3D Tiles Property LOD River

<!--
  示例：河流的属性驱动样式
  说明：河宽与颜色随流量属性变化，流量按正弦涨落，分段标注按视距显隐
  核心逻辑：CallbackProperty 驱动 width 与 color → postRender 更新流量 → 视距控制标注
  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>河流的属性驱动样式</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}
  .hud{position:absolute;right:12px;top:12px;z-index:5;background:rgba(23,28,35,.9);
       border:1px solid #2a313b;border-radius:6px;padding:6px 10px;font-size:12px;color:#a2a9b6}
</style>
</head>
<body>
<div id="c"></div>
<div class="tip">河流的宽度与颜色在缓慢变化。拉远到 900 米以上，分段流量标注会消失。</div>
<div id="hud" class="hud">近景：显示分段流量</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;

  /* 河道中心线，从西向东 */
  var path = [
    [116.3852, 39.9042], [116.3872, 39.9048], [116.3892, 39.9052],
    [116.3912, 39.9050], [116.3932, 39.9056], [116.3952, 39.9062],
    [116.3972, 39.9060]
  ];
  /* 每段的流量属性，随时间起伏 */
  var flow = [12, 26, 44, 30, 52, 34, 18];
  var t = 0;

  var positions = [];
  path.forEach(function (p) {
    positions.push(Cesium.Cartesian3.fromDegrees(p[0], p[1], 0.5));
  });

  /* 主河道：宽度与颜色都由流量属性驱动 */
  viewer.entities.add({
    polyline: {
      positions: positions,
      width: new Cesium.CallbackProperty(function () {
        return 4 + flow[3] / 8;
      }, false),
      material: new Cesium.ColorMaterialProperty(
        new Cesium.CallbackProperty(function () {
          var v = flow[3] / 60;
          return Cesium.Color.fromHsl(0.55, 0.7, 0.35 + v * 0.3, 1);
        }, false)
      ),
      clampToGround: false
    }
  });

  /* 分段标注，只在近处显示（属性 LOD） */
  var labels = [];
  path.forEach(function (p, i) {
    labels.push(viewer.entities.add({
      position: Cesium.Cartesian3.fromDegrees(p[0], p[1], 6),
      label: {
        text: flow[i] + ' m³/s',
        font: '500 11px sans-serif',
        fillColor: Cesium.Color.WHITE,
        showBackground: true,
        backgroundColor: Cesium.Color.fromCssColorString('#1b232e').withAlpha(0.9),
        disableDepthTestDistance: Number.POSITIVE_INFINITY
      }
    }));
  });

  scene.postRender.addEventListener(function () {
    t += 0.016;
    for (var i = 0; i < flow.length; i++) {
      flow[i] = 20 + Math.sin(t * 0.8 + i * 0.7) * 16 + i * 3;
    }
    /* 视距超过 900 米就隐藏分段数值，模拟属性随距离降级 */
    var d = Cesium.Cartesian3.distance(viewer.camera.positionWC,
      Cesium.Cartesian3.fromDegrees(116.3912, 39.9050, 0));
    var show = d < 900;
    labels.forEach(function (l) { l.label.show = show; });
    document.getElementById('hud').textContent = show ? '近景：显示分段流量' : '远景：只保留河流主体';
  });

  viewer.camera.flyTo({
    destination: Cesium.Cartesian3.fromDegrees(116.3888, 39.8998, 420),
    orientation: { heading: Cesium.Math.toRadians(8), pitch: Cesium.Math.toRadians(-14), roll: 0 },
    duration: 0
  });
})();
</script>
</body>
</html>