Using Free* Game Assets and Code
Disclaimer: This information is provided “as is” and does not constitute legal advice and should not be taken as such. This information explicitly has NO WARRANTY, to the maximum extent permissible by law. Always consult an attorney for legal assistance regarding copyright law and other legal advice. The author is not liable for the use or misuse of this information.
What’s an Asset?
Games contain all sorts of audio and visual media, such as photos, map textures, sounds and videos. These are all ‘assets’ which are owned by someone, who mandates in what way their media may be used. The license dictates the author’s terms for use, reuse, redistribution, terms for modification, and specify any terms for commercial use (primarily to make a profit), or specify that it’s only personal use. For open source games, the license for assets is almost always licensed separately from the source code for the game.
So where are the ‘FREE’ assets?
Sometimes “free” refers to the price of something; free as in ‘free beer’, and sometimes this refers to what rights you have been granted; free as in ‘freedom’. When artists share their stuff on the internet, they specify how others may and may not use it. Most ‘free’ art works found around the internet are free because you are not being charged to access it. However, if you read the fine print, they are copyrighted © and you might not have any right to reuse, modify or redistribute it.
Reuse and Modification
When creating new content for games, it is very common to reuse the works of other people, as ‘source’ material. There are many ‘Free’ asset sites online, however, not everything you find online can be used freely. You may have seen rookie developers or people making tutorial videos reuse copyrighted works, this is very unprofessional. Stealing is wrong, and hurts the artists who make a living off their digital works. Misusing assets that violates copyright could have very serious legal ramifications (laws and trade agreements vary in each country, depending on where you live, copyright violation can bring lawsuits, fines and imprisonment).
What is Copyright?
In the United States, copyright is defined (according to copyright.gov):
“A form of protection provided by the laws of the United States for "original works of authorship", including literary, dramatic, musical, architectural, cartographic, choreographic, pantomimic, pictorial, graphic, sculptural, and audiovisual creations. "Copyright" literally means the right to copy but has come to mean that body of exclusive rights granted by law to copyright owners for protection of their work."
And goes on to outline things that are not covered by copyright law:
"Copyright protection does not extend to any idea, procedure, process, system, title, principle, or discovery. Similarly, names, titles, short phrases, slogans, familiar symbols, mere variations of typographic ornamentation, lettering, coloring, and listings of contents or ingredients are not subject to copyright.”
"Burger Führer" logo © by Cameron Fraser. Licensed CC-BY-SA-4.0
What is Copyleft?
Works that are considered to be ‘Open content’ and other ‘Free Cultural Works’ that are intended for people to ‘freely’ reuse, modify, remix and redistribute, are often released under a ‘copyleft’ (ɔ) or ‘Free Culture’ license, such as a Creative Commons (CC) license.
SOME (but not all) of these licenses allow works to be relicensed, converted, ported to other compatible free culture licenses.
About the Creative Commons Licenses
CC BY - Literally the word ‘by’. You must ‘attribute’ the source work’s author, by giving them credit, usually including:
1. The name or title of their original work (Sometimes this is just a filename).
2. The original author (This will be their first & last name and might include a ‘username’ too).
3. Contact info (Probably an email address, but could be their website or twitter username..).
4. The URL of the permanent link to their original work (Where you got it from).
5. The exact license used, specifically. (Don’t change licenses or terms! This is very important!).
6. The date you accessed the source work.
Make a checklist to make sure you are giving credit and attributing things correctly. Some examples might be:
<Original Source Work> by Original Otto otto@example http://example/original-asset under CC-BY-4.0 license 2017/04/20
Or, for a derivative work that you are sharing:
<New Asset> by Derivative Dan uses <Original Work> by Original Otto otto@example http://example/original-asset under CC-BY-4.0 license 2017/04/20
CC BY-NC - Non-Commercial. Commercial use is where the asset would be used primarily to make money. This does not mean a commercial entity cannot use your work in other ways (like everyone else is allowed).
CC BY-NC assets are incompatible for release with GPL code or binaries, and must be packaged and distributed separately.
CC BY-ND - No-Derivatives. The work can only be used or redistributed in whole, unmodified.
CC BY-SA - Share-alike. When reusing the original or sharing any derivative work you must license it under identical terms**
Other Licenses and compatibility (In order of Freedom!)
CC0 1.0 - Public Domain. No copyright, no rights reserved by the author, giving others the freedom to use it almost any way.
WTFPL - ‘Do What The F*ck You Want To Public License’. Very permissive and recognized as compatible with the GPL. (WTFPL works can be relicensed as CC0 public domain if needed.)
Unlicense - Another very permissive mechanism to waive copyright and provide the work with no-warranty.
OGA-BY-3.0 - The OpenGameArt.org Attribution 3.0 Unported license was created to be more permissive for redistribution than CC and potentially allows the assets to be used by digital rights management (DRM) systems. However, OGA-BY-3.0 works can later be converted to a CC-BY 3.0 license (if desired).
Picking an appropriate license for your own works
When you create new artwork, from scratch (using no copyrighted sources), you own the copyright. When you modify the works of others, you only own the copyright for what you added, not the original. When you create new artwork, from scratch, you can ‘copyleft’ license your own work and release it for others to use under the free license of your choosing.
For source code
Note: ioquake3 engine, and Tremulous are under GPLv2+ license (Not GPLv3!).
Contributing your code and assets:
To contribute code and assets to Tremulous and also to the GrangerHub project, see our GitLab page.
GrangerHub will release and distribute the Game’s client, Tremulous dedicated server, (tremded and QVMs) under GPLv2+ separately, where people would be free to relicense as GPLv3 for their own projects. New assets will be re-licensed upon re-release as CC-BY-3.0.
Any update / commit from people working on a GPLv3 source code base will need to explicitly grant permission to port their code to the GPLv2+ master.
GNU GPL version differences
GNU and The Free Software Foundation maintain the General Public License, GNU GPL, or GPL for short. It is important to understand compatibility with the GPL and other licenses and even between various versions of the GPL. Many free-software licenses are compatible with others, but many are incompatible! There’s conflicts with different versions of the GPL, as well as incompatibility with all Non-Free software.
For instance, you cannot use GPLv3 code inside of software released as GPLv2, unless you get explicit permission from ALL authors of the GPLv3 code. The inverse is not true, GPLv2+ code can be used inside of GPLv3 projects where the ‘or any later version’ of the license applies, where you then relicense the original GPLv2+ source and re-release as GPLv3.
Note: GPLv2-ONLY licensed works lack the ‘or any later versions’ clause and cannot be relicensed as GPLv3. GPL works cannot be combined with closed source and non-free software. For this, the GNU created the LGPL.
Here’s a pretty in-depth article about how other licenses interface with the GPL: http://www.dwheeler.com/essays/floss-license-slide.html
Sources for Open Content textures and images
burningwell - Texture sources from Public Domain images.
OpenGameArt - Has a lot of textures and 3d models from regular contributors.
flickr - Lets you specify Creative Commons licensed content in image searches.
Advanced Google Image Search - Lets you filter by usage rights. See Google’s support topic: Find images you can reuse.
Non-free - Special restrictions apply
Other sites exist where images are sold under a commercial license where you pay to have rights to edit and redistribute. Always know what you are paying for and read the terms carefully.
Commercial Texture Sources
CGTextures.com - Textures on CGTextures.com should only be used in the creation of other textures and for texturing models, must be modified to be distributed, and may not be released in ‘texture packs’ http://www.cgtextures.com/content.php?action=license
Be aware. Be smart. Be responsible. Make stuff. Know your rights, and share with others in good faith.