Spatial subdivision within a glTF model

[pjcozzi]

From #519

Spatial subdivision within a glTF file with glTF nodes as the payload

I think this is a big enough topic that its discussion should be broken out into a separate issue here.

With 3D Tiles Next, a 3D tileset can point directly to glTF files for tile content. Given interest in a glTF Geospatial profile, KhronosGroup/glTF#2092, it would be interested to explore how:

• a glTF file could reference 3D Tiles explicit or implicit tiling
• How implicit tiling could be elegantly stored in glTF buffers
• The granularity of what tile content points to in this context, e.g., a glTF node/subtree and/or a separate external glTF file
• How tile/tileset metadata (3DTILES_metadata) is stored in a glTF node/subtree

Today 3D Tiles is a container for many glTF files; there could be a future where 3D Tiles is - or is part of - a glTF Geospatial Profile.
Exploring how spatial subdivision is integrated is a key step.

I don't think this necessarily needs to happen before 3D Tiles Next goes from an open specification to an open standard, but it could be helpful to have some initial ideas on a reasonable path to get here and if the community wants to get here. I would think yes to the later.

[pjcozzi]

This may be able to be used to implement Nest spatial data structure to optimize additive refinement, #11

[pjcozzi]

The granularity of what tile content points to in this context, e.g., a glTF node/subtree and/or a separate external glTF file

CC KhronosGroup/glTF#37

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USD can also point to other USD files via Composition. CC #555

[dbuck]

We've started looking at how we might do this. We've got a.. non-standard spatial layout based roughly on rooms in very large photogrammetry scans.

The initial thought was to add a 3d-tiles extension for content which can reference specific scenes in a gltf file uri: lod0.glb#15, this would allow a single roundtrip for the low detail mesh levels while continuing to use the main tile schema for the individual tile bounds, which let's the next levels down replace the sub-scenes from the courser level.

I'd not considered any of the implications of implicit tiling or multiple contents in this context at all yet though?