obj-exporter

name: obj-exporter description: Three.js OBJExporter utility for exporting 3D geometry to Wavefront OBJ format. Use when converting Three.js scenes, meshes, or geometries to OBJ files for use in other 3D software like Blender, Maya, or MeshLab.

OBJExporter Guide

Basic Structure

OBJ is a text-based 3D geometry format:

# Comment
v x y z        # Vertex position
vn x y z       # Vertex normal
f v1 v2 v3     # Face (triangle)
f v1/vt1/vn1 v2/vt2/vn2 v3/vt3/vn3  # Face with texture/normal indices

Example:

# Cube
v 0 0 0
v 1 0 0
v 1 1 0
v 0 1 0
f 1 2 3
f 1 3 4

Three.js OBJExporter

Three.js provides OBJExporter in examples:

import { OBJExporter } from 'three/examples/jsm/exporters/OBJExporter.js';

const exporter = new OBJExporter();
const objString = exporter.parse(object3D);

// Write to file (Node.js)
import fs from 'fs';
fs.writeFileSync('output.obj', objString);

Exporting with World Transforms

To export geometry in world coordinates:

// Update world matrices first
root.updateMatrixWorld(true);

// Clone and transform geometry
const worldGeometry = mesh.geometry.clone();
worldGeometry.applyMatrix4(mesh.matrixWorld);

// Create new mesh for export
const exportMesh = new THREE.Mesh(worldGeometry);
const objData = exporter.parse(exportMesh);

Merging Multiple Geometries

import { mergeGeometries } from 'three/examples/jsm/utils/BufferGeometryUtils.js';

const geometries = [];
root.traverse((obj) => {
  if (obj instanceof THREE.Mesh) {
    const geom = obj.geometry.clone();
    geom.applyMatrix4(obj.matrixWorld);
    geometries.push(geom);
  }
});

const merged = mergeGeometries(geometries);
const mergedMesh = new THREE.Mesh(merged);
const objData = exporter.parse(mergedMesh);

Node.js ES Module Setup

For running Three.js in Node.js:

// package.json
{ "type": "module" }

// script.js
import * as THREE from 'three';
import { OBJExporter } from 'three/examples/jsm/exporters/OBJExporter.js';

OBJ Export for Scene Components

Component Definition (Hierarchy-Aware)

Treat each named THREE.Group as a component/part. Preserve hierarchy by using the nearest named parent group in the ancestor chain as the owning component for each mesh. Call root.updateMatrixWorld(true) before exporting so matrixWorld is correct.

function collectComponentMeshes(root) {
    const componentMap = {};
    root.traverse(obj => {
        if (obj instanceof THREE.Group && obj.name) {
            componentMap[obj.name] = { group: obj, meshes: [] };
        }
    });

    root.traverse(obj => {
        if (obj instanceof THREE.Mesh) {
            let parent = obj.parent;
            while (parent && !(parent instanceof THREE.Group && parent.name)) {
                parent = parent.parent;
            }
            if (parent && componentMap[parent.name]) {
                componentMap[parent.name].meshes.push(obj);
            }
        }
    });

    return Object.fromEntries(
        Object.entries(componentMap).filter(([_, data]) => data.meshes.length > 0)
    );
}

Export Individual Meshes (World Transforms)

import { OBJExporter } from 'three/examples/jsm/exporters/OBJExporter.js';

const exporter = new OBJExporter();

function exportMesh(mesh, filepath) {
    let geom = mesh.geometry.clone();
    geom.applyMatrix4(mesh.matrixWorld);
    if (geom.index) {
        geom = geom.toNonIndexed();
    }
    if (!geom.attributes.normal) {
        geom.computeVertexNormals();
    }
    const tempMesh = new THREE.Mesh(geom);
    tempMesh.name = mesh.name;
    const objData = exporter.parse(tempMesh);
    fs.writeFileSync(filepath, objData);
}

Export Merged Component Meshes (Optional)

import { mergeGeometries } from 'three/examples/jsm/utils/BufferGeometryUtils.js';

function mergeMeshes(meshes) {
    const geometries = [];
    for (const mesh of meshes) {
        let geom = mesh.geometry.clone();
        geom.applyMatrix4(mesh.matrixWorld);
        if (geom.index) {
            geom = geom.toNonIndexed();
        }
        if (!geom.attributes.normal) {
            geom.computeVertexNormals();
        }
        geometries.push(geom);
    }
    if (geometries.length === 0) return null;
    return new THREE.Mesh(mergeGeometries(geometries, false));
}

Output Paths

Write OBJ files to the paths specified by the task instructions or a provided output root variable. Avoid hardcoding fixed directories in the skill itself.