Should the refine property inherit from its parent tile?

[pjcozzi]

Currently refine is optional and defaults to "add" (as opposed to "replace").

Instead, if it is omitted, should it inherit its parent's value recursively up the tree?

[pjcozzi]

Instead, if it is omitted, should it inherit its parent's value recursively up the tree?

Yes, this is consistent with the spatial coherence inheritance, and it is hard to say that either "add" or "replace" is a sensible default given the diverse use cases 3D Tiles serve.

I also thought about putting this in the top-level 3D Tiles object, e.g., as a sibling to root, but then that would force putting a subtree tree in a separate tiles.json to change refinement strategy, which I believe would take away from a tileset's flexibility for heterogeneous data.

@lilleyse I'll make this change in the spec and probably Cesium if time allows.