Layer Clipping

<!--
  说明：图层裁剪
  核心逻辑：prerender 里 clip 圆形路径，postrender 里 restore 恢复画布状态
  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>图层裁剪</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}
</style>
</head>
<body>
<div class="bar">
  <label><input type="checkbox" id="on" checked> 启用裁剪</label>
  <label>半径 <input type="range" id="r" min="40" max="260" step="10" value="140"></label>
  <span id="rv">140 px</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;

  function pattern() {
    var canvas = document.createElement('canvas');
    canvas.width = 480;
    canvas.height = 360;
    var ctx = canvas.getContext('2d');
    ctx.fillStyle = '#12314f';
    ctx.fillRect(0, 0, 480, 360);

    for (var y = 0; y < 360; y += 24) {
      for (var x = 0; x < 480; x += 24) {
        var d = Math.sqrt(Math.pow(x - 240, 2) + Math.pow(y - 180, 2));
        ctx.beginPath();
        ctx.arc(x + 12, y + 12, 8, 0, Math.PI * 2);
        ctx.fillStyle = 'hsl(' + (d * 1.2) % 360 + ', 70%, 55%)';
        ctx.globalAlpha = 0.55;
        ctx.fill();
      }
    }
    ctx.globalAlpha = 1;
    return canvas.toDataURL();
  }

  var extent = ol.proj.transformExtent([115.6, 39.4, 117.2, 40.4], 'EPSG:4326', 'EPSG:3857');

  var clipLayer = new ol.layer.Image({
    source: new ol.source.ImageStatic({
      url: pattern(),
      imageExtent: extent,
      projection: 'EPSG:3857'
    }),
    opacity: 0.95
  });

  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'}) }), clipLayer],
    view: new ol.View({ center: fromLonLat([116.391, 39.907]), zoom: 9, minZoom: 2, maxZoom: 19 }),
    controls: ol.control.defaults.defaults({ attribution: false, rotate: false })
  });

  var clipping = true;
  var radius = 140;

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

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

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

  document.getElementById('on').addEventListener('change', function () {
    clipping = this.checked;
    map.render();
    info.textContent = clipping ? '已启用圆形裁剪' : '裁剪关闭，显示完整矩形';
  });

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

  info.textContent = '已启用圆形裁剪';
})();
</script>
</body>
</html>