Threejs

Three.js Scene Graph + Export

Quick start

1. Load the module, call createScene(), then updateMatrixWorld(true).
2. Treat named THREE.Group nodes as parts/links.
3. Assign each mesh to its nearest named ancestor.
4. Bake world transforms into geometry before export.
5. Export per-part OBJ (and optionally URDF) using deterministic ordering.

Scene graph essentials

Object3D (root)
├── Group (part)
│   ├── Mesh
│   └── Group (child part)
│       └── Mesh
└── Group (another part)
    └── Mesh

• THREE.Scene / THREE.Object3D: containers
• THREE.Group: logical part (no geometry)
• THREE.Mesh: geometry + material

Part partitioning rules

• Parts are named groups at any depth.
• Each mesh belongs to the nearest named ancestor.
• Nested named groups are separate parts; do not merge their meshes into parents.
• Skip empty parts; sort part names for determinism.

function buildPartMap(root) {
  const parts = new Map();
  root.traverse((obj) => {
    if (obj.isGroup && obj.name) parts.set(obj.name, { group: obj, meshes: [] });
  });
  root.traverse((obj) => {
    if (!obj.isMesh) return;
    let parent = obj.parent;
    while (parent && !(parent.isGroup && parent.name)) parent = parent.parent;
    if (parent && parts.has(parent.name)) parts.get(parent.name).meshes.push(obj);
  });
  return Array.from(parts.values()).filter((p) => p.meshes.length > 0);
}

Mesh baking (world transforms)

Always bake transforms before export:

root.updateMatrixWorld(true);
let geom = mesh.geometry.clone();
geom.applyMatrix4(mesh.matrixWorld);
if (geom.index) geom = geom.toNonIndexed();
if (!geom.attributes.normal) geom.computeVertexNormals();

InstancedMesh expansion

const tempMatrix = new THREE.Matrix4();
const instanceMatrix = new THREE.Matrix4();
obj.getMatrixAt(i, instanceMatrix);
tempMatrix.copy(obj.matrixWorld).multiply(instanceMatrix);

Axis conversion (Y-up to Z-up)

const axisMatrix = new THREE.Matrix4().makeRotationX(-Math.PI / 2);
geom.applyMatrix4(axisMatrix);

Per-part OBJ export

• Collect meshes owned by each part.
• Do not traverse into child named groups when exporting a part.
• Merge baked geometries per part and write <part>.obj.

Reference: references/link-export-rules.md.

Relation between Three.js and URDF articulation

• Use named groups as links.
• Parent is the nearest named ancestor link.
• Joint type defaults to fixed unless evidence suggests revolute/prismatic.
• Sort link and joint names for determinism.

References:

• references/joint-type-heuristics.md
• references/urdf-minimal.md

Scripts

• InstancedMesh OBJ exporter: node scripts/export_instanced_obj.mjs
• Per-link OBJ exporter: node scripts/export_link_objs.mjs --input <scene_js> --out-dir <dir>
• URDF builder: node scripts/build_urdf_from_scene.mjs --input <scene_js> --output <file.urdf> --mesh-dir <mesh_dir>

Adjust inputs/outputs inside scripts as needed.