Can we keep the components of a derivative work open source, but allow the work as a whole to be closed?

Question

We are creating a repository of resources so that people can build software or create videos or artwork using our material as their basis. We are making a virtual Moon colony and a website of related material, and inviting others to use that to make games or shows that take place in the colony, use our library to create objects or events that are placed within our hosted version of the colony as a 3d environment, write stories that draw on our material, et cetera. To facilitate that, all our libraries are open source and copy left. Currently they use GPL-3.

However, people who go to the effort to make things using our libraries are likely going to want to hold copyright on the derivative work, including making the source code closed. I would like to make it a license condition that they add models, textures, animations, and other similar component pieces they make to our libraries. But I am fine with it if they want to put a different license on their work as a whole so they can charge for it. I seriously doubt we will have many takers using our libraries otherwise.

It seems that if we change the license on the one folder in our GitHub repo that has 3d content, the one called 'blenderstuff', this might be achievable. I am the sole copyleft holder on all that material so I can make that change easily right now.

The project is at an early stage and I know very little about this realm. I just want to help people make cool stuff and educate the public about space. When it comes to legalities, I need a lot of help navigating it all.

Answer 1

You can dual-license your project under a GPL3 license and a non-free license. Or you can add an exception to the GPL3 license as per section 7 (Additional terms) of the license (the two solutions would achieve the same result).

The non-free license / additional exception to GPL3 would contain language to indicate that "using the library in closed-source works is allowed provided that new models, textures and animations are contributed back through pull requests on [URL of your repo]" (don't say "and similar" because this would only create juridical uncertainty and this is bad for users of your library).

Side note: Of course this means that people could still use your library and not contribute back new textures if they chose to keep the resulting work GPL3. Moreover, if they do so, you couldn't take their new textures (even if they share them publicly) and add them to your repository because you wouldn't be able to grant a similar exception on these new textures. But this is only a small risk as most people who value free and open source software are generally happy to send their contributions to the original repository, under the original terms.