ShadowMap阴影控制是怎么做的？

1尝试在帖子和其他网站搜索相关问题，但没有结果

2尝试过如下代码


var shadowMap = viewer.shadowMap;
shadowMap.cascadesEnabled = false;

//创建相机
var camera = new Cesium.Camera(viewer.scene);
camera.direction = Cesium.Cartesian3.negate(Cesium.Cartesian3.UNIT_Z, new Cesium.Cartesian3());
camera.frustum.fov = Cesium.Math.PI_OVER_THREE;
camera.frustum.near = 1.0;
camera.frustum.far = 5.0;
//为shadowMap添加相机
shadowMap._lightCamera = camera;


//改变位置无效，如何正确改变光源位置呢？
shadowMap._lightCamera.position = Cesium.Cartesian3.fromDegrees(90,180);