chitin

Open-source physics asset compiler for scanned, generated, splat, and rigged 3D assets.

Chitin is a free MIT-licensed compiler that bridges the gap between visual capture (gaussian splats, photogrammetry, LiDAR) and physics simulation. Feed it a point cloud, mesh, or skinned model and get back portable convex hulls that any engine can load. The primary output is the .phys binary sidecar -- a compact format with readers for Python, TypeScript, C#, and C++.

It is not a splat viewer feature or a single-engine import button. Viewer collision tools are great for making one splat scene walkable; Chitin's job is to turn messy 3D assets into deterministic, validated physics artifacts that can ship through web, engine, simulation, and CI pipelines.

Why Chitin

• Free MIT infrastructure: audit it, vendor it, modify it, or run it offline without service lock-in.
• Portable artifact contract: .phys stores quantized convex hulls, bind transforms, and collision LOD tiers instead of engine-owned caches or viewer-only collision data.
• Attachable to any viewer: load a .phys sidecar next to a splat, mesh, or generated scene and feed the hulls into your runtime physics API.
• Broader input surface: splats, point clouds, static meshes, USD assets, and experimental rigged GLB support.
• Thin runtime readers: Python, TypeScript, C#, and C++ consumers load the same binary format while the heavy reconstruction/decomposition work stays in the compiler.
• Pipeline-friendly checks: chitin check, inspect, and validate make collision generation scriptable and reviewable.

Use cases

• Gaussian splat scenes: extract collision geometry from PLY point clouds with opacity filtering
• Robotics simulation: generate colliders for scanned environments (Isaac Sim, Gazebo, MuJoCo)
• Web/XR: load .phys sidecars in the browser alongside your 3D viewer (Three.js + Rapier)
• Game engines: Unity's ScriptedImporter turns .phys into MeshColliders on drop; the Unreal plugin imports the hull data as an asset
• Rigged characters: per-bone convex hulls in bone-local space, ready for ragdoll or hit detection

Compared with viewer collision

Splat viewer pipelines usually voxelize a gaussian splat, fill or carve navigable space, and feed that occupancy data to a specific viewer/runtime. That is the right shape for immediate walk mode.

Chitin reconstructs surfaces and decomposes them into convex hulls. That is the right shape when the output needs to become a reusable physics asset: versioned, validated, loadable by multiple engines, and independent of the original viewer. A splat viewer can keep its visual format and load Chitin collision as a sidecar.

Ecosystem

Chitin is the asset layer: a compiler that emits .phys and nothing more. It stays independent, but is designed to sit under a scene-level registry — the layer that owns coordinate frames, stable object identity, provenance, and layout, and that references each asset's .phys sidecar rather than producing physics itself.

Visual-only viewers (mesh or gaussian-splat) render read-only geometry; Chitin colliders fill the collision/raycast gap via the Three.js + Rapier reader (@autarkis/chitin-web).

Integration is by artifact (.phys) and reference type, never by package import.

Install

pip install chitin              # mesh extraction (OBJ, GLB, STL)
pip install chitin[splat]       # + point cloud / gaussian splat extraction
pip install chitin[usd]         # + USD Physics output
pip install chitin[service]     # + local build service
pip install chitin[all]         # everything

The base install handles mesh inputs with just trimesh + CoACD. The [splat] extra adds Open3D for Poisson surface reconstruction from point clouds and gaussian splats.

Requires Python 3.12. (chitin[splat] requires open3d, which does not yet have a 3.13 wheel. The base install works on 3.13+.)

Browser path

For manifold meshes (OBJ, GLB, STL from standard modeling tools), you can skip Python entirely and run decomposition in the browser:

npm install @autarkis/chitin-lite

This drives CoACD compiled to WebAssembly (558KB) in the browser and writes the same .phys format the Python compiler produces. The npm package ships the JavaScript/TypeScript wrapper only; you supply the CoACD WASM module — build it from integrations/wasm/ or host the prebuilt coacd.js + coacd.wasm — and point the loader at it. See integrations/wasm-lite/ for usage.

Use chitin check <file> to see which path a given input needs:

$ chitin check model.glb
file:       model.glb
format:     glb
vertices:   12,847
faces:      25,102
manifold:   yes
path:       either
  server:   pip install chitin
  browser:  npm install @autarkis/chitin-lite
reason:     manifold mesh, eligible for browser-side decomposition

Point clouds, gaussian splats, and non-manifold meshes require the Python pipeline.

