Extensions properties should point to extensions schema

[javagl]

The extensions property in the schema files of extensions is currently defined as

"extensions": {},

(for example, here, but it appears to be done consistently like that for the extensions).

In the core schema, the extensions are pointing to the extension.schema.json, via

    "extensions" : {
        "$ref": "extension.schema.json"
    },

(for example, here, but it appears to be done consistently like that for the core)

---

I think that the extensions should also point to the extensions.schema.json (despite the difficulty that might arise from "nested extensions" - they are currently required, e.g for combining 3DTILES_multiple_contents and 3DTILES_metadata).

---

The same is currently true for the glTF gltf.EXT_mesh_features.schema.json, which should similarly point to the glTF extension.schema.json.

[lilleyse]

@javagl good timing - I already did this in https://github.com/CesiumGS/3d-tiles/tree/3d-tiles-next-json-schema-2020-12 and plan to open a PR soon.

[lilleyse]

Fixed in #497

[javagl]

I think they are still explicitly listed, e.g. https://github.com/CesiumGS/3d-tiles/blob/main/specification/schema/boundingVolume.schema.json#L40 - I hope it's OK to reopen

EDIT: On a second thought, this issue was mainly about referring to the actual schema, and not so much about repeating the extensions property in each thing that is "derived" from tilesetProperty. So feel free to close if this should be tracked as a separate issue.

[lilleyse]

3D Tiles schemas are mostly followed what glTF is doing. If glTF changes we should change here as well, but until then maybe not worry about it?

[javagl]

Based on a recent comment, ... we can stop putting empty name, extras, and extensions everywhere", but the next comment indicated that this would likely happen after the existing glTF extension schemas have been updated to the latest JSON Schema version

We could do that change here and remove the extensions and extras, because everything here already uses the latest schema. But it may make sense to wait until this is done for glTF, because... if there are any issues or caveats with removing these properties, then they will be found and solved there.

I think it's OK to leave this issue open as a reminder, but again: If it should be closed because there is no immediate action, then that's also fine for me.

[javagl]

I think this issue is resolved. If there are cases where it was not resolved, this issue can be reopened.