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Erin Catto

@erin_catto@mastodon.gamedev.place

mastodon 4.7.3

Game physics and animation. Creator of Box2D. Working on The Legend of California.

1659 Followers

218 Following

7 Posts

Joined November 07, 2022

blog:

https://box2d.org/

twitter:

https://twitter.com/erin_catto

blue sky:

erincatto.bsky.social

game:

https://www.kintsugiyama.com/

Erin Catto @erin_catto@mastodon.gamedev.place

· 1w ago

New blog post on speeding up the separating axis test (SAT) using inscribed spheres: https://box2d.org/posts/2026/09/inscribed-spheres/

box2d.org

Inscribed Spheres

The separating axis test (SAT) works well for computing contact manifolds. Unlike GJK, it is robust when shapes are very close together and it works when shapes are overlapped. Box3D uses SAT for computing the contact manifolds between convex hulls (polytopes). I like using SAT because I can get all the contact points at once and it doesn’t need a fallback algorithm for overlap like GJK does. So I don’t need to waste time calling GJK only to have it fail and then drop to a fallback algorithm lik

Erin Catto @erin_catto@mastodon.gamedev.place

· 3mo ago

Boosted by @gvenema@fairmove.net

I’m happy to announce the release of a new open source 3D physics engine called Box3D. I’ve been working on this project for a few years now, but it represents over 20 years of experience writing physics engines for games. Read more here: https://box2d.org/posts/2026/06/announcing-box3d/

box2d.org

Announcing Box3D :: Box2D

I’m happy to announce the release of Box3D, an open source 3D physics engine. It is now available on GitHub. Box3D repository You can think of Box3D as a fork of Box2D, extended with many features needed for 3D games. Some additions: Triangle mesh collision Height-field collision Baked compound collision The core architecture of Box3D remains almost identical to Box2D. C API All library source is C17 Sub-stepping solver Continuous collision Graph coloring for large islands Wide SIMD contact solv

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Erin Catto @erin_catto@mastodon.gamedev.place

· 2mo ago

New blog post. Box3D is getting faster. https://box2d.org/posts/2026/07/simd-for-collision/

box2d.org

SIMD for Collision

In a previous post SIMD Matters I discussed the big wins I got from graph coloring to make the contact solver faster. That same approach exists in Box3D. I like to call this approach “wide SIMD”. The idea is to process multiple work units at the same time. In the contact solver this means solving four contact points at the same time. This is different from “narrow SIMD” where a 3-vector (xyz) is put into a SIMD register and then standard vector math is expressed as SIMD intrinsics.

Erin Catto @erin_catto@mastodon.gamedev.place

· 3mo ago

Pushing Box3D to the limit. A pyramid with 16290 boxes. Stabilized by contact recycling.
https://www.youtube.com/watch?v=vG4HOKcQFxM

Pushing Box3D to the limit

Erin Catto @erin_catto@mastodon.gamedev.place

· 1mo ago

I added a new experimental rigid body character mover to Box2D. https://www.youtube.com/watch?v=MWnTQUKUc0s

Rigid body character mover

Erin Catto @erin_catto@mastodon.gamedev.place

· 4mo ago

I added a new feature to Box2D that lets you record the physics simulation in your game and then play it back in a replay viewer. This can be used to debug your simulation and includes recording of ray casts and other queries. The system makes use of determinism to exactly reproduce the simulation from your game. This feature also brings the possibility of deterministic rollback. Here's the blog post which also links to a video showing the feature in action.

https://box2d.org/posts/2026/06/replay/

box2d.org

Replay

Replay is a new system I developed for Box2D. The main goal is to allow users to send me files I can use to reproduce bugs. Because Box2D is open source, I can rarely run a user’s game or see their code in full context. This makes it hard to track down problems. Previously in v2.4 a user could use b2World::Dump to save a text file that let me reproduce bugs.

Erin Catto @erin_catto@mastodon.gamedev.place

· 3mo ago

Large world support is coming to Box2D v3.2. Uses doubles for body positions but all math is done in relative coordinates using single precision. Around 3% performance loss.
https://www.youtube.com/watch?v=RwQrQafZU6Y

Erin Catto

Inscribed Spheres

Announcing Box3D :: Box2D

SIMD for Collision

Pushing Box3D to the limit

Rigid body character mover

Replay

Double precision for larger worlds