Consider creating a "relative to 3D Tiles" height reference

[OmarShehata]

Right now your options for a height reference are absolute, relative to ground or clamp to ground.

Since 3D Tiles are often "the ground" in many applications (such as photogrammetry of cities), it can be useful to be able to position things relative to this ground. It is already possible to clamp entities to 3D Tiles as shown by this Sandcastle:

https://sandcastle.cesium.com/index.html?src=Clamp%20to%203D%20Tiles.html

But this is done in a postRender call that continually adjusts the height. It would be nice if this were a built-in height reference that can be used from the Entity API/with CZMLs.

My use case for this is placing labels on buildings in a photogrammetry 3D Tileset. To do this currently with CZML I need to manually specify the height for each label depending on where it is to make sure it's visible.

[lilleyse]

I'm a little worried that scene.clampToGround won't scale super well. Terrain keeps triangle data in CPU memory, wonder if we need to do the same for 3D Tiles at some point. And then optimize ray/triangle intersection tests.

[ggetz]

I believe this is a duplicate of #7044. Closing to consolidate.