Cesium 基本概念：创建实体和相机控制

基本概念

Entity

// 创建一个实体
const entity_1 = viewer.entities.add({position: new Cesium.Cartesian3(0, 0, 10000000),point: {pixelSize: 10,color: Cesium.Color.BLUE}
});

// 通过经纬度创建实体
const position = Cesium.Cartesian3.fromDegrees(180.0, 0.0);
// 创建一个恒星实体
let entity_2 = viewer.entities.add({name: 'entity_2',position: position,point: {pixelSize: 10,color: Cesium.Color.YELLOW,outlineColor: Cesium.Color.BLACK,outlineWidth: 2}
});

    // 创建PolylineVolume实体const positions = Cesium.Cartesian3.fromDegreesArrayHeights([0, 0, 11742000,20, 0, 11742000,40, 0., 11742000,60, 0, 11742000,80, 0., 11742000,100, 0, 11742000,120, 0., 11742000,]);const polyline = viewer.entities.add({polyline: {width: 4,positions: positions, // 路径点数组material: Cesium.Color.RED.withAlpha(0.5) // 设置半透明的红色材质}});

Cesium.Cartesian3代表固连在地球上的笛卡尔坐标系（大地坐标系）上的点。
大地坐标系x轴正方向为从地心指向经纬度（0，0）点方向，y轴从地心指向（90，0）方向，z轴从地心指向（0，90）方向。
Cesium提供（经度，纬度，高度）转大地坐标的方法Cesium.Cartographic.fromCartesian，和大地坐标转经纬高坐标的方法Cesium.Cartesian3.fromDegrees

DataSource

const czml = [{id: "document",name: "CZML Point",version: "1.0",clock: {"interval": "2012-03-15T10:00:00Z/2012-03-15T15:00:00Z","currentTime": "2012-03-15T10:00:00Z","multiplier": 60,"range": "LOOP_STOP","step": "SYSTEM_CLOCK_MULTIPLIER"}},{id: "point 1",name: "point",label: {fillColor: {rgba: [255, 255, 255, 255],},font: "13pt Lucida Console",horizontalOrigin: "LEFT",pixelOffset: {cartesian2: [20, 0],},style: "FILL",text: "referencing fillColor",},position: {"interpolationAlgorithm": "LAGRANGE","interpolationDegree": 5,"referenceFrame": "INERTIAL","epoch": "2012-03-15T10:00:00Z",cartographicDegrees: [0, .0, .0, 12742000,2000, 20.0, .0, 12742000,4000, 40.0, .0, 12742000,6000, 60.0, .0, 12742000,8000, 80.0, .0, 12742000,10000, 100.0, .0, 12742000,12000, 120.0, .0, 12742000,14000, 140.0, .0, 12742000,16000, 160.0, .0, 12742000,18000, 180.0, .0, 12742000,],},point: {color: {rgba: [255, 255, 255, 255],},pixelSize: 5,},path: {"show": [{"interval": "2012-03-15T10:00:00Z/2012-03-16T10:00:00Z","boolean": true}],"width": 1,"material": {"solidColor": {"color": {"rgba": [255, 255, 255, 100]}}},"resolution": 120,leadTime: 0,trailTime: 1e10}
];

const dataSourcePromise = Cesium.CzmlDataSource.load(czml)
viewer.dataSources.add(dataSourcePromise)
dataSourcePromise.then(() => {viewer.zoomTo(dataSourcePromise)
})

czml中，第一个元素为描述信息，从第二个元素开始为实体。
其中path属性表示运动轨迹，可以通过leadTime和trailTime控制前后长度。

相机控制

采用以下方式控制相机绕地球自转
center为地心向z轴偏移1个单位的点，viewer.camera.lookAt(center, offfset)offset参数为以center为原点的局部坐标系，z轴正方向为地心到center的方向。

let distance = 50000000
let height = 10000000
let a = 0
let lastTime: number | undefined = undefined
viewer.scene.preRender.addEventListener(() => {// 抵消地球自转let currentTime = viewer.clock.currentTimelet days = currentTime.dayNumber + currentTime.secondsOfDay / 24 / 3600if (lastTime != undefined) {a += (days - lastTime) * 2 * Math.PI}lastTime = daysif (viewer.scene.mode === Cesium.SceneMode.SCENE3D) {viewer.camera.lookAt(center,new Cesium.Cartesian3(distance * Math.sin(a), distance * Math.cos(a), height))}
})