I have delved a bit into the "replaceentity" feature in order to be able to replace models/entities I don't personally own referenced in fpms another user sent me.
The outcome was a little tool that walks the .log file created by GG when referenced models are not found locally, stops at every such line and lets me search interactively in a list of "my" entities.
Currently, I simply enter a partial string (like "build" for "building") , getting a list output of all entities whose names contain "build". A single click shows me the corresponding .bmp, a double click takes over the required name parts into the replace file.
It would also be easy to put a database like sqlite behind it, so that arbitrary tags could be assigned to any entity by each user; this would make modifications to the existing structures inside GG unnecessary.
The search program could certainly run without having to use a log/replace file, all that would be needed were a means of communicating with a running GG (API Call, RFC or whatever) to tell GG to fetch the selected entity into the "Library" window. Since this is a VERY basic function, I would expect it to be very stable and robust for the forseeable future.
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