Thanks for your reply.
To answer your question :
My project is a RPG 3D project, build with Unity. I have a library of 3D buildings, houses, fences, towers, ... I want to use the PNG to present a 2D village or a 2D city to the Dungeon Master. He selects one, and I build a 3D village or city with my Unity assets. And I build that with the JSON file, quite like your City Viewer. And RPG players can play in my 3D village or city after 3D generation.
The first check for me was/is to superpose PNG and JSON, to assure a minimal consistency for the DM. If the DM adds some assets to his village (a tavern, a casern, ...) or remove one, I need superposition between PNG and JSON.