Cesium Sensor terrain culling

I have developed cesium rectangle sensor 1.32 with cesium 1.32. I have placed sensor at some height looking towards ground.

I found that white rectangle always shows rectangle. If ground is not flat, it should follow ground surface.

Also I think pyramid is always shows in front of everything.
If pyramid side height is enough tall, bottom should go underground, and hidden by ground.

In this way, I can see where is the intersection between pyramid (camera visibility range) and ground surface.