Offscreen Canvas

<!--
  示例：离屏画布生成图标
  要点：OffscreenCanvas 绘制后转成 ImageBitmap，交给 ol.style.Icon 使用
  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>离屏画布生成图标</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:16px;align-items:center;font-size:12px;color:#58606e;flex-wrap:wrap}
  .bar b{color:#16181d;font-weight:600}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3}
</style>
</head>
<body>
<div class="bar">
  <span>OffscreenCanvas：<b id="sup">—</b></span>
  <span>图标由 48×48 画布绘制，同一份位图复用到多个要素</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 (!window.ol) {
    document.getElementById('map').textContent = 'OpenLayers 加载失败，请检查 cdn.jsdelivr.net 是否可访问';
    return;
  }
  var fromLonLat = ol.proj.fromLonLat;

  var hasOffscreen = typeof OffscreenCanvas !== 'undefined';
  document.getElementById('sup').textContent = hasOffscreen ? '可用' : '不可用，退回普通画布';

  function drawPin(ctx, size, fill) {
    ctx.clearRect(0, 0, size, size);
    ctx.beginPath();
    ctx.arc(size / 2, size * 0.36, size * 0.3, Math.PI, 0);
    ctx.lineTo(size / 2, size * 0.96);
    ctx.closePath();
    ctx.fillStyle = fill;
    ctx.fill();
    ctx.lineWidth = 2.5;
    ctx.strokeStyle = '#ffffff';
    ctx.stroke();

    ctx.beginPath();
    ctx.arc(size / 2, size * 0.36, size * 0.11, 0, Math.PI * 2);
    ctx.fillStyle = '#ffffff';
    ctx.fill();
  }

  /* 每种颜色只画一次，位图缓存在 map 里重复使用 */
  var iconCache = {};

  function iconFor(color) {
    if (iconCache[color]) return iconCache[color];

    var size = 48;
    var source;

    if (hasOffscreen) {
      var oc = new OffscreenCanvas(size, size);
      drawPin(oc.getContext('2d'), size, color);
      source = oc.transferToImageBitmap ? oc.transferToImageBitmap() : oc;
    } else {
      var c = document.createElement('canvas');
      c.width = c.height = size;
      drawPin(c.getContext('2d'), size, color);
      source = c;
    }

    var icon = new ol.style.Icon({
      img: source,
      anchor: [0.5, 0.96],
      anchorXUnits: 'fraction',
      anchorYUnits: 'fraction'
    });

    iconCache[color] = icon;
    return icon;
  }

  var spots = [
    [116.32, 39.93, '枢纽', '#1f5fd0'],
    [116.40, 39.905, '中心', '#b42318'],
    [116.47, 39.945, '仓储', '#2f9e6e'],
    [116.44, 39.862, '南站', '#a9700f'],
    [116.36, 39.875, '西片', '#6b4fa0']
  ];

  var features = spots.map(function (s) {
    return new ol.Feature({
      geometry: new ol.geom.Point(fromLonLat([s[0], s[1]])),
      name: s[2],
      style: new ol.style.Style({
        image: iconFor(s[3]),
        text: new ol.style.Text({
          text: s[2],
          offsetY: 6,
          font: '600 12px sans-serif',
          fill: new ol.style.Fill({ color: '#16181d' }),
          stroke: new ol.style.Stroke({ color: '#ffffff', width: 3 })
        })
      })
    });
  });

  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: features }) })
    ],
    view: new ol.View({ center: fromLonLat([116.40, 39.905]), zoom: 12 }),
    controls: ol.control.defaults.defaults({ rotate: false })
  });
})();
</script>
</body>
</html>