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Erin Catto

@erincatto.bsky.social

630 followers 190 following 37 posts

I created Box2D. I enjoy all kinds of programming, but specialize in physics and animation. I work in Unreal for my day job (like everyone else). C/C++ and whatever Claude spits out for me. https://box2d.org/

box2d.org

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Erin Catto @erincatto.bsky.social · 6h

You can have AI walking, running, sitting, climbing, etc. They can do anything out of view.

Erin Catto @erincatto.bsky.social · 11h

Pre-rolling is good for character screens in the UI. For in game cloth this could lead to frame spikes, so my favorite solution is to simulate cloth at a lower rate off-screen.

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Erin Catto @erincatto.bsky.social · 1d

When your graphics programmer tells you to stop simulating off-screen cloth, show them this video.

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Erin Catto @erincatto.bsky.social · 4d

We use this in our CI. We have some tests that run our game and test various game play systems. Catches a lot of bugs before code gets merged.

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Erin Catto @erincatto.bsky.social · 9d

Very cool! Have you considered using voxels? Lots of procedural generation options with those.

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Reposted by Erin Catto

WereCoder @werecoder.gamedev.blue · 10d

I'm an #IndieDev building a custom engine for my medieval fantasy survival game.

Some things I've done so far:
Simulation with client / server networking
Basic DX12 renderer
Asset pipeline
Character movement and animation
#ProcGen world creation
Basic UI
Basic Audio
Basic Health / Combat

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Erin Catto @erincatto.bsky.social · 13d

Box2D version 3 has a new API written in C to make it easier to wrap. For the revision I also decided to write the internals in C to see if I liked it. It worked out well.

Erin Catto @erincatto.bsky.social · 13d

A couple barriers to C development are lack of a fast sorting library and a fast hash set and map. I've found these two libraries to be excellent at providing those. They are macro based, but I think the trade-off is worth it. Both work with C99.
github.com/svpv/qsort
github.com/JacksonAllan...

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Erin Catto @erincatto.bsky.social · 27d

Pinched wrists on ragdolls are caused by twist bone poses living in the animation data. Twist bones are procedural and computed based on the pose of animated bones, so they should computed as a post process at runtime to fix this. See www.youtube.com/watch?v=Jkv0... by @bobbyanguelov.bsky.social

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Erin Catto @erincatto.bsky.social · Sep 4

Silksong is using Unity? So using Box2D?

Erin Catto @erincatto.bsky.social · Aug 30

I love physics based walking simulations. I'm also pleased the author was successful using Box2D (v3.1?) www.youtube.com/watch?v=aBp-...

AI Cat Learning to Run

YouTube video by Pezzza's Work

www.youtube.com

Erin Catto @erincatto.bsky.social · Aug 23

I revoked your ban. Please make sure your discord account is secure.

Erin Catto @erincatto.bsky.social · Jul 13

I tried this many years ago. I didn't like it. Lots of wasteful code subtracting the origin to make stuff work.
Want:
point = 0.5 * point1 + 0.5 * point2
Have:
point = 0.5 * (point1 - zero) + 0.5 * (point2 - zero) + zero

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Erin Catto @erincatto.bsky.social · Jul 11

Casey presents some optimizations that seem intuitive and they work correctly for determining separation. However, when computing distance and closest points there are some cases which should not be skipped.

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Erin Catto @erincatto.bsky.social · Jul 11

This video is great for teaching the basics of GJK. However, the approach is flawed for computing distance and only works for determining separation true/false. Unfortunately the author has disabled comments and people keep using this and ending up with broken code. www.youtube.com/watch?v=Qupq...

Implementing GJK - 2006

YouTube video by Casey Muratori

www.youtube.com

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Erin Catto @erincatto.bsky.social · Apr 20

Box2D version 3.1 is released!
Blog post: box2d.org/posts/2025/0...
Release notes: github.com/erincatto/bo...
Documentation: box2d.org/documentation/

Box2D 3.1

Version 3.0 was released about 8 months ago. It was a huge update and it needed some refinement. Version 3.1 addresses most of the bugs and build problems. It also introduces some new features and ref...

box2d.org

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Erin Catto @erincatto.bsky.social · Apr 6

Better quality here: www.youtube.com/watch?v=aiFu...

Gate Lift Physics with High Mass Ratio

YouTube video by Erin Catto

www.youtube.com

Erin Catto @erincatto.bsky.social · Apr 6

Conventional wisdom is that you need a complex and expensive direct solver to handle high mass ratios and long chains.

Box2D has an iterative SoftStep solver that uses sub-stepping and soft constraints to handle high mass ratios and long chains.

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Erin Catto @erincatto.bsky.social · Mar 29

Also recommend this for hot reloading code in VS. It works quit well these days and great for UI work.

add_compile_options("$<$<CONFIG:Debug>:/ZI>")
add_link_options("$<$<CONFIG:Debug>:/INCREMENTAL>")

Erin Catto @erincatto.bsky.social · Mar 29

tip: if you work with the generated sln you can edit the cmake files in the sln and hit f5/f7 and it will regenerate the solution in place

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Erin Catto @erincatto.bsky.social · Mar 29

I use this to avoid bringing in stuff for samples if someone is just using my library.
`if(PROJECT_IS_TOP_LEVEL)`

Erin Catto @erincatto.bsky.social · Mar 28

The nice thing about FetchContent is users can pin to a specific version of a library easily using a tag or git commit hash.

Erin Catto @erincatto.bsky.social · Mar 28

I recommend cmake these days. I don't like it either but it gets the job done and I can point users at all the cmake info out there. I made an example app with raylib and box2d using FetchContent. github.com/erincatto/bo...

GitHub - erincatto/box2d-raylib: Shows how to integrate Box2D with raylib

Shows how to integrate Box2D with raylib. Contribute to erincatto/box2d-raylib development by creating an account on GitHub.

github.com

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Erin Catto @erincatto.bsky.social · Feb 18

Nice! Do you have large island threading?

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Erin Catto @erincatto.bsky.social · Feb 13

Tagging @officialgdc.bsky.social