In July 2009, 0 A.D. became free, open-source software (FOSS), where "free" means both gratis and, more importantly, "free" as in software and cultural freedom.
(Specifically, we, the developers of 0 A.D., have made the code and data of the game available under the GPL license, version 2 or later, and released the art, sound and documentation under CC-BY-SA, version 3.0.)
Briefly, this means that we allowed anyone to reuse components of 0 A.D. for any purpose, redistribute them and remix them without having to ask us for permission. They are free to make new games using the 0 A.D. engine, they may create remixes of 0 A.D. music, you name it.
We did this, among other reasons, because we believe free, open-source software like 0 A.D. encourages sharing, learning and creativity, and we want to encourage people to remix, mod and tinker with 0 A.D. in the future.
But there's an important condition that comes with these licenses. The main condition we ask from anyone who wants to make their own works derived from 0 A.D. (or parts of it) is that they release those derivative works only under the same licenses we did, or under similar ones.
In other words, let's say you want to take a 3d elephant from 0 A.D. and you want to add a funny hat on its head and show everyone on the internet that you made a funny elephant with a hat.
If you do this using an elephant from 0 A.D., you must allow other people to reuse the "elephant with the hat" image as much as they want, to redistribute and to remix it, etc., and even print it on posters and sell them if they want to. If you remix an elephant image from 0 A.D., you cannot legally tell people, "you can't use this 'elephant with hat' image without my permission", or require money for the permission to print it, etc.
In this way, the creator of the "elephant" image makes sure that the "elephant and hat" work, the "elephant in a dress" work etc. all remain free cultural works. Nobody can legally rip off 0 A.D. works to make their own proprietary art. Rather, the 0 A.D. artwork and all its derivatives will remain forever free for everyone to share and enjoy.
For many reasons, it is important that we make sure that everything that is bundled as a part of 0 A.D. is released freely.
From the codebase and development tools, through the unit statistics, the artwork, the music score and the sound effects, to the stories of the scenarios and campaigns - Everything must be free (as in "freedom").
We must be very strict about this, or we risk disappointing some people who really want to make sure everything they use is free and trust us to keep tabs on these matters. This helps us maintain effective ways for 0 A.D.'s distribution and our public image.
So, if you want us to bundle your art contributions with 0 A.D., we ask you to post in this thread to release your artwork under CC BY-SA 3.0.
To do this, please copy-paste the following sentence and post in this thread:
Please remember this action is irrevocable - Once you release a work under this license, you can never take it back and say, "I don't allow you to remix this 3d giraffe without asking for my permission".
Thanks in advance from all of us.
Feel free to post in this thread if you have any questions.
And may your contribution to free software and culture be everlasting.