Calculate the viewing location in Cesium

 Here I summarise two approach for calculating the viewing location in Cesium viewer:

Method 1 (using Camera.computeViewRectangle() method) 

var projection = new Cesium.WebMercatorProjection();

var rect = new Cesium.Rectangle()

viewer.camera.computeViewRectangle(projection.ellipsoid, rect)

let rect_x = (rect.east - rect.west)/2 + rect.west;

let rect_y = (rect.north - rect.south)/2 + rect.south;

var rect_lon = Cesium.Math.toDegrees(rect_y);

var rect_lat = Cesium.Math.toDegrees(rect_x);

console.log(rect_lat, rect_lon)

Note: The computeViewRectangle( ) could be very large when the view pitch is very flat, resulting in a very far point being calculated.

Method 2 (using Camera.computeViewRectangle() method) 

var cc = document.getElementById("cesiumContainer");

var screenCenterPt = new Cesium.Cartesian2(cc.offsetWidth/2, cc.offsetHeight/2);

var pick = viewer.scene.globe.pick(viewer.camera.getPickRay(screenCenterPt), viewer.scene);

if (pick) {

  var geoPt = viewer.scene.globe.ellipsoid.cartesianToCartographic(pick);

  var point = [geoPt.longitude/ Math.PI * 180, geoPt.latitude/ Math.PI * 180];

  

  var rect_lon = point[0];

  var rect_lat = point[1];

  

  console.log(rect_lat, rect_lon);

}

Note: It uses screen center to calculate the point, seems to be more useful.