The complete guide for open sourcing video games March 23, 2021 on Drew DeVault's blog

Video games are an interesting class of software. Unlike most software, they are a creative endeavour, rather than a practical utility. Where most software calls for new features to address practical needs of their users, video games call for new features to serve the creative vision of their makers. Similarly, matters like refactoring and paying down tech debt are often heavily de-prioritized in favor of shipping something ASAP. Many of the collaborative benefits of open source are less applicable to video games. It is perhaps for these reasons that there are very few commercial open source games.

However, there are some examples of such games, and they have had a great deal of influence on gaming. Id is famous for this, having released the source code for several versions of DOOM. The Quake engine was also released under the GPL, and went on to be highly influential, serving as the basis for dozens of games, including time-honored favorites such as the Half Life series. Large swaths of the gaming canon were made possible thanks to the generous contributions of open source game publishers.

Publishing open source games is also a matter of historical preservation. Proprietary games tend to atrophy. Long after their heyday, with suitable platforms scarce and physical copies difficult to obtain, many games die a slow and quiet death, forgotten to the annals of time. Some games have overcome this by releasing their source code, making it easier for fans to port the game to new platforms and keep it alive.

What will your game’s legacy be? Will it be forgotten entirely, unable to run on contemporary platforms? Will it be source-available, occasionally useful to the devoted player, but with little reach beyond? Perhaps it goes the way of DOOM, living forever in ports to hundreds of devices and operating systems. Maybe it goes the way of Quake, its soul forever a part of the beloved classics of the future. If you keep the source code closed, the only conclusion is the first: enjoyed once, now forgotten.

With this in mind, how do you go about securing your game’s legacy?

Source available: the bare minimum

The bare minimum is to make your game “source available”. Be aware that this is not the same thing as making it open source! Some of your famous peers in this category include Alien 3, Civilization IV and V, Crysis, Deus Ex, Prince of Persia, Unreal Tournament, and VVVVVV.

This approach makes your source code available to view and perhaps to compile and run, but prohibits derivative works. This is definitely better than leaving it closed source: it provides helpful resources for modders, speedrunners, and other fans; and devoted players may be able to use it as the basis for getting the game running on future platforms, albeit alone and unable to share their work.

If you choose a minimal enforcement approach, then some players might ultimately share their work, but you’re leaving them on tenuous legal grounds. I would recommend this if you’re very protective of your IP, but know that you’re limiting the potential second life of your game if you take this approach.

Copyleft with proprietary assets

The next step up is to make your game open source using a copyleft license, but refraining from extending the license to the assets — anyone who wants to get the source code working would either need to buy the game from you and extract the assets, or supply their own community-made assets. This is a popular approach among open source games, and gives you most of the benefits and few of the drawbacks. You’ll join the ranks of our DOOM and Quake examples, as well as Amnesia: the Dark Descent, System Shock, Duke Nukem 3D, and Wolfenstein 3D.

Games like this enjoy a long life as their software is more easily ported to new platforms and shared with other users. DOOM runs on phones, digital cameras, ATMs, even toasters! Its legacy is secure without any ongoing commitment from the original developers. This also allows derivatives works — new games based on your code — though it may turn some developers away. Using a copyleft license like the GPL requires derivative works to also be made open source. The community generally has no problem with this, but it may affect the willingness of future developers to incorporate your work into their own commercial games. I personally think that the proliferation of open source software that’s implied in the use of a copyleft license is a positive thing — but you may want to use another approach.

Permissive license, proprietary assets

If you want to allow your source code to find its way into as many future games as possible, a permissive open source license like MIT is the way to go. Flotilla is an example of a game which went with this approach. It allows developers to incorporate your source code into their own games with little restriction, either by creating a direct derivative, or by taking little samples of your code and incorporating it into their own project. This comes with no obligation to release their own changes or works in a similar fashion: they can just take it, with very few strings attached. Such an approach makes it very easy to incorporate into new commercial games.

This is the most selfless way to release your code. I would recommend this if you don’t care about what happens to your code later, and you just want to make it open source and move on. Though this will definitely enable the largest number of future projects to make use of your work, the copyleft approach is better for ensuring that the largest possible number of future games are also open source.

Open assets

If you’re feeling especially generous, you could release the assets, too. Good licenses for this includes the Creative Commons licenses. All of them permit free redistribution of your assets, so future players won’t have to buy your game to get them. This could be important if the distribution platform you used is defunct, or if you’re not around to buy it from — consider this well before deciding that you’d rather keep your share of the dwindling asset sales as your game ages.

Using Creative Commons also allows you to tune the degree to which your assets may be re-used. You can choose different CC licenses to control the commercialization of your assets and use in derivative works. To allow free redistribution and nothing else, the CC-NC-ND license (noncommercial, no derivatives) will do the trick. The CC-BY-SA license is the copyleft of creative commons: it will allow free redistribution, commercialization, and derivative works, if the derivatives are also shared with the same rights. The permissive approach is CC-0, which is equivalent to releasing your assets into the public domain.

