Public domain consists of works that aren't protected by intellectual property laws, most notably copyright. Anyone can use this material in any way and for any purpose. Works enter public domain typically when the copyright expires or when the author explicitly waives their rights.
Definition of public domain differs in each jurisdiction, .e.g by time after which the copyright expires (e.g. 70 years after author's death in the USA). Some countries allow authors to give up some rights while others don't. This makes it difficult for creators to deliberately put their work into public domain worldwide. For this reason, waivers (similar to licenses), most notably CC0, have been created. Some licenses, such as WTFPL or Unlicense, are sometimes labeled as near public domain, or public domain equivalent, but are mostly discouraged due to vague wording. CC0 is the preferred way to place works (including source code) in the public domain.
From the game making perspective public domain offers a huge resource of material for game media, and sometimes even code, that the creator can use, without any legal burden (e.g. giving appropriate credit), for any purpose (e.g. the assets can be used in a commercial game).
Note that game in public domain is free software only as long as the source code is available.
Public domain resources
There exists a number of repositories that include, or contain exclusively public domain material. Some of these include:
- OpenClipart – Vector graphics, all public domain under CC0.
- Pixabay – High quality pictures, all public domain under CC0.
- Wikimedia Commons – Media under various free licenses. It is possible to search for public domain only, e.g. this query searches for images of penguins in the public domain:
penguin filetype:image incategory:"cc-zero|PD-user|CC-PD-Mark"
- FreeSound – Sound resources under various free licenses, CC0 can be selected in the filter.
- OpenGameArt – Game resources under various free licenses, CC0 can be selected in the filter.
- FreeMusicArchive – Music, public domain can be selected in the filter.
- Internet Archive – General archive of various material, it is possible to construct queries that try to search within the public domain, e.g.:
penguin possible-copyright-status:(NOT_IN_COPYRIGHT) OR licenseurl:("http://creativecommons.org/publicdomain/mark/1.0/") OR licenseurl:("https://creativecommons.org/publicdomain/mark/1.0/")