CLI

# extract colliders from a splat point cloud
chitin extract scene.ply -o scene.phys --opacity-threshold 0.5

# extract from a mesh
chitin extract model.obj -o colliders.phys --concavity 0.05

# environment scan (room, cave, outdoor scene)
chitin extract room.ply -o room.phys --density-quantile 0.3 --proximity-filter 5.0 --thin-shell

# multi-LOD: generate tiers at different concavity thresholds
chitin extract model.obj -o colliders.phys --concavity 0.05 --lod-concavities 0.1,0.3,0.5

# inspect a .phys file (shows LOD tiers if present)
chitin inspect colliders.phys

# validate binary integrity
chitin validate colliders.phys

Library

from chitin import extract, Config

config = Config(concavity=0.05, opacity_threshold=0.5)
result = extract("scene.ply", config)

result.to_phys("colliders.phys")   # primary output
result.to_json("colliders.json")   # debug companion
result.to_usd("colliders.usda")    # USD Physics (Isaac Sim, Omniverse)

# multi-LOD output (v3 .phys with tiered collision hulls)
config = Config(concavity=0.05, lod_concavities=[0.1, 0.3, 0.5])
result = extract("model.obj", config)
result.to_phys("colliders.phys")   # LOD 0 at 0.05, then tiers at 0.1, 0.3, 0.5

From numpy arrays

import numpy as np
from chitin import extract_from_arrays, Config

positions = np.random.randn(10000, 3).astype(np.float32)
result = extract_from_arrays(positions, config=Config())

Read .phys back

from chitin import read_phys, validate_phys

phys = read_phys("colliders.phys")
for hull in phys.hulls:              # LOD 0 (highest detail)
    print(hull.vertices.shape, hull.indices.shape)

# LOD tiers (if present)
for tier in phys.lod_tiers:
    print(f"concavity={tier.concavity}: {tier.hull_count} hulls")

# pick the tier closest to a target concavity
coarse = phys.lod_tier(0.3)

issues = validate_phys("colliders.phys")

.phys format

The .phys binary sidecar is the primary output. It stores quantized convex hulls with optional per-bone bind transforms and collision LOD tiers in a single file that loads in microseconds. Full spec in docs/phys.md.

Collision LOD

A single decomposition forces a tradeoff between fidelity and cost. Multi-LOD solves this: the producer generates tiers at different concavity thresholds, the consumer picks based on distance, platform budget, or simulation context. LOD 0 is always the highest-detail decomposition. Additional tiers are coarser and cheaper. v2 readers open a v3 file and get LOD 0 without changes.

Runtime tier selection by nearest concavity is available in every reader: phys.lod_tier(...) (Python), selectLodHulls(phys, ...) / createColliders(..., { lodConcavity }) (TypeScript), PhysAsset.SelectLod(...) (Unity), and UChitinPhysAsset::SelectLod(...) (Unreal). Readers with no target pick default to LOD 0.

Format	Extension	Use
Binary sidecar	.phys	Web, native engines, Rapier, custom loaders
JSON	.json	Debug companion, lightweight consumers
USD Physics	.usda	Isaac Sim, Omniverse, Kit-based tools

Engine integrations

All integrations read the same .phys binary with identical dequantization.

Engine	Package	Path
Web (Three.js + Rapier)	@autarkis/chitin-web	integrations/web/
Unity	com.chitin.physics	integrations/unity/
Unreal Engine	ChitinImporter plugin	integrations/unreal/

Web: Three.js + Rapier

import RAPIER from "@dimforge/rapier3d-compat";
import { parsePhys } from "@autarkis/chitin-web";          // format only, no deps
import { addToWorld } from "@autarkis/chitin-web/rapier";  // Rapier bindings

const buffer = await fetch("/assets/scene.phys").then((r) => r.arrayBuffer());
const phys = parsePhys(buffer);
addToWorld(RAPIER, world, phys);

The package root (@autarkis/chitin-web) is dependency-free — just the .phys parser and validator. The Rapier collider bindings live at @autarkis/chitin-web/rapier and the Three.js debug meshes at @autarkis/chitin-web/three, so a format-only consumer never pulls in Three or Rapier.

A full working example with capsule walk controller and Playwright tests lives in integrations/walktest/. See docs/usage.md for the end-to-end walkthrough.

