EXT_structural_metadata - JSON encoding

[lilleyse]

Metadata assigned to glTF objects - like nodes and materials - is now encoded in JSON rather than in binary in a property table. This is similar to tileset and tile metadata in 3D Tiles. This is a breaking change that shouldn't have a wide impact.

[lilleyse]

Comments from @javagl in #70

The first part is a breaking change for any existing data that uses the per-object metadata that originally pointed to the property table row. I think that this was not actively or widely used, so the impact might be minimal. In terms of alleviating the possible impact, one could consider different options. In theory, just on a "brainstorming" level, the new structure could "emulate" the old one: Given a data set that defines a node with the extension object...

"EXT_structural_metadata": {
  "propertyTable": 1,
  "index": 4
}

one could add a new class to the schema, say propertyTableRow, with the properties propertyTable and index, and then just define

"EXT_structural_metadata": {
  "class": "propertyTableRow",
  "properties": {
    "propertyTable": 1,
    "index": 4
  }
}

The client application would have to be adjusted to handle this. I.e. a possibly existing function like
const metadata = node.getStructuralMetadataValue();
would now be something like

const info = node.getStructuralMetadataValue();
const metadata = gltf.getPropertyTableRow(info.propertyTable, info.index);

But again: This is just brainstorming about how existing data could be "upgraded" to the new structure.

[lilleyse]

@javagl that could be a potential upgrade path. It reminds me of an approach that Kevin suggested for creating relationships (parent/child) between property table elements.