Distribution of media content together with GPLv2 code in one package?

[Rudolf Polzer <divVerent@alientrap.org>]

Hello,

speaking for a new game that will aim to be included in Debian, I wonder how
certain media content can be legally distributed together with GPLv2 in one
package.

The main problem is that applying the GPL to certain types of media seems quite
unclear, especially because of the question

  What exactly is source code?

E.g.:

Textures usually are edited destructively, in applications like GIMP. Typically
further editing is done based on the last saved finished version. However,
sometimes they may even be based on screenshots from a (free) game and possibly
custom content for it, which afterwards is edited in various ways. Also, a
typical manual destructive editing steps is retouching.

Music is typically done using a large range of applications, also using
destructive editing steps. A typical pipeline might be:
1. handwritten musical score on paper
2. playing that on a MIDI keyboard into a notation application (e.g. Rosegarden)
3. quantizing and edits in that notation application
4. export to MIDI format from the notation application
5. rendering to an audio file by MIDI synthesizer applications, or even
   external hardware devices
6. editing that audio file with a wave editing application (e.g. Audacity)
What is to be considered the source code here? Is it allowed to use a
commercially bought MIDI synthesizer application and samples, or a physical
device (like an electronically controlled piano) for the recording?

Basically, in both cases the big problem is destructive editing steps. Just
changing a note of the music is simply not possible without repeating all the
editing steps after it. Which is something the author would not even remember,
and most likely do a different way next time.

So, as the GPL raises a big uncertainty on what exactly constitutes source for
non-code assets, I wonder whether it would be wiser to instead choose a different
license for these assets. Because of the source code requirement of the GPL,
there cannot be any GPL compliant license fulfilling this.

On the other hand, many games that have been accepted in Debian "main" contain
music and textures without accompanying source code for these assets. For
example, a similar case can be found in Frozen Bubble - the directory snd/ of
the source distributiion contains multiple opaque (source-less) audio files,
including introzik.ogg and frozen-mainzig-1p.ogg. Yet still, the game is
licensed under the GPLv2.

Furthermore, Enigma is released under "GPLv2 or later", and contains, in the
directory data/ of the source distribution, a file models-2d.lua that
references DejaVuSansCondensed.ttf, which actually is provided in the
data/fonts/ directory. The Deja Vu fonts come with a license restriction that
is not GPL compatible:

  The Font Software may be sold as part of a larger software package but no
  copy of one or more of the Font Software typefaces may be sold by itself.

Also, for none of the graphics any more than a PNG file is provided.

So, apparently these games - and many others - release source-less media
content under the GPL, in case of Enigma even with clearly non-GPL-compatible
origin.

Therefore, I am asking - how can a game contain GPL code, but still use "more
artist friendly" assets (especially because the source code requirement is NOT
clear, as often there IS, because of manual destructive editing steps, no such
thing as "complete corresponding machine-readable source code" source as the
GPL demands)?

Best regards,

Rudolf Polzer