Unity: drag-and-drop import

Drop a .phys file into your Assets folder. The ScriptedImporter creates a GameObject hierarchy with convex MeshColliders -- no code required. For rigged assets, hulls are grouped under bone-named parents.

Install via Package Manager: "Add package from disk" -> integrations/unity/package.json. See docs/usage.md for runtime loading code.

Unreal: asset import

The ChitinImporter plugin imports a .phys into a UChitinPhysAsset (dequantized hulls + bind poses) exposed to Blueprints. It does not yet build UBodySetup collision for you -- read the hull vertices/indices from the asset and create the collision bodies (FKConvexElem / UBodySetup) in C++ or Blueprint.

Other engines

Use parsePhys(buffer) (TypeScript) or PhysReader.Read(data) (C#/C++) directly and pass each hull's vertices/indices to the engine's convex-collider API.

Local build service

A single-machine build server with content-addressed caching. Jobs run synchronously in the current process -- suitable for local/CI use, not production.

pip install chitin[service]
chitin-server serve --port 8400
chitin-server submit model.glb
chitin-server download <job_id> -o ./output

How it works

1. Loads input (PLY, OBJ, STL, GLB, USD, or raw arrays)
2. Filters by opacity for gaussian splat point clouds
3. Derives oriented normals from gaussian covariance (scale + rotation) when available, falls back to KD-tree estimation
4. Auto-detects environment scans (hollow-shell point distributions) and enables thin-shell + proximity filter. Use --no-auto-environment to disable
5. Partitions large scenes into octree cells (threshold: 50K points) with ghost-zone overlap for boundary continuity
6. Optionally inflates gaussian centers into disk samples along their two largest axes for better surface coverage
7. Reconstructs surface mesh via Poisson reconstruction (Open3D), with auto-selected depth per cell and subprocess crash isolation
8. For environment scans: proximity-filters closure surfaces and optionally extrudes a thin shell to prevent interior volume fill
9. PCA-based flatness detection replaces near-flat octree cells with oriented boxes instead of running CoACD
10. Decomposes remaining cells into convex hulls (CoACD)
11. Seam repair: detects height discontinuities at octree cell boundaries, merges affected cells, and re-extracts for seamless coverage
12. Deduplicates cross-cell hulls by AABB IOU
13. If lod_concavities is set, runs additional decompositions at each threshold to produce LOD tiers
14. For rigged GLTF assets (experimental): reads joint weights directly from GLB binary, segments by dominant bone, generates per-bone hulls in bone-local space

Limitations

• Environment scan auto-detection can misfire. Chitin auto-detects hollow-shell point distributions and enables --thin-shell and --proximity-filter automatically. Use --no-auto-environment to disable if the heuristic is wrong for your scene.
• Rigged GLTF support is experimental. Skinning is read directly from GLB binary (trimesh drops these attributes). Currently supports single-primitive meshes. Interleaved byteStride is handled, but multiple primitives and vertex reordering may produce incorrect bone segmentation.
• Flat surfaces over-decompose (mitigated). A PCA-based flatness detector (--flatness-threshold, default 0.9) replaces near-flat octree cells with oriented boxes instead of running CoACD. On the Mip-NeRF 360 Garden scene this reduced hulls from 1,725 to 579 and build time from 27 min to 9 min. Scenes with unusual ground geometry may need threshold tuning or --flatness-threshold 0 to disable.
• No sparse voxel collision output yet. Chitin currently emits convex hull artifacts, not viewer-native SVO/voxel grids for walk-mode raycasts.
• Python 3.12 only until open3d ships a 3.13 wheel.
• FBX needs Blender. trimesh has no FBX loader, so chitin extract model.fbx auto-converts the file to GLB via headless Blender (Blender must be on PATH) and extracts from that. chitin convert runs the same step explicitly. Without Blender, FBX extraction raises a clear error.
• No physics material metadata. Input formats (USD, GLTF) may carry material properties (friction, density, restitution) that chitin does not propagate to the output. Consumers must assign material properties manually.
• Rigged runtime helpers are Python/data only. The web addToWorld() helper places rigged hulls on a single body without applying per-bone bind transforms. For rigged runtime placement, read the per-hull boneIndex and the bone bind transforms and position the hulls yourself.
• Target normalization does not rescale splat covariance. target_height / target_footprint scale point and mesh positions uniformly, but gaussian-splat covariance (scale) is left unchanged, so splat inflation and ghost-zone radii stay in the source scale. Normalize splats at the source if metric splat sizing matters.

License

MIT