Particle Fire

<!--
  示例：用粒子系统堆出火焰
  要点：SphereEmitter 向上喷射，updateCallback 里加浮力让粒子加速上升
  依赖：CesiumJS 1.121 · jsDelivr CDN，火焰贴图由 canvas 径向渐变生成
-->
<!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>火焰粒子</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%}
  .hud{position:absolute;left:12px;top:12px;z-index:5;background:rgba(23,28,35,.92);
       border:1px solid #2a313b;border-radius:6px;padding:9px 12px;font-size:12px;
       color:#a2a9b6;width:250px}
  .hud b{color:#e6e8ec;font-weight:600}
  .hud .row{margin-top:7px}
  input[type=range]{width:100%}
  .cesium-viewer-bottom .cesium-widget-credits,.cesium-viewer-animationContainer,
  .cesium-viewer-timelineContainer{display:none!important}
</style>
</head>
<body>
<div id="c"></div>
<div class="hud">
  <div><b>火焰</b></div>
  <div class="row">发射率 <span id="eVal">240</span> 粒/秒</div>
  <div class="row"><input id="rate" type="range" min="40" max="800" step="20" value="240"></div>
  <div class="row">三处火点共用一套贴图</div>
</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;

  /* 火苗贴图：中心亮黄，边缘透明 */
  function flameTexture() {
    var c = document.createElement('canvas');
    c.width = c.height = 64;
    var ctx = c.getContext('2d');
    var g = ctx.createRadialGradient(32, 40, 2, 32, 32, 32);
    g.addColorStop(0, 'rgba(255,255,235,1)');
    g.addColorStop(0.35, 'rgba(255,196,88,0.95)');
    g.addColorStop(0.7, 'rgba(255,110,40,0.45)');
    g.addColorStop(1, 'rgba(255,60,20,0)');
    ctx.fillStyle = g;
    ctx.beginPath();
    ctx.arc(32, 32, 32, 0, Math.PI * 2);
    ctx.fill();
    return c;
  }

  var TEX = flameTexture();

  /* 粒子在局部坐标系里，Z 轴朝上，updateCallback 直接改 velocity.z */
  function buoyancy(particle, dt) {
    particle.velocity.z += 9 * dt;
    particle.velocity.x *= 0.97;
    particle.velocity.y *= 0.97;
  }

  var systems = [];

  var SPOTS = [
    [116.383, 39.903], [116.391, 39.903], [116.399, 39.903]
  ];

  SPOTS.forEach(function (p) {
    var matrix = Cesium.Transforms.eastNorthUpToFixedFrame(
      Cesium.Cartesian3.fromDegrees(p[0], p[1], 20)
    );
    matrix = Cesium.Matrix4.multiply(
      matrix, Cesium.Matrix4.fromUniformScale(6), matrix
    );

    var system = new Cesium.ParticleSystem({
      image: TEX,
      startColor: Cesium.Color.fromCssColorString('#ffd166').withAlpha(0.95),
      endColor: Cesium.Color.fromCssColorString('#ff3d1f').withAlpha(0.0),
      startScale: 1.1,
      endScale: 0.35,
      minimumParticleLife: 0.7,
      maximumParticleLife: 1.4,
      minimumSpeed: 1.2,
      maximumSpeed: 3.4,
      imageSize: new Cesium.Cartesian2(46, 46),
      emissionRate: 240,
      emitter: new Cesium.SphereEmitter(1.4),
      modelMatrix: matrix,
      lifetime: 32,
      updateCallback: buoyancy
    });

    systems.push(system);
    scene.primitives.add(system);
  });

  /* 承火平台 */
  viewer.entities.add({
    position: Cesium.Cartesian3.fromDegrees(116.391, 39.903, 4),
    box: {
      dimensions: new Cesium.Cartesian3(2400, 400, 8),
      material: Cesium.Color.fromCssColorString('#243140')
    }
  });

  document.getElementById('rate').addEventListener('input', function () {
    var v = parseInt(this.value, 10);
    document.getElementById('eVal').textContent = v;
    systems.forEach(function (s) { s.emissionRate = v; });
  });

  viewer.camera.flyTo({
    destination: Cesium.Cartesian3.fromDegrees(116.378, 39.893, 700),
    orientation: {
      heading: Cesium.Math.toRadians(12),
      pitch: Cesium.Math.toRadians(-10),
      roll: 0
    },
    duration: 0
  });
})();
</script>
</body>
</html>