Particle Weather

<!--
  示例：粒子系统模拟雨雪
  要点：ParticleSystem 配 BoxEmitter 挂在相机附近，updateCallback 里按地心方向施加重力
  依赖：CesiumJS 1.121 · jsDelivr CDN，粒子贴图由 canvas 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>粒子雨雪</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,.9);
       border:1px solid #2a313b;border-radius:6px;padding:9px 12px;font-size:12px;
       color:#a2a9b6;max-width:250px}
  .hud b{color:#e6e8ec;font-weight:600}
  .hud .row{margin-top:7px}
  .hud button{font:inherit;font-size:12px;padding:3px 9px;margin-right:4px;border-radius:4px;
       cursor:pointer;border:1px solid #343b46;background:#1d232b;color:#cfd6e2}
  .hud button.on{background:#1d3a5c;border-color:#3d6f9e;color:#cfe4ff}
  .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>粒子天气</b></div>
  <div class="row">
    <button id="rain" class="on">雨</button>
    <button id="snow">雪</button>
    <button id="off">关闭</button>
  </div>
  <div class="row" id="info">发射区跟随相机移动</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;

  /* ---------- 两张程序生成的粒子贴图 ---------- */
  function rainTexture() {
    var c = document.createElement('canvas');
    c.width = 4; c.height = 24;
    var ctx = c.getContext('2d');
    var g = ctx.createLinearGradient(0, 0, 0, 24);
    g.addColorStop(0, 'rgba(255,255,255,0)');
    g.addColorStop(0.6, 'rgba(255,255,255,0.9)');
    g.addColorStop(1, 'rgba(255,255,255,0.1)');
    ctx.fillStyle = g;
    ctx.fillRect(1, 0, 2, 24);
    return c;
  }

  function snowTexture() {
    var c = document.createElement('canvas');
    c.width = c.height = 16;
    var ctx = c.getContext('2d');
    var g = ctx.createRadialGradient(8, 8, 0, 8, 8, 8);
    g.addColorStop(0, 'rgba(255,255,255,1)');
    g.addColorStop(0.5, 'rgba(255,255,255,0.7)');
    g.addColorStop(1, 'rgba(255,255,255,0)');
    ctx.fillStyle = g;
    ctx.fillRect(0, 0, 16, 16);
    return c;
  }

  var RAIN_TEX = rainTexture();
  var SNOW_TEX = snowTexture();

  var gravity = new Cesium.Cartesian3();
  function applyGravity(particle, dt) {
    Cesium.Cartesian3.normalize(particle.position, gravity);
    Cesium.Cartesian3.multiplyByScalar(gravity, -9.8 * dt, gravity);
    particle.velocity = Cesium.Cartesian3.add(particle.velocity, gravity, particle.velocity);
  }

  var emitterBox = new Cesium.Cartesian3(700, 700, 260);

  var rain = new Cesium.ParticleSystem({
    image: RAIN_TEX,
    startColor: Cesium.Color.fromCssColorString('#8fb7ff').withAlpha(0.55),
    endColor: Cesium.Color.fromCssColorString('#8fb7ff').withAlpha(0.05),
    minimumParticleLife: 0.9,
    maximumParticleLife: 1.4,
    minimumSpeed: 26,
    maximumSpeed: 38,
    imageSize: new Cesium.Cartesian2(2.5, 20),
    emissionRate: 900,
    emitter: new Cesium.BoxEmitter(emitterBox),
    modelMatrix: Cesium.Matrix4.IDENTITY,
    lifetime: 60,
    updateCallback: applyGravity
  });

  var snow = new Cesium.ParticleSystem({
    image: SNOW_TEX,
    startColor: Cesium.Color.WHITE.withAlpha(0.9),
    endColor: Cesium.Color.WHITE.withAlpha(0.1),
    minimumParticleLife: 4,
    maximumParticleLife: 7,
    minimumSpeed: 1.5,
    maximumSpeed: 4,
    imageSize: new Cesium.Cartesian2(8, 8),
    emissionRate: 260,
    emitter: new Cesium.BoxEmitter(emitterBox),
    modelMatrix: Cesium.Matrix4.IDENTITY,
    lifetime: 60,
    updateCallback: applyGravity
  });

  snow.show = false;
  scene.primitives.add(rain);
  scene.primitives.add(snow);

  /* 每帧把发射区搬到相机前方，粒子始终填满镜头 */
  var forward = new Cesium.Cartesian3();
  var center = new Cesium.Cartesian3();
  var matrix = new Cesium.Matrix4();

  scene.preUpdate.addEventListener(function () {
    var cam = viewer.camera;
    Cesium.Cartesian3.multiplyByScalar(cam.direction, 220, forward);
    Cesium.Cartesian3.add(cam.positionWC, forward, center);
    matrix = Cesium.Transforms.eastNorthUpToFixedFrame(center);
    rain.modelMatrix = matrix;
    snow.modelMatrix = matrix;
  });

  var btnRain = document.getElementById('rain');
  var btnSnow = document.getElementById('snow');
  var btnOff = document.getElementById('off');
  var info = document.getElementById('info');

  function setMode(mode) {
    rain.show = mode === 'rain';
    snow.show = mode === 'snow';
    btnRain.className = mode === 'rain' ? 'on' : '';
    btnSnow.className = mode === 'snow' ? 'on' : '';
    btnOff.className = mode === 'off' ? 'on' : '';
    info.textContent = mode === 'rain' ? '900 粒/秒 · 竖直下落'
      : mode === 'snow' ? '260 粒/秒 · 缓慢飘落'
      : '粒子已停止发射';
  }

  btnRain.addEventListener('click', function () { setMode('rain'); });
  btnSnow.addEventListener('click', function () { setMode('snow'); });
  btnOff.addEventListener('click', function () { setMode('off'); });

  viewer.camera.flyTo({
    destination: Cesium.Cartesian3.fromDegrees(116.391263, 39.907359, 1500),
    orientation: {
      heading: Cesium.Math.toRadians(30),
      pitch: Cesium.Math.toRadians(-18),
      roll: 0
    },
    duration: 0
  });
})();
</script>
</body>
</html>