Years ago (whenever Qt 5.7 was current), as a contractor I developed a closed source tablet app for a company with (at the time) a commercial license for Qt (they assigned me a license seat). The app has been sitting in the iOS appstore ever since.

Since then a recent iOS update broke it (graphics issues) and the client has been in touch about getting it fixed. I still have the ability to build and debug it - in fact I have my own Qt for Small Businesses/Startups or whatever it's called these days license - and the simplest fix seemed to be to simply rebuild the app with a more recent Qt (it works fine again when rebuilt with Qt 5.15).

However the client has let their Qt license lapse and cannot justify paying for a Qt commercial license again (and they are too big to qualify for the Qt for Small Businesses one themselves). But they are quite open to the idea of releasing the code under an open source license! (Which is a good thing IMHO! Even if I'm somewhat embarrassed by some of the code now.)

I have checked that the app works correctly when built with the last Qt5 Qt open-source version, and I could easily flip the visibility on the bitbucket git repository hosting the code to public and plaster the files with with an appropriate license (to be decided).

However, the app is basically an "ebook" with a lot of text, images, videos and also data for some interactive visualisations (the app download is almost 2GB!), and the client seems more squeamish about "losing control" of that "content" than they are the source code for the app logic/implementation.

My question(s) are:

  • If we open source the code of the app but continue to claim copyright over the "content" (images, text, videos, data used by interactive visualisation widgets), is that still allowed by the Qt open source license (which I think basically means GPL as apps are static linked on iOS)? Does the content also need to be under some permissive "open" license to satisfy the Qt open source license?

  • Do we even have to make the data in the app available? Would some minimial placeholders suffice (Lorem Ipsum text or whatever); enough to build the "ebook" app, but in a "blank", "demo version" form? Or do we have to provide everything someone exercising their right to build from source would need to build the app exactly as-is in the appstore with all the content in that version? (Even if the content has more restrictive licensing attached than the code).

Thanks for any pointers. I have released LGPL & GPL-ed Qt-based software before, and also proprietary closed source Qt applications under commercial licensing... but I've not had to deal with this sort of "hybrid" before where there seem to be opposing forces pulling in different directions.

Update: The question suggested as a duplicate - Is data part of corresponding source for GPL 3? - proved to be quite useful. That led to the idea of dual licensing (CC data & GPL code), and then further reading on the compatibility (or not) of the various CC licenses with the GPL helped clarify what's considered acceptable. I think the content author/owner was hoping to maintain an "Attribution-NonCommercial-NoDerivs (CC BY-NC-ND)" level of control, but it's pretty clear that's completely incompatible with the GPL. That - combined with the issue raised in the comments below on the incompatibility of the GPL with the AppStore - has been enough to switch focus back to a commercially licensed update.

  • 1
    It is generally possible to license your program under (L)GPL but data under another (incompatible) license. This is often done for video games. Quick search found this: opensource.stackexchange.com/questions/13317/…
    – ecm
    Apr 12 at 20:57
  • 4
    I agree with @ecm in that I feel this is covered by Is data part of corresponding source for GPL 3?; if you feel it doesn't, please explain what you still need clarity on. Apr 13 at 10:23
  • 2
    Separately from the "GPL and data" issue, you probably also need to worry about whether you can put GPL software on the App Store at all. Apr 13 at 10:24
  • 2
    If the end-users rebuild the open source app from sources themselves, then would they be able to still open the proprietary content (assuming they have a valid license/account/subscription)?
    – Brandin
    Apr 16 at 11:55
  • 1
    "further reading on the compatibility (or not) of the various CC licenses with the GPL helped clarify what's considered acceptable." Actually, if you read my first comment I noted the data license need not be GPL-compatible. The free software Doom games are a good example of this; their level data is incompatible to the GPL.
    – ecm
    Apr 18 at 16:53