Clamp Entities to Ground

<!--
  示例：实体贴地
  说明：两组标注一个在 0 米、一个在 200 米，都用贴地模式，最终都落在地表
  核心逻辑：heightReference = CLAMP_TO_GROUND → label / point / polyline 都支持
  依赖：CesiumJS 1.121 · 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>实体贴地</title>
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/cesium@1.121.0/Build/Cesium/Widgets/widgets.css">
<style>
  html,body{height:100%;margin:0}
  body{background:#0e1116;color:#e6e8ec;overflow:hidden;
       font:13px/1.6 -apple-system,"Segoe UI","PingFang SC",sans-serif}
  #c{height:100%}
  .tip{position:absolute;left:12px;top:12px;z-index:5;
       background:rgba(23,28,35,.9);border:1px solid #2a313b;border-radius:6px;
       padding:8px 12px;font-size:12px;color:#a2a9b6;max-width:320px;line-height:1.6}

  .cesium-viewer-bottom .cesium-widget-credits,.cesium-viewer-animationContainer,
  .cesium-viewer-timelineContainer{display:none!important}
</style>
</head>
<body>
<div id="c"></div>
<div class="tip">左边一组用绝对高度浮空，右边一组贴地，标注都落在地面上。</div>
<script src="https://cdn.jsdelivr.net/npm/cesium@1.121.0/Build/Cesium/Cesium.js"></script>
<script>
(function () {
  'use strict';

  var viewer = new Cesium.Viewer('c', {
    baseLayer: false,
    baseLayerPicker: false, geocoder: false, homeButton: false,
    sceneModePicker: false, navigationHelpButton: false,
    animation: false, timeline: false, fullscreenButton: false,
    infoBox: false, selectionIndicator: false
  });

  var scene = viewer.scene;
  scene.globe.baseColor = Cesium.Color.fromCssColorString('#1c2733');
  scene.backgroundColor = Cesium.Color.fromCssColorString('#0e1116');
  scene.skyBox.show = false;
  scene.sun.show = false;
  scene.moon.show = false;
  if (scene.skyAtmosphere) scene.skyAtmosphere.show = false;

  var lon = 116.3865, lat = 39.9030;

  function label(dx, height, ref, text, color) {
    viewer.entities.add({
      position: Cesium.Cartesian3.fromDegrees(lon + dx, lat, height),
      point: {
        pixelSize: 12,
        color: Cesium.Color.fromCssColorString(color),
        heightReference: ref,
        disableDepthTestDistance: Number.POSITIVE_INFINITY
      },
      label: {
        text: text,
        font: '600 13px sans-serif',
        fillColor: Cesium.Color.WHITE,
        pixelOffset: new Cesium.Cartesian2(0, -22),
        heightReference: ref,
        disableDepthTestDistance: Number.POSITIVE_INFINITY
      }
    });
  }

  label(0.0000, 200, Cesium.HeightReference.CLAMP_TO_GROUND, '贴地 200 米', '#4d9bff');
  label(0.0024, 0, Cesium.HeightReference.CLAMP_TO_GROUND, '贴地 0 米', '#5bb08a');
  label(0.0048, 200, Cesium.HeightReference.NONE, '绝对 200 米', '#d08a4e');

  /* 贴地折线，与标注组成网格 */
  viewer.entities.add({
    polyline: {
      positions: Cesium.Cartesian3.fromDegreesArray([
        lon - 0.001, lat - 0.0015, lon + 0.006, lat - 0.0015
      ]),
      width: 3,
      clampToGround: true,
      material: Cesium.Color.fromCssColorString('#8fb4d8')
    }
  });
  viewer.camera.flyTo({
    destination: Cesium.Cartesian3.fromDegrees(116.3905, 39.8990, 420),
    orientation: { heading: 0, pitch: Cesium.Math.toRadians(-22), roll: 0 },
    duration: 0
  });
})();
</script>
</body>
</html>