Is there a copyleft license that applies specifically to assets created using my software?

Question

I'm developing a new kind of graphical library for the web. It's roughly analogous to an animation library such as Spine in that it will ship with an editor for creating graphical assets and a runtime "binary" for using those assets in e.g. a game. I'd like to open-source my code in such a way that anyone who publishes a game using it must also release the assets created or manipulated by my library under a copyleft license like CC-BY-SA. The purpose would be to encourage the community to grow a collection of free assets to go along with my free tools.

Later I may provide an alternative commercial license for those that find my conditions too restrictive (i.e. dual licensing). Commercial license holders would not be required to release their assets. I don't believe this is relevant for my question but I'm mentioning it here just in case I'm wrong.

Is there a software license that specifically covers derived assets in this way?

EDIT (followup based on comments): If I cannot dictate terms for the created assets, could I instead dictate a license for the released game (which by necessity integrates my runtime code)? Wouldn't this work assuming I release the runtime under e.g. the GPL?

Answer 1

Is there a software license that specifically covers derived assets in this way?

Probably not.

Unless those created assets contain a copyrightable portion of your software, the assets are not derived works of your software (just like a document you write in MS Word is not a derived work of MS Word). If it is not your work nor derived from your work, you have no rights to dictate licensing terms.

If I cannot dictate terms for the created assets, could I instead dictate a license for the released game (which by necessity integrates my runtime code)?

Yes. If you use the GPL or AGPL license, then that forces the game engine to be under that same license. It might still be possible to construct the game engine in such a way that the game-play aspects (rules, visuals, audio, etc.) are still independent works of authorship as far as copyrights go and would not be affected by your (A)GPL license.