So I thought I would post some thing interesting while in the process of importing parts of the sewer, especially with the recent debate, uv maps and mapping large objects.
While my method is limited, especially if you want apply, mask as such, been using this method for a while now.I am not using a single atlas texture to render instead use various texture sources, I am able merge the mesh and render it out as a unique texture and uvmap which combines all the textures in to a single file.However for large objects texture sized gets reduce and results in numerous issues.
The way around this, is to delete exact duplicates and only render out each copy, because they then all share the same texture space, you can simply extract and duplicate faces and paste them were they are needed to complete the final mesh, merge and export, because they then all share the same uv space it is handy for dealing with seamless textures. Only down side is it does increase polygon count slightly, and texture variation is a bit lacking, plus side is that you don't have to worry about orientation or grouping of uv meshes, it also allows using a 100 different textures and you can also add masks directly in the model editor if you wanted to
.What I do like is that each mesh part uv map is automatically scaled based on mesh size.
PBR can then be rendered, by extracting information needed from the color, height, AO, and specular map, downside of that is that the specular map needs to be 100% perfect for the gloss map and metalness map, and may require manually editing.
Rendered texture, there is still duplicates that can be removed, but not really needed as the texture sizes is correct and will match up with the rest of the meshes
Polygon faces duplicated and the final mesh, seamlessly assembled.
Win7 64bit----iCore5 4590 @ 3.7GHZ----AMD RX460 2gb----16gig ram