// 两点之间抛物线绘制函数，twoPoints是一个数组：[lon1,lat1,lon2,lat2]
function animatedParabola(twoPoints) { //动态抛物线绘制
 let startPoint = [twoPoints[0],twoPoints[1],0]; //起点的经度、纬度
 let end = [twoPoints[2],twoPoints[3]]; //终点的经度、纬度
 let step = 80; //线的数量，越多则越平滑
 let heightProportion = 0.125; //最高点和总距离的比值(即图中H比上AB的值)
 let dLon = (end[0] - startPoint[0])/step; //经度差值
 let dLat = (end[1] - startPoint[1])/step; //纬度差值
 let deltaLon = dLon * Math.abs(111000*Math.cos(twoPoints[1])); //经度差(米级)
 let deltaLat = dLat * 111000; //纬度差(米),1纬度相差约111000米
 let endPoint = [0,0,0]; //定义一个端点（后面将进行startPoint和endPoint两点画线）
 let heigh = (step * Math.sqrt(deltaLon*deltaLon+deltaLat*deltaLat) * heightProportion).toFixed(0);
 let x2 = (10000*Math.sqrt(dLon*dLon+dLat*dLat)).toFixed(0); //小数点扩大10000倍，提高精确度
 let a = (heigh/(x2*x2)); //抛物线函数中的a
 function y(x,height) { //模拟抛物线函数求高度
  //此处模拟的函数为y = H - a*x^2 (H为高度常数)
  return height - a*x*x;
 }
 for(let i = 1;i <= step; i++){ //逐“帧”画线
  endPoint[0] = startPoint[0] + dLon; //更新end点经度
  endPoint[1] = startPoint[1] + dLat; //更新end点纬度
  let x = x2*(2*i/step-1); //求抛物线函数x
  endPoint[2] = (y(x,heigh)).toFixed(0); //求end点高度
  viewer.clock.currentTime = Cesium.JulianDate.now(); //将时钟指针移到当前时间
  //这里viewer是容器初始化时new Cesium.Viewer构造的: var viewer = new Cesium.Viewer('mapContainer', {...});
  let IsoTime = Cesium.JulianDate.now(); //获取当前时间
  viewer.entities.add({ //添加动态线
   polyline: {
    positions: Cesium.Cartesian3.fromDegreesArrayHeights(startPoint.concat(endPoint)),
    width: 4,
    material: new Cesium.PolylineOutlineMaterialProperty({
     color: Cesium.Color.GOLD,
     outlineWidth: 0.3,
    })
   },
   availability: new Cesium.TimeIntervalCollection([new Cesium.TimeInterval({ //设置显示的时间区间
    start: {
     dayNumber: IsoTime.dayNumber,
     secondsOfDay: IsoTime.secondsOfDay+((i-1)*300),
    },
    stop: {
     dayNumber: IsoTime.dayNumber,
     secondsOfDay: IsoTime.secondsOfDay+(i*300),
    },
   })]),
  });
  viewer.entities.add({ //添加静态线
   polyline: {
    positions: Cesium.Cartesian3.fromDegreesArrayHeights(startPoint.concat(endPoint)),
    width: 4,
    material: new Cesium.PolylineGlowMaterialProperty({
     color: Cesium.Color.AQUA.withAlpha(0.9),
     outlineWidth: 0.3,
     glowPower : 0.3,
    })
   },
  });
  // end点变为start点
  startPoint[0] = endPoint[0];
  startPoint[1] = endPoint[1];
  startPoint[2] = endPoint[2];
 }
 viewer.clock.shouldAnimate = true; //启动时钟开始转动
 viewer.clock.multiplier = 1600; //时钟转动速度
}

function parabola(twoPoints) { //抛物线绘制
 let startPoint = [twoPoints[0],twoPoints[1],0]; //起点的经度、纬度
 let end = [twoPoints[2],twoPoints[3]]; //终点的经度、纬度
 let step = 80; //线的多少，越多则越平滑(但过多浏览器缓存也会占用越多)
 let heightProportion = 0.125; //最高点和总距离的比值
 let dLon = (end[0] - startPoint[0])/step; //经度差值
 let dLat = (end[1] - startPoint[1])/step; //纬度差值
 let deltaLon = dLon * Math.abs(111000*Math.cos(twoPoints[1])); //经度差(米级)
 let deltaLat = dLat * 111000; //纬度差(米),1纬度相差约111000米
 let endPoint = [0,0,0]; //定义一个端点（后面将进行startPoint和endPoint两点画线）
 let heigh = (step * Math.sqrt(deltaLon*deltaLon+deltaLat*deltaLat) * heightProportion).toFixed(0);
 let x2 = (10000*Math.sqrt(dLon*dLon+dLat*dLat)).toFixed(0); //小数点扩大10000倍，提高精确度
 let a = (heigh/(x2*x2));
 function y(x,height) { return height - a*x*x; }
 for(var i = 1;i <= step; i++){ //逐“帧”画线
  endPoint[0] = startPoint[0] + dLon; //更新end点经度
  endPoint[1] = startPoint[1] + dLat; //更新end点纬度
  let x = x2*(2*i/step-1); //求抛物线函数x
  endPoint[2] = (y(x,heigh)).toFixed(0); //求end点高度
  viewer.entities.add({ //添加静态线
   polyline: {
    positions: Cesium.Cartesian3.fromDegreesArrayHeights(startPoint.concat(endPoint)),
    width: 4,
    material: new Cesium.PolylineGlowMaterialProperty({
     color: Cesium.Color.AQUA.withAlpha(0.9),
     outlineWidth: 0.3,
     glowPower : 0.3,
    })
   },
  });
  // end点变为start点
  startPoint[0] = endPoint[0];
  startPoint[1] = endPoint[1];
  startPoint[2] = endPoint[2];
 }
}

// An Example
var viewer = new Cesium.Viewer('mapContainer');
var twoPoints = [114.3698, 22.6139, 114.2135, 22.6127];
animatedParabola(twoPoints);