Extra extension attributes for semantic surface objects?

[clausnagel]

I'd like to add extra attributes to semantic surface objects like, for instance, a +material to "WallSurface". I understand that I can simply add attributes to a semantic surface in a schema-less way. But I'd like to define a schema for these additional attributes in the same way I can define a schema for additional attributes of city objects.

Is defining extra attributes for semantic surface objects supported by the extension mechanism? At least it seems that the spec only covers the definition of new semantic objects.

And why are JSON objects not allowed as attributes of semantic surface objects but only simple values? For example, if I had a value and a unit of measurement, the following attribute would be possible for city objects but not for semantic surfaces:

"myAttribute": {
  "value": 10
  "uom": "m"
}

[hugoledoux]

Is defining extra attributes for semantic surface objects supported by the extension mechanism?

Yes! If you look at the specs you can see that you can define extra attributes and make them "required".

Here's a simple example, cjval cube.city.json -e thermal.ext.json --verbose to test/validate.

And why are JSON objects not allowed as attributes of semantic surface objects but only simple values?

Hmmmm, I guess we wanted to keep it simple, but I agree that it could be useful. Maybe @balazsdukai remembers why we/I did this? This could I guess be changed easily for future releases

[balazsdukai]

No, I don't remember unfortunately, I can only guess. That guess would be the same as yours, to keep parsing as simple as possible and avoid deeply nested objects as attribute values. I have some vague memory that we might have discussed scalar-only semantic attributes with @hugoledoux at some point, years ago, but maybe I'm just dreaming... :-D But yeah, if objects are allowed values for CityObject attributes, then I don't see a reason why not allow them for Semantic surfaces too.

[balazsdukai]

I just did some digging and the array/object CityObject attribute type was made explicit with 63fa0085c1d44b0256ec7bc9883f9d9e1cffca80, as agreed in https://github.com/cityjson/specs/issues/141 .

[clausnagel]

Thanks @hugoledoux for your example. I would prefer not to define a new semantic object like "+ThermalSurface" though but just to add new attributes to the predefined semantic objects of CityJSON. I have slightly adapted your example to illustrate this: cube-adapted.city.json

I know that you can add arbitrary attributes to semantic objects in a schema-less way like, for instance, the "slope" attribute of the "RoofSurface" in the example. But this "slope" attribute is not defined in any schema and, thus, cannot be automatically tested (e.g. correct spelling, correct data type and value). I would rather like to define an attribute like "+azimuth" of the "WallSurface" in the example and define this attribute in an Extension schema similar to extra attributes for city objects. But this is not covered by the spec at the moment, is it?

@balazsdukai, thanks for pointing to #141. I guess a similar clarification as added with 63fa0085c1d44b0256ec7bc9883f9d9e1cffca80 would be nice for semantic objects as well.

[hugoledoux]

oh now I understand what you mean, sorry.

No it's not covered by it at the moment, but I guess this could (should?) be added to the next release, I see that it could be useful indeed!