
      while (lower.length >= 2 &&
        cross(lower[lower.length - 2], lower[lower.length - 1], p) <= 0) {
        lower.pop();
      }
      lower.push(p);
    });

    var upper = [];
    pts.slice().reverse().forEach(function (p) {
      while (upper.length >= 2 &&
        cross(upper[upper.length - 2], upper[upper.length - 1], p) <= 0) {
        upper.pop();
      }
      upper.push(p);
    });

    lower.pop();
    upper.pop();
    return lower.concat(upper);
  }

  /* 顶点圆周采样后求凸包，等价于外扩缓冲的近似 */
  function makeBuffer(polygon, distance) {
    var ring = polygon.getCoordinates()[0];
    var samples = [];
    var SEG = 24;

    ring.forEach(function (coord) {
      for (var k = 0; k < SEG; k++) {
        var a = k / SEG * Math.PI * 2;
        samples.push([
          coord[0] + Math.cos(a) * distance,
          coord[1] + Math.sin(a) * distance
        ]);
      }
    });

    var hull = convexHull(samples);
    if (hull.length < 3) return null;
    hull.push(hull[0]);
    return new ol.geom.Polygon([hull]);
  }

  function rebuild() {
    var distance = Number(document.getElementById('dist').value);
    document.getElementById('distVal').textContent = distance + ' m';
    bufferSource.clear();

    source.getFeatures().forEach(function (f) {
      var geom = f.getGeometry();
      if (!geom || geom.getType() !== 'Polygon') return;
      var buf = makeBuffer(geom, distance);
      if (buf) {
        var feature = new ol.Feature({ geometry: buf });
        feature.setStyle(bufferStyle);
        bufferSource.addFeature(feature);
      }
    });
  }

  var draw = new ol.interaction.Draw({
    source: source,
    type: 'Polygon'
  });

  draw.on('drawend', function (evt) {
    var geom = evt.feature.getGeometry().clone();
    source.clear();
    source.addFeature(new ol.Feature({ geometry: geom }));
    rebuild();
    document.getElementById('info').textContent = '缓冲区半径 ' +
      document.getElementById('dist').value + ' m';
  });

  map.addInteraction(draw);

  /* 预置一个多边形，打开即可看到缓冲效果 */
  var seed = new ol.geom.Polygon([[
    [116.380, 39.900], [116.402, 39.900], [116.405, 39.914],
    [116.386, 39.916], [116.380, 39.900]
  ]]).transform('EPSG:4326', 'EPSG:3857');
  source.addFeature(new ol.Feature({ geometry: seed }));
  rebuild();

  document.getElementById('dist').addEventListener('input', rebuild);

  document.getElementById('clear').addEventListener('click', function () {
    source.clear();
    bufferSource.clear();
    document.getElementById('info').textContent = '在地图上画一个多边形';
  });
})();
</script>
</body>
</html>