Cog Blob

<!--
  说明：从 Blob 挂载影像的中文示例，数据在页面内生成，可独立运行。
  核心逻辑：canvas.toBlob + createObjectURL/revokeObjectURL + ImageStatic
  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>从 Blob 挂载影像</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:10px;align-items:center;flex-wrap:wrap;font-size:12px;color:#58606e}
  .bar label{display:flex;gap:5px;align-items:center;cursor:pointer}
  .bar button{font:inherit;padding:3px 9px;border:1px solid #c9ced6;background:#fff;
       border-radius:5px;cursor:pointer;color:#333}
  .bar button:hover{background:#eef2f8}
  .bar input[type=range]{width:110px;vertical-align:middle}
  .bar input[type=text]{font:inherit;padding:2px 6px;border:1px solid #c9ced6;border-radius:4px}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3;position:relative}
  .panel{flex:0 0 auto;max-height:34%;overflow:auto;padding:8px 12px;font-size:12px;
       background:#fff;border-top:1px solid #e3e6ea;color:#3d4451}
  .panel pre{margin:0;font-family:Consolas,"Courier New",monospace;font-size:11px;
       white-space:pre-wrap;word-break:break-all;line-height:1.5}
  .ol-attribution{font-size:11px}

</style>
</head>
<body>
<div class="bar">
<button id="make" type="button">生成影像 Blob</button>
<button id="drop" type="button">释放 Blob</button>
<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') {
    var errBox = document.getElementById('map');
    if (errBox) {
      errBox.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 W = 512;
  var canvas = document.createElement('canvas');
  canvas.width = canvas.height = W;
  var ctx = canvas.getContext('2d');

  function paint() {
    var g = ctx.createLinearGradient(0, 0, W, W);
    g.addColorStop(0, '#1b4f9c');
    g.addColorStop(0.5, '#2f8f6b');
    g.addColorStop(1, '#d8b04a');
    ctx.fillStyle = g;
    ctx.fillRect(0, 0, W, W);
    ctx.strokeStyle = 'rgba(255,255,255,0.25)';
    ctx.lineWidth = 1;
    for (var i = 0; i <= W; i += 32) {
      ctx.beginPath(); ctx.moveTo(i, 0); ctx.lineTo(i, W); ctx.stroke();
      ctx.beginPath(); ctx.moveTo(0, i); ctx.lineTo(W, i); ctx.stroke();
    }
    ctx.fillStyle = 'rgba(255,255,255,0.9)';
    ctx.font = 'bold 22px sans-serif';
    ctx.fillText('Blob 影像 512 × 512', 36, 60);
  }

  var extent = ol.proj.transformExtent([115.9, 39.5, 116.9, 40.4], 'EPSG:4326', 'EPSG:3857');
  var layer = new ol.layer.Image();
  var objectUrl = null;

  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'})}), layer],
    view: new ol.View({center: fromLonLat([116.4, 39.9]), zoom: 11}),
    controls: ol.control.defaults.defaults({attribution: false, zoom: true, rotate: false})
  });

  var info = document.getElementById('info');

  document.getElementById('make').addEventListener('click', function () {
    paint();
    canvas.toBlob(function (blob) {
      if (objectUrl) { URL.revokeObjectURL(objectUrl); }
      objectUrl = URL.createObjectURL(blob);
      layer.setSource(new ol.source.ImageStatic({
        url: objectUrl,
        imageExtent: extent,
        projection: 'EPSG:3857'
      }));
      info.textContent = 'Blob ' + (blob.size / 1024).toFixed(1) + ' KB · 类型 ' + blob.type +
        ' · objectURL 已挂载';
    }, 'image/png');
  });

  document.getElementById('drop').addEventListener('click', function () {
    if (!objectUrl) {
      info.textContent = '当前没有挂载中的 Blob';
      return;
    }
    URL.revokeObjectURL(objectUrl);
    objectUrl = null;
    layer.setSource(null);
    info.textContent = 'Blob 已释放，图层源已移除';
  });
})();
</script>
</body>
</html>