OK, I think I'm ready for this. Am I mad? Yes! Will I get even madder after I've done this? Definitely!
I've essayed in the craft with a few smaller cities and towns...it's time for the big one. If Serif Drawplus takes one look at the number of nodes involved, it will laugh at me, thank me very kindly for my interest and shut down, so I'm going to do this one in Photoshop.
Here's the base image which is an RPG City creator generation (that program just rocks), the guy who wrote it was a genius. I've exported a sliced image with the final composite being 6400 px on each side.
First win! There's no antialiasing in the RPG city export! This means to separate all the elements into their own layers is a simple magic wand job. Fantastic, I can work without anti-aliasing and deal with that at the end if I need to by using a bit of gaussian blur.
Wish me luck! I'm going in!