Box2D Creator Erin Catto Releases Box3D, an Open-Source 3D Physics Engine With Roots in Half-Life: Alyx's Rubikon
Erin Catto, creator of the Box2D physics engine, has released Box3D v0.1.0, an MIT-licensed 3D physics engine for games that blends Box2D code with remnants of Rubikon-Lite, a hobby version of the Half-Life: Alyx physics engine, and already powers Facepunch's s&box.
Overview
Erin Catto, the developer of the Box2D physics engine, has released Box3D, an open-source 3D physics engine for games. “I’m happy to announce the release of Box3D, an open source 3D physics engine. It is now available on GitHub,” Catto wrote in an announcement on Box2D.org dated June 30, 2026. The project’s v0.1.0 build, titled “Initial Release,” was published the same day on GitHub, and the code is open source under an MIT license, as reported by Phoronix.
What We Know
“You can think of Box3D as a fork of Box2D, extended with many features needed for 3D games,” Catto wrote in the announcement. The additions include triangle mesh collision, height-field collision, and baked compound collision, while the core architecture, in Catto’s words, “remains almost identical to Box2D”: a C API, library source written in C17, continuous collision, a wide SIMD contact solver, multi-threading hooks, large world support with doubles for position, cross-platform determinism, and recording and replay.
According to the project’s README, the core library has no dependencies beyond the C runtime (plus libm on Unix), and the bundled samples use the sokol library to run with D3D11 on Windows, Metal on macOS, and OpenGL 4.5 on Linux. The engine can also be built for the web, where it “uses SSE2 with WebAssembly.”
Box3D grew out of Catto’s work on The Legend of California, a game he has been building at the studio Kintsugiyama since 2022 using Unreal Engine, starting with version 5.0, according to the announcement. He wrote that experiments with Unreal’s native Chaos physics engine ran into problems: it had no support for simulating gyroscopic torques at the time — a feature he says Epic added in late 2024 — and, in a survival game where chopping down trees was one of the first things he worked on, “the falling trees moved erratically, teleporting around the screen.” Phoronix similarly reported that Box3D “began development after having issues with the native physics built into the Unreal engine.”
Rather than adopting an existing open-source engine such as Jolt, which Catto wrote he was considering, he took a suggestion from Dirk Gregorius, a physics programmer who, per the announcement, shipped a custom physics engine called Rubikon in Half-Life: Alyx and maintains a hobby version Catto calls “Rubikon-Lite.” Catto forked Rubikon-Lite, hooked it into Unreal, and later replaced “almost all the APIs, data structures, and algorithms” in the fork with Box2D code. “Eventually my Rubikon-Lite fork transformed into Box3D,” he wrote, adding that Rubikon-Lite code remains in the convex hull generation and some collision algorithms. Phoronix noted that the new Box3D code is “derived in part from ‘Rubikon-Lite’ as an offshoot of the physics engine for Half-Life: Alyx.” On the Valve side, according to the announcement, Rubikon continues to evolve and Gregorius has developed similar optimizations in a new engine called Ragnarok, which Catto said will appear in future Valve games.
Beyond The Legend of California, the announcement lists three early adopters: s&box, a game platform by Facepunch Studios; Esoterica, an open source game engine led by Bobby Anguelov; and a 1000-player multiplayer space game by Glenn Fiedler. Phoronix wrote that the engine “has already seen uptake by a few games and game engines/platforms.”
Catto, who wrote that he has been making physics engines for games since 2004, said the team at Kintsugiyama agreed to let him release Box3D as open source and work on it as part of his day job, per the announcement. “Open source is not a business for me. I develop Box2D and Box3D because I love working on game physics,” he wrote.
What We Don’t Know
- There is no timeline for a stable release. Catto still considers Box3D “alpha software,” both in the announcement and in the v0.1.0 release notes, and wrote that the engine “needs more testing and more complete documentation” on the way to a v1.0 release.
- Roadmap items — enhanced character movement features, improved ghost collision mitigation, optimizations, and an improved joint solver — are described in the announcement only as tasks Catto is “considering,” not as commitments.
- The contribution policy is not final: unlike with Box2D, Catto wrote in the announcement that he expects to open Box3D to pull requests, “probably using a CLA.”
- The announcement does not include performance comparisons with other physics engines; Catto wrote that he is “not trying to compete with other physics engines.”