Nor I know much about Nanite. This is pretty new method of polygon controlling. No answers last 3 months, so here’s what I know.
Example:
Even if you merge meshes, that’s basically not meaning each vertacies are even connected. As an example, let’s say you made 2 triangled objects. Total of 2 faces, 3 edges and 3 vertexes. Now let’s say you now join them over. Now you see, 2 objects merged into one. But still, these are one object, but 2 meshes. No mesh have a vertacy and edge connected to the other one. So they together basically still are two disconnected meshes over one object.
Nanite:
But in Nanite, these two meshes need to have a connection of atleast a vertex or a face connected to the other one in order to polerize them into one single mesh over long distances.
Your Case:
Since your building and door are same object after merge, but not connected through vertacy or edges with each others, they are actually low-polerized differently, not polerized into one.
My own research:
I hope my explaination was not wrong. I am pretty confused myself about it. Thing is this connection between objects are hard to make than saying. Such as look at my PKP setup:
You see the gun I previously made. It’s PKP, 3 core parts here. The stock, body and muzzle pipe. Here, even if I merge them, you see their connection is faulty. There’s no vertacy stock connected to body. Nor muzzle has any vertacy connected to body or stock. Which still makes them separate meshes. In nanite, it gonna polerize them into 3 parts inside Unreal Engine 5. But we want to polerize them into one.
Solution:
The solution is hard to accept. I need to connect a vertacy of stock into body and another vertacy from muzzle into body. This makes them connected and UE5 nanite ready to merge into one.
Cons:
Nanite mesh will still be expansive in performance in close perspective view. I would like to quadrize the topology method while also connecting each parts. There are more parts I didn’t shown in the picture that I would topolize and merge.
I wish Nanite to have an option to auto merge meshes in per object. Also, personally texture creation is never been autonomized. Simply, it’s pretty impossible to complete the process inside any free softwares available to hand paint to mesh properly like professional people actually do.