Open-source code and free assets

One command, dozens of packages

Kai wants sparkly explosions in Hyper Shards and runs one command: npm install with a particle library. Seconds later the node_modules folder holds dozens of new packages. Kai only asked for one.

Every package is code someone else wrote, so it is copyrighted. We can still ship most of it in a game we sell, because it is open source: code whose authors publish it with a license that lets anyone use, change and share it. Each license has its own rules, though.

Permissive licenses: MIT and Apache 2.0

• MITUse, change, share and sell. The condition: keep the copyright notice and license text with every copy. The code comes with no warranty, so if it breaks, that is on us.
• Apache 2.0The same freedoms, plus a patent license from contributors for their code. We must mark files we changed, and it gives no right to use the project's trademarks.

Permissive licenses allow almost anything, including use in a closed game we sell, as long as we follow a few conditions.

Copyleft: the GPL shares alike

Copyleft licenses pass their freedom forward. The best known is the GPL (GNU General Public License). It allows commercial use, even selling. But if we distribute a program that includes GPL code, the whole program must be released under the GPL, with its complete source code available to the people who get it.

That is "share alike" for code. For Hyper Shards, a closed game we sell, one GPL package inside would mean publishing all our source code, or taking the package out.

Packages have packages

Every npm package declares its license in its package.json with a short standard code called an SPDX identifier: "MIT", "Apache-2.0", "GPL-3.0-only". "(MIT OR Apache-2.0)" means we may pick either. "UNLICENSED" means the author grants nobody the right to use it.

Packages depend on other packages, which have their own dependencies: a whole tree. npm ls --all prints the tree, and license-checking tools list every license in it. We check before installing, and again before each release.

Notices travel with the game

Our credits and license list already tracks every asset (the Copyright and licenses lesson explains it). Code adds a duty: MIT, Apache and most open-source licenses require their copyright notice and license text to travel with the game, not just sit in our repo.

So the studio ships a THIRD-PARTY-NOTICES.txt file and an "Open-source licenses" screen in the settings menu. An Apache package may include a NOTICE file, whose credits we pass on too. Build tools can collect all this, and we read the result before release.

Free asset licenses, element by element

CC0 and the license basics are in the Copyright and licenses lesson. Here is what each Creative Commons element means for a real sprite, sound or font:

• BY: credit as askedCreative Commons recommends TASL: Title, Author, Source (a link) and License. For example: "Laser Pack" by Ana Ruiz, OpenGameArt, CC BY 4.0.
• SA: share alikeIf we change the asset, our changed version must use the same license. Creative Commons counts music synced to video, like a trailer, as a change too.
• ND: no derivativesUse it exactly as it is. No recolouring, cropping, speeding up or cutting a loop.
• NC: non-commercialNot for a work mainly aimed at earning money. A game that earns from sales or ads is commercial.
• Fonts: SIL Open Font LicenseOFL fonts can be built into a game we sell. We may not sell the font by itself, and an edited version must drop its reserved name.

Where good free assets come from

Free to download is not the same as free to use in a game we sell. These sources are popular with game makers, and each works differently:

• KenneyGame asset packs released as CC0: free even for commercial games, no credit required (giving one is kind). The Kenney logo is not included.
• OpenGameArtEach asset has its own license: CC0, CC BY, CC BY-SA, OGA-BY or GPL. Read its license notice, and treat previews as all rights reserved unless it says otherwise.
• itch.io asset packsEach creator sets their own terms, on the page or in a file in the download. No license stated? Ask the creator in writing and keep the reply.

Bolt says: Save each license page (a screenshot or PDF, with the date) next to the asset on our list. If a license changes later, we can show what we agreed to.