User extensions: empty collections and null

Any key-value pair whose value is {}, [], or None gets scrubbed out of the glTF data when exporting. That means a user-extension can’t do things like this:

    "extensions": { "KHR_materials_unlit": {} }
    // gets turned into "extensions": {}

    "extensions": { "my_ext": { "foo": [] } }
    // gets turned into "extensions": { "my_ext": {} }

The way the in-tree code works around this is to use a hard-coded whitelist of key-names that shouldn’t be scrubbed.

https://github.com/KhronosGroup/glTF-Blender-IO/blob/3ab37ba38a3ae483d69a029f979286ded8b9b94b/addons/io_scene_gltf2/blender/exp/gltf2_blender_export.py#L108-L115

That’s not very flexible and it doesn’t help user extensions anyway. Here’s my proposed fix:

• Add some new objects you should use wherever you want an empty collection/null

  EmptyDict = object()  # use this where you want an empty dict
  EmptyList = object()  # use this where you want an empty list
  Null = object()  # use this where you want a null
  

• Make from_extension believe that EmptyDict is a dict so it will allow it in the "extensions" object.

• Change __fix_json to convert EmptyDict into {} etc. Get rid of the whitelist.

• Wherever you want to put an empty dict, just use EmptyDict instead. Example:

  extensions['KHR_materials_unlit'] = Extension(
      'KHR_materials_unlit',
      EmptyDict,
      required=False,
  )
  

What’s everyone think?

cc @jjcasmar

[scurest]

They just do it the same way they import everything else

class glTF2ExportUserExtension:
    def __init__(self):
        import io_scene_gltf2
        self.Extension = io_scene_gltf2.io.com.gltf2_io_extensions.Extension
        self.EmptyDict = io_scene_gltf2.io.com.gltf2_io.EmptyDict
        
    def gather_material_hook(self, material, blender_material, export_settings):
        if material.extensions is None:
            material.extensions = {}
        material.extensions['KHR_materials_unlit'] = self.Extension(
            name='KHR_materials_unlit',
            extension=self.EmptyDict,
            required=False,
        )

[jjcasmar]

I agree. I have the same doubts when I first looks at the __fix_json. I think your solution is a good one.