Art licensing guide

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Which free license to chose? How to include art assets of different licenses by different authors? These guidelines will make it easy for artists, copyright holders and project maintainers to deal with asset license questions.

Choosing a free license for your work

As the owner of a work of a game art asset, you can chose to license it in a way that allows easy usage in open source games. Some of the recommended[1] licenses for assets are:

License comparison

Requirements and acceptance of free art asset licenses
License Attribution Copyleft Copysource1 FCD DFSG FSF
CC0 No No No Yes Yes Yes
CC BY 3.0 Yes No No Yes Yes Yes
CC BY SA 3.0 Yes Yes No Yes Yes[4] Yes[5]
GPLv3, v22 Yes Yes Yes Yes Yes Yes

1 Copysource requires sources of derivative works to be released under same terms (example: releasing the rendered image of a modified copysource-licensed 3d model would require the modified 3d model to be made available as well).

Multi-licensing and later versions

It is common to combine license choices (multi-licensing), especially when it comes to CC BY SA and GPL. As an example, Wikipedia uses both GFDL and CC BY SA for content.

Permission to use under "any later version of that license" is often given. This can be indicated by using a "+" sign (example: GPLv2+), if declared in the license information text first.

Remixing third party assets (creating derivative works)

When licensing a derivative work that is a remix of other work or incorporates other works, some questions need to be addressed:

  • Is there permission to use the work in the way it is being used?
  • Is there permission to license the derivative work under a free license?

Many pieces of "free to use" work prohibit commercial use or specific kinds of distribution, which makes it illegal to license derivative works under free licenses. Read and understand the terms of use. Feel free to ask for clarification on the FGD forums, Freesound forums or OGA forums.

Giving credit

Whether you are distributing your own work or using the work of others, proper credit to author, source and terms is usually required and always recommended.

Text files containing full license texts should be located in doc/licenses/.

Single attribution

There are guidelines and tools available for simple licensing of single works under CC0, CC BY 3.0, CC BY SA 3.0 and GPL.

The minimal way of giving credit to the author is by providing a text file which is clearly related to the file in question. If it is called BrownTableTexture.png, a file called BrownTableTexture.png-license.txt in the same directory should contain:

 Brown texture for wooden table models
 Copyright 2011 Wina Tschivo <http://winashomepage.example.org>
 
 Licensed under CC BY SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>

Ideally there also would be a cc-by-sa-3_0.txt file included, containing a copy of the full license text.

Attribution of derivative works

 Brown texture for wooden table models
 Copyright 2008 Wina Tschivo <http://winashomepage.example.org>
 Copyright 2011 Foob Baroofboo <http://foobar.example.com>
   Made texture seamlessly tiling
 Copyright 2012 Maar Sooooon <http://maar.example.com>
   Added scratch visuals to surface
 Licensed under CC BY SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>
 and GPLv2 <http://gnu.org/licenses/gpl.html> or (at your option) any later version.

Centralized attribution text

When the quantity of used art grows, it is recommended to centralize author and license information, for example in a license.txt file in a hypothetical doc directory.

 List of authors and licenses. "+" indicates permission to use later version of license.
 License info can be found in doc/licenses/
 
 Happy sounds and music
 Copyright: 2008 Wina Tschivo <http://winashomepage.example.org>
 License: GPLv2+ / CC BY SA 3+
   assets/sounds/jump.wav
   assets/sounds/success.flac
   assets/music/grasslands.ogg
 
 Texture for grass landscapes
 Copyright: 1992-1993 Nomen Testuser <http://ntu.example.com>
 License: CC-BY 3.0+
   assets/textures/grass.png
 
 Models of ghost character and heart pickup
 Copyright: 2008 2008 Wina Tschivo <http://winashomepage.example.org>
 License: CC0/PD
   assets/models/ghost.obj
   assets/models/heart.obj

Information about changes applied to derivative art can also be included:

 Brown ground texture
 Copyright: 2008 2008 Wina Tschivo <http://winashomepage.example.org>
 Copyright: 2011 Foob Baroofboo <http://foobar.example.com>
   Made texture seamlessly tiling
 License: CC BY SA 3.0+ / GPLv2+
   assets/textures/rock.png

Accepting art asset contributions

When artists create assets and send it to a project, you have to receive explicit permission to use it under a free license. The best way to do this is to request from artist that they include a permission statement in the message, in which they share the assets. Also make sure to ask for sources used in the asset (for example textures, music clips or sound effects), as these might make free licensing impossible.

An example statement would be:

This work is provided by me under the Attribution-ShareAlike 3.0 license <http://creativecommons.org/licenses/by-sa/3.0/>. In its creation, I used textures from BurningWell <http://burningwell.org/> (Public Domain).

For more information and discussion, see Art licensing and usage against the copyright holders rights forum thread.

An easy alternative, which ensures that licenses are clearly stated and hosting is secure is to ask contributors to upload to OpenGameArt. This also exposes the work to other artists and can draw attention to your project if the asset description includes its purpose.

References

  1. Both DFSG and OpenGameArt.org accept a set of art asset licenses, which can be considered as 'common'. Both Freesound and deviantArt support that selection. GPL stays relevant by being used by famous projects like OpenArena and Wesnoth.
  2. Since the GPL was written with software/code in mind, it does talk about "source" which is a concept not 100% convertable to the world of art. It's definition of "source" however is that of the most easily modifiable version of the licensed work. What that means is up to you, but generally speaking it should be a lossless format and include things like layers if available (in the case of 2D artwork). Logic also dictates that you should be able to open the file with a programm that is freely available, which is however often not possible (Photoshop's .psd, 3DSMax files etc). There have also been concerns voiced from musicians that these lossless "source" files can be huge in size (several hundreds of megabytes) which is the reason why they are usually only available as .mp3s or .oggs. In this case a good compromise would be to supply the "source" files only upon personal request (as specified in the GPL). The GNU foundation did clarify the effects of using GPL for something other than software.
  3. GPLv2 and GPLv3 are not compatible with each other, so it is advisable to add the wording "version 2 or (at your option) any later version" in the licensing text for maximum compatibility. Read about the differences for more information.
  4. DFSG Licenses: CC-SA
  5. http://www.fsf.org/news/fdl-1.3-pr.html