Polylines on 3D Tiles

<!--
  示例：瓦片表面画线
  说明：沿台阶表面采样出一条折线，线贴着瓦片表面起伏
  核心逻辑：在瓦片表面采样高度 → Polyline 使用采样后的坐标 → 与贴地折线对比
  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>瓦片表面画线</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">白线贴着台阶起伏，它的每个顶点都是从瓦片表面采样出来的。</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;

  function glbBytes(json, bin) {
    var jb = new TextEncoder().encode(JSON.stringify(json));
    var jpad = (4 - jb.length % 4) % 4, bpad = (4 - bin.length % 4) % 4;
    var jl = jb.length + jpad, bl = bin.length + bpad;
    var out = new Uint8Array(12 + 8 + jl + 8 + bl), dv = new DataView(out.buffer);
    dv.setUint32(0, 0x46546c67, true); dv.setUint32(4, 2, true); dv.setUint32(8, out.length, true);
    dv.setUint32(12, jl, true); dv.setUint32(16, 0x4e4f534a, true); out.set(jb, 20);
    for (var i = 0; i < jpad; i++) out[20 + jb.length + i] = 0x20;
    dv.setUint32(20 + jl, bl, true); dv.setUint32(24 + jl, 0x004e4942, true); out.set(bin, 28 + jl);
    return out;
  }

  function stepsGlb() {
    var boxes = [[0, 0, 140, 30], [40, 60, 140, 30], [80, 120, 140, 30]];
    var pos = [], nor = [], idx = [], base = 0;
    boxes.forEach(function (b) {
      var ox = b[0], oy = b[1], w = b[2], h = b[3], hx = w / 2, hz = w / 2;
      var faces = [
        [[hx, 0, hz], [hx, 0, -hz], [hx, h, -hz], [hx, h, hz], [1, 0, 0]],
        [[-hx, 0, -hz], [-hx, 0, hz], [-hx, h, hz], [-hx, h, -hz], [-1, 0, 0]],
        [[-hx, h, hz], [hx, h, hz], [hx, h, -hz], [-hx, h, -hz], [0, 1, 0]],
        [[-hx, 0, -hz], [hx, 0, -hz], [hx, 0, hz], [-hx, 0, hz], [0, -1, 0]],
        [[-hx, 0, hz], [hx, 0, hz], [hx, h, hz], [-hx, h, hz], [0, 0, 1]],
        [[hx, 0, -hz], [-hx, 0, -hz], [-hx, h, -hz], [hx, h, -hz], [0, 0, -1]]
      ];
      faces.forEach(function (f) {
        for (var i = 0; i < 4; i++) {
          pos.push(f[i][0] + ox, f[i][1] + oy, f[i][2]);
          nor.push(f[4][0], f[4][1], f[4][2]);
        }
        idx.push(base, base + 1, base + 2, base, base + 2, base + 3);
        base += 4;
      });
    });
    var p = new Float32Array(pos), n = new Float32Array(nor), ix = new Uint16Array(idx);
    var pb = p.byteLength, nb = n.byteLength, ib = ix.byteLength;
    var bin = new Uint8Array(pb + nb + ib);
    bin.set(new Uint8Array(p.buffer), 0);
    bin.set(new Uint8Array(n.buffer), pb);
    bin.set(new Uint8Array(ix.buffer), pb + nb);
    var json = {
      asset: { version: '2.0' }, scene: 0, scenes: [{ nodes: [0] }], nodes: [{ mesh: 0 }],
      meshes: [{ primitives: [{ attributes: { POSITION: 0, NORMAL: 1 }, indices: 2, material: 0 }] }],
      materials: [{ pbrMetallicRoughness: {
        baseColorFactor: [0.33, 0.36, 0.42, 1], metallicFactor: 0, roughnessFactor: 0.95
      }, doubleSided: true }],
      accessors: [
        { bufferView: 0, componentType: 5126, count: pos.length / 3, type: 'VEC3', min: [-70, 0, -70], max: [150, 160, 70] },
        { bufferView: 1, componentType: 5126, count: nor.length / 3, type: 'VEC3' },
        { bufferView: 2, componentType: 5123, count: idx.length, type: 'SCALAR' }
      ],
      bufferViews: [
        { buffer: 0, byteOffset: 0, byteLength: pb, target: 34962 },
        { buffer: 0, byteOffset: pb, byteLength: nb, target: 34962 },
        { buffer: 0, byteOffset: pb + nb, byteLength: ib, target: 34963 }
      ],
      buffers: [{ byteLength: pb + nb + ib }]
    };
    return new Blob([glbBytes(json, bin)], { type: 'model/gltf-binary' });
  }

  var origin = Cesium.Cartesian3.fromDegrees(116.3895, 39.9035, 0);
  var enu = Cesium.Transforms.eastNorthUpToFixedFrame(origin);
  var json = {
    asset: { version: '1.1' },
    geometricError: 300,
    root: {
      transform: Cesium.Matrix4.toArray(enu),
      boundingVolume: { box: [70, 80, 0, 130, 0, 0, 0, 100, 0, 0, 0, 70] },
      geometricError: 0,
      content: { uri: URL.createObjectURL(stepsGlb()) }
    }
  };
  var tsUrl = URL.createObjectURL(new Blob([JSON.stringify(json)], { type: 'application/json' }));

  Cesium.Cesium3DTileset.fromUrl(tsUrl).then(function (tileset) {
    scene.primitives.add(tileset);
    viewer.zoomTo(tileset, new Cesium.HeadingPitchRange(
      Cesium.Math.toRadians(28), Cesium.Math.toRadians(-12), 420
    ));

    scene.postRender.addEventListener(function tick() {
      scene.postRender.removeEventListener(tick);

      var carto = new Cesium.Cartographic();
      var positions = [];
      for (var x = -50; x <= 150; x += 4) {
        var local = Cesium.Matrix4.multiplyByPoint(enu, new Cesium.Cartesian3(x, 0, 0), new Cesium.Cartesian3());
        Cesium.Cartographic.fromCartesian(local, Cesium.Ellipsoid.WGS84, carto);
        var h = scene.sampleHeight(carto);
        positions.push(Cesium.Cartesian3.fromRadians(carto.longitude, carto.latitude, (h || 0) + 1.2));
      }

      viewer.entities.add({
        polyline: {
          positions: positions,
          width: 4,
          material: Cesium.Color.fromCssColorString('#ffb13d')
        }
      });
    });
  });
  /* 相机在瓦片集加载完成后由 zoomTo 设置 */
})();
</script>
</body>
</html>