[Christopho]

Okay, so CC-BY-NC-SA is non-free and cannot be distributed in the same package as GPL software. Thanks Zefk for the clarification, I was not aware of the problem.

But it would be terrible to separate the sample quest from the engine and editor distribution (especially not from the editor distribution), because the goal is to provide assets to users who create a new quest.
If we separate it, only a few poeple will use it. If it is non-free, it cannot be included in Linux distributions and will get much less visilibity.

Diarandor, I think you should consider switching to a CC-BY-SA license for your art First, you told that one day you would want to charge some money for your art. This implies changing the license anyway.
If we put a CC-BY-SA license, not only it will simplify the licensing of the sample quest and Children of Solarus, but more importantly, the art would be free as in freedom, distributed with Solarus: more people will contribute in the end, making awesome games, improving the art again. Just like people contribute to the engine and editor and improve them a lot.
I don't think it is a problem to allow commercial redistributions. If people redistribute existing games for money without modification, it won't work for them, our games are already gratis. If they do some real work and want to charge money, then good for them, and they might as well get something from it. Keep in mind that their buyers will then have maximum freedom, including the right to redistribute for no money what they just purchased.

See http://www.gnu.org/philosophy/selling.html
"Distributing free software is an opportunity to raise funds for development. Don't waste it!"

Imagine that after the Children of Solarus release, someone makes an Android port and sells Children of Solarus on Google Play. Are they getting illegitimate money by doing that? No, they deserve some compensation, this is some real work. But think about it. If it is good, I will totally buy it. Thanks to GPL and CC-BY-SA, I will also get the source code of their work and the right to redistribute it like I want. So we would redistribute it on the Solarus website, and probably on Google Play under the name "Solarus team", possibly for money. Who benefits more? Solarus!

[Zefk]

Okay, so CC-BY-NC-SA is non-free and cannot be distributed in the same package as GPL software. Thanks Zefk for the clarification, I was not aware of the problem.

I believe GPL should change a little and allow non-commercial art in free distributions. I think it is ridiculous.

Diarandor,

Another thing you can do is make a license of your own allowing non-commercial and commercial use, but require them to pay you if their sells exceed a certain number and to share derivatives. That will be compatible with GPL.

For example,
Diarandor-SA

Diarandor license allows one to use art for commercial and non-commercial uses. If sells exceed [insert number], then you must pay [insert number] for each sell exceeding the selling that number. You must put Diarandor's name in the visible credits or text file linking to his works.

You must also link to the Diarandor website and notify of any changes you make to the art for your game because others are allowed to use your edits or reworks as long as credit to you is given for those modifications.

Also, you can set that you are able to choose or pick [insert number] sprites for free, but must purchase a Diarandor-SA-E (E=extended) at the price of [insert number] for using more sprites.

That type of license would be compatible with GPL because you are allowing commercial and non-commercial use for a free distribution with Solarus.

[Diarandor]

This is what it says here: http://www.gnu.org/distros/free-system-distribution-guidelines.en.html

Non-functional Data

Data that isn't functional, that doesn't do a practical job, is more of an adornment to the system's software than a part of it. Thus, we don't insist on the free license criteria for non-functional data. It can be included in a free system distribution as long as its license gives you permission to copy and redistribute, both for commercial and non-commercial purposes. For example, some game engines released under the GNU GPL have accompanying game information—a fictional world map, game graphics, and so on—released under such a verbatim-distribution license. This kind of data can be part of a free system distribution, even though its license does not qualify as free, because it is non-functional.

-------------

So it should be possible to distribute the assets (non-functional data) with non-free licenses with the GPL functional data. If my non-commercial license does not allow it, I will have to find another non-commercial license allowing it.

EDIT: I still don't understand completely the situation since I do not understand what does "distribution" mean in this context. I always thought that the art license was independent of the software and code.

My intention is to keep exclusivity of my graphics keeping them non-commercial, at least until a commercial game is released, just to be sure that I receive some money (I don't mind if it's not that much). My current salary is too low and I need some money to live.

[Zefk]

The Diarandor-SA example is considered not free, but it is compatible. You still get money if they use too many of your sprites or they exceed certain selling numbers. The loophole is that it is for commercial and non-commercial use.

Edit: Free system distribution refers to the package the GPL work is in. Solarus engine is GPLv3, so anything package inside the archive with Solarus must be for both commercial and non-commercial uses to be considered free distribution.

[Diarandor]

For example,
Diarandor-SA

Diarandor license allows one to use art for commercial and non-commercial uses. If sells exceed [insert number], then you must pay [insert number] for each sell exceeding the selling that number. You must put Diarandor's name in the visible credits or text file linking to his works.

You must also link to the Diarandor website and notify of any changes you make to the art for your game because others are allowed to use your edits or reworks as long as credit to you is given for those modifications.

Also, you can set that you are able to choose or pick [insert number] sprites for free, but must purchase a Diarandor-SA-E (E=extended)  at the price of [insert number] for using more sprites.

That type of license would be compatible with GPL because you are allowing commercial and non-commercial use.

Yes, some customized license like this is what I need. It is like the one used by the creators of Unity.
I prefer to avoid inventing a license if possible (which is too risky since I am not a lawyer and do not know the consequences), I will make some investigations to see if that license has a known name, and re-license all my art with it.

EDIT: a different solution would be to say that we use a freeware distribution instead of a free one. Although probably Christopho will disagree, so better to forget about this possibility.

[Zefk]

You can state in the license that you are not liable for any damages of any kinda done by your art and that they will agree to be held accountable by their countries laws or the country they currently reside in if they illegally use your work in anyway. That way they will not use the...."I am not in your country excuse." At most....you can shutdown their project at no cost, but you might have to go through court to get anything else done. sending them to jail, etc.

Edit: Or as you said....you can find a license that goes by the terms you want, but I do not know much beyond MIT, creative commons, or GPL. I hope I was able to help a little.