Permitting derivatives and re-commercialization of your assets can save a lot of time for new game developers, especially indie devs with a small budget. It’s also cool for making derivative games, similar to modding, where creative players can remix your assets to make a new game or expansion pack.

What if I don’t completely own my game?

You can’t give away the rights to anything you don’t own. If you rely on proprietary libraries, or a third-party level editor, or you don’t own the rights to the music or sprites, you cannot make them open source.

In this situation, I recommend open sourcing everything that you’re able to. This might mean that you open source an ultimately broken game — it simply might not work, or not even compile, without these resources. This is unfortunate, but by releasing everything you can, you leave your community in a good position to fill in the gaps themselves, perhaps by refactoring your code to work around them, or by replacing the proprietary bits with free alternatives. This also allows the parts of your game which are open to be reused in future games.

But cheaters could use it!

This is true. And it’s worth noting that if your game has a mandatory online component based on your own servers, then making it open source doesn’t make nearly as much sense, especially if you ultimately decide to shut those servers off.

There is a trade-off to be made here. In truth, it’s very difficult to prevent cheating in your game. If you’ve made a popular competitive multiplayer game, you and I both know that there are still cheaters using it despite your best efforts. Keeping it proprietary is not going to stave off cheaters. Social solutions are better — like a system to report cheaters, or to let friends play on private servers.

Making your game open source might help less skilled script kiddie figure out how to cheat more easily in your game. I can’t decide for you if the trade-off is worth it for your game, but I can tell you that the benefits of making it open are vast, and the efficacy of keeping it closed to prevent cheating is questionable.

But my code is embarrassing!

So is everyone else’s. 🙂 We all know that games are running up against tight deadlines and clean code is not going to be the #1 priority. I assure you that your community will be too busy having fun to judge you for the quality of your code. The idea that it just needs to be “cleaned up” first is the death of many projects which would otherwise have been made open source. If you feel this way, you will probably never be satisfied, and thus you’ll never open it. I assure you: your game is ready to make open source, no matter what state it’s in!

Bonus: Ethan Lee tipped me off to some truly awful code which was left in VVVVVV, which you can freely browse on the 2.2 tag. It’s not great, but you probably didn’t know that — you only remember VVVVVV as a critically acclaimed game. Game developers are working under tight constraints and no one is judging them for that — we just want to have fun!

So what do I need to do?

Let’s lay out the specific steps. You need to answer the following questions first:

If you’re not sure what’s best, I would recommend using the GPL for your code, and CC-BY-SA for the assets. This allows for derivative works, so long as they’re also made open with a similar license. This enables the community to build on your work, porting it to new platforms, building a thriving modding community, and freely sharing your assets, ensuring an enduring legacy for your game. If you’d like to decide the details for yourself, review the comments above once again and pick out the licenses you’d like to use for each before moving on.

If you need help with any of these steps, or have any questions, please send me an email, and I will help you to the best of my ability.

Publishing the source code

Prepare an archive of your source code, and add the license file. If you went with the source-available approach, simply write “Copyright © <you> <current year>. All rights reserved.” into a text file named LICENSE. If you chose something else, copy the license text into a LICENSE file.

If you want this over with quickly, just stick the code and license into a zip file or a tarball and drop it on your website. A better approach, if you have the patience, would be to publish it as a git repository. If you already use version control, you may want to consider carefully if you want to publish your full version control history — the answer might be “yes”, but if you’re unsure, the answer is probably “no”. Just make a copy of the code, delete the .git directory, and import it into a new repository if you need to.

Double check that you aren’t checking in any artifacts — assets, executables, libraries, etc — and then push it to the hosting service of your choice. GitHub is a popular choice, but I would selfishly recommend sourcehut as well. If you have time, write a little README file which gives an introduction to the project as well.

Publishing the assets

If you choose to leave the assets proprietary, then there are no further steps. Players can figure out how to extract the assets from their purchased game.

If you choose to make them open, prepare an archive of your assets. Include a copy of the license you choose — e.g. which Creative Commons license you used — and drop it into a zip file or a tarball or something similar. Stick this on your website, and if you’re feeling generous, prepare some instructions for how to incorporate the asset bundle into the game once a player compiles your code.

Tell the world!

Let everyone know that you’ve made your game open source! Write a little blog post, link to the source and assets, and enjoy a little bit more of the limelight while the press and the community thanks you for your contribution.

One final request on this note: if you choose the source-available approach, please refer to it as such in your public statements. Source available is not the same thing as “open source”, and the distinction is important.

And now it’s my turn to thank you: I’m so happy that you’ve released your game as an open source project! The community is much richer for your contribution to it, and I hope that your game will live on for many years to come, both in self through ports and mods, and in spirit through its contributions to future games. You’ve done a wonderful thing. Thank you!

If you found this guide helpful in publishing your game, please email me so I can play it!

List of FOSS games inspired by this guide:

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