Render Geometry

<!--
  示例：几何图形渲染
  要点：ol.render.toContext 拿到离屏渲染器，直接把几何画进普通 canvas
  依赖：OpenLayers 10.3.1 · jsDelivr CDN（底图 OSM，无 Key）
-->
<!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>几何图形渲染</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}
  .wrap{position:relative;flex:1 1 auto;min-height:0}
  #map{position:absolute;inset:0;background:#eef0f3}
  .panel{position:absolute;left:12px;bottom:12px;background:#fff;border:1px solid #e3e6ea;
         border-radius:8px;box-shadow:0 4px 16px rgba(16,24,40,.12);padding:8px}
  .panel h5{margin:0 0 6px;font-size:12px;color:#58606e;font-weight:600}
  .panel canvas{display:block;border-radius:4px}
</style>
</head>
<body>
<div class="bar"><span>左下角图例由离屏渲染器直接绘制，与地图共用同一套样式</span></div>
<div class="wrap">
  <div id="map"></div>
  <div class="panel">
    <h5>图例 · offscreen render</h5>
    <canvas id="legend" width="220" height="120"></canvas>
  </div>
</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 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.Vector({
        source: new ol.source.Vector({
          features: [
            new ol.Feature(new ol.geom.Point(ol.proj.fromLonLat([116.391, 39.907]))),
            new ol.Feature(new ol.geom.Point(ol.proj.fromLonLat([116.44, 39.93])))
          ]
        }),
        style: new ol.style.Style({
          image: new ol.style.Circle({
            radius: 6,
            fill: new ol.style.Fill({ color: '#1f5fd0' }),
            stroke: new ol.style.Stroke({ color: '#fff', width: 2 })
          })
        })
      })
    ],
    view: new ol.View({ center: ol.proj.fromLonLat([116.40, 39.92]), zoom: 13 }),
    controls: ol.control.defaults.defaults({ attribution: false, rotate: false })
  });

  /* 离屏渲染器：坐标以 canvas 左上角为原点，y 轴向下 */
  var canvas = document.getElementById('legend');
  var renderer = ol.render.toContext(canvas.getContext('2d'), {
    size: [220, 120],
    pixelRatio: 1
  });

  var areaStyle = new ol.style.Style({
    fill: new ol.style.Fill({ color: 'rgba(31,95,208,.20)' }),
    stroke: new ol.style.Stroke({ color: '#1f5fd0', width: 2 })
  });

  var pointStyle = new ol.style.Style({
    image: new ol.style.Circle({
      radius: 6,
      fill: new ol.style.Fill({ color: '#1f5fd0' }),
      stroke: new ol.style.Stroke({ color: '#fff', width: 2 })
    })
  });

  /* 在 canvas 坐标系里画多边形与点，不经过地图 */
  renderer.setStyle(areaStyle);
  renderer.drawGeometry(new ol.geom.Polygon([[
    [12, 60], [64, 30], [104, 74], [46, 100]
  ]]));

  renderer.setStyle(pointStyle);
  renderer.drawGeometry(new ol.geom.Point([138, 52]));
  renderer.drawGeometry(new ol.geom.Point([172, 82]));

  renderer.setStyle(new ol.style.Style({
    stroke: new ol.style.Stroke({ color: '#d64545', width: 2, lineDash: [6, 4] })
  }));
  renderer.drawGeometry(new ol.geom.LineString([[126, 20], [206, 20]]));
})();
</script>
</body>
</html>