three.js glTF Loading

Load .gltf/.glb models and play their animations in three.js, including
compressed geometry (DRACO/Meshopt) and textures (KTX2). Patterns target
r165+, verified against r184.

When to use

• Use to import a 3D model, add it to the scene, inspect its node hierarchy, and
  play baked/skinned animation clips with an AnimationMixer.
• Use when files are .gltf/.glb, or code imports GLTFLoader /
  DRACOLoader / KTX2Loader from three/addons/loaders/....

When not to use: creating the renderer/camera/loop → threejs-scene-setup.
Tuning surface look, lights, or shadows on the loaded model →
threejs-materials-lighting. Authoring/exporting the model itself (Blender) is out
of scope; prefer glTF over OBJ/FBX for runtime.

Core workflow

1. Why glTF. It's a transmission format: binary vertex data, PBR materials, and
   animations are ready to render with minimal parsing. Prefer it over OBJ (no scene
   graph, no animation) and FBX (heavy) for the web.
2. Load with GLTFLoader. loader.load(url, onLoad, onProgress, onError). The
   result gltf has gltf.scene (the Object3D root), gltf.animations
   (AnimationClip[]), gltf.cameras, and gltf.asset.
3. Add gltf.scene to your scene and frame it. Inspect the hierarchy with
   traverse / getObjectByName to find the parts you'll control.
4. Play animations with an AnimationMixer. One mixer per animated root;
   mixer.clipAction(clip).play(); advance with mixer.update(delta) every frame.
5. Decode compressed assets. Attach a DRACOLoader (and/or KTX2Loader +
   Meshopt) so DRACO meshes and KTX2 textures load; point the decoders at their
   files.
6. Verify what loaded — log the scene graph and gltf.animations, and confirm
   the model is visible (right scale, lit) and the clip actually plays.

Patterns

1. Load a model and frame it

import { GLTFLoader } from 'three/addons/loaders/GLTFLoader.js';

const loader = new GLTFLoader();
loader.load(
  'assets/robot.glb',
  (gltf) => {
    const root = gltf.scene;
    scene.add(root);
    // Inspect: gltf.animations is an array of AnimationClip.
    console.log('clips:', gltf.animations.map((c) => c.name));
  },
  (event) => console.log(`${(event.loaded / event.total) * 100}% loaded`),
  (error) => console.error('glTF load failed:', error)
);

2. Play a skinned animation with AnimationMixer

import * as THREE from 'three';
import { GLTFLoader } from 'three/addons/loaders/GLTFLoader.js';

let mixer;                                  // declare outside so the loop can see it
const clock = new THREE.Clock();

new GLTFLoader().load('assets/character.glb', (gltf) => {
  scene.add(gltf.scene);
  mixer = new THREE.AnimationMixer(gltf.scene);          // one mixer per animated root
  const clip = THREE.AnimationClip.findByName(gltf.animations, 'Run')
            ?? gltf.animations[0];
  mixer.clipAction(clip).play();
});

renderer.setAnimationLoop(() => {
  const dt = clock.getDelta();
  if (mixer) mixer.update(dt);              // advance the animation by real seconds
  renderer.render(scene, camera);
});

3. Cross-fade between two clips

const actions = {};
mixer = new THREE.AnimationMixer(gltf.scene);
for (const clip of gltf.animations) {
  actions[clip.name] = mixer.clipAction(clip);
}
actions['Idle'].play();

function transitionTo(name, duration = 0.3) {
  const next = actions[name];
  next.reset().play();
  for (const [n, action] of Object.entries(actions)) {
    if (n !== name) action.crossFadeTo(next, duration, false);
  }
}

4. DRACO-compressed geometry

import { GLTFLoader } from 'three/addons/loaders/GLTFLoader.js';
import { DRACOLoader } from 'three/addons/loaders/DRACOLoader.js';

const draco = new DRACOLoader();
// Point at the decoder files you ship (or a pinned CDN copy of the same version).
draco.setDecoderPath('https://cdn.jsdelivr.net/npm/three@0.184.0/examples/jsm/libs/draco/');

const loader = new GLTFLoader();
loader.setDRACOLoader(draco);
loader.load('assets/city-draco.glb', (gltf) => scene.add(gltf.scene));

5. Find and animate a named part

new GLTFLoader().load('assets/car.glb', (gltf) => {
  scene.add(gltf.scene);
  const wheels = [];
  gltf.scene.traverse((node) => {
    if (node.name.startsWith('Wheel')) wheels.push(node);
  });
  renderer.setAnimationLoop(() => {
    const dt = clock.getDelta();
    for (const w of wheels) w.rotation.x += dt * 4;
    renderer.render(scene, camera);
  });
});

Pitfalls

• Model loads but is invisible → it has lit (PBR) materials and the scene has no
  light or environment. Add a light or scene.environment (see
  threejs-materials-lighting), and check scale — glTF is in metres, so a 0.01-scaled
  asset is tiny.
• load is async → gltf only exists inside the callback; declare mixer/refs
  outside and assign them in the callback, or use await loader.loadAsync(url).
• Animation never moves → you didn't call mixer.update(delta) each frame, or you
  passed milliseconds instead of seconds (use clock.getDelta()), or you forgot
  action.play().
• DRACO/KTX2 model fails → the decoder/transcoder path is wrong or version-
  mismatched. setDecoderPath/setTranscoderPath must point at files matching your
  three.js version.
• Multiple mixers fighting → use one AnimationMixer per animated root and
  create all actions from it; don't make a new mixer per clip.
• Baked-in transforms surprise you → exporters sometimes bake scale/rotation onto
  child nodes. Dump the hierarchy (names + position/rotation/scale) before relying on
  a node's local transform; re-export from the source if the rig is unusable.
• Origins are off → re-parent a part under a fresh Object3D to give it a clean
  pivot rather than fighting baked offsets.

References

• For the full decode/transcode setup (DRACO + Meshopt + KTX2 together), loadAsync
• a LoadingManager progress bar, reusing models with SkeletonUtils.clone, and
  exporter guidance (apply transforms, one clean root), read
  references/loaders-and-animation.md.

Related skills

• threejs-scene-setup — the renderer, camera, and loop this model renders into.
• threejs-materials-lighting — lighting/environment so PBR models look right.
• fps-shooter — a 3D genre that composes three.js skills.