Threejs Ai Skill

Three.js AI Skill

AI-generated Three.js code almost always compiles — and then looks and feels broken. Models load at the wrong scale facing random directions; objects spawn at arbitrary coordinates, float, or clip through everything; the camera drifts unframed; there are never any collisions; controls are frame-rate-dependent and don't handle diagonals; and characters slide across the floor while their legs move out of sync.

The root cause is always the same: the model writes code without reasoning about the physical and spatial rules of a real 3D world. This skill encodes those rules as layers. Read the layer(s) relevant to the request; each builds on the one before.

The layer stack

1. Scene sanity (this file, §1) — scale, orientation, lighting, camera, placement, renderer setup. Every scene needs this.
2. World coherence → references/world-coherence.md — no infinite spawns, relative scale, everything on the ground, framed camera, intentional layout.
3. Collisions → references/collision-bounds.md — objects that don't pass through everything; test-before-move; when to use a physics engine.
4. Input & controls → references/input-controls.md — deltaTime, normalized diagonals, multi-key, clamped mouse-look.
5. Locomotion → references/locomotion.md — characters that walk without sliding or floating; turn toward movement; blend idle/walk/run. (Verify visually — hard to get right.)
6. Animation handling → references/animation-handling.md — play clips, retarget animation from another file, and procedural motion for models with no animation. (Procedural is experimental — always test the visual result before trusting it.)

Read references/model-normalization.md for the drop-in model-loading helpers referenced throughout.

react-three-fiber: every rule below applies unchanged — r3f is a renderer binding, not a different engine. Prefer drei equivalents where they exist (for framing, for IBL, ``), and do movement/physics work in useFrame((_, delta) => ...), never in useEffect or render.

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§1 Scene Sanity (core — always apply)

Model scale & orientation — never trust the file

GLTF/GLB/FBX files carry arbitrary units and forward axes (FBX is frequently in centimeters — 100× too big). Always normalize after load: measure with new THREE.Box3().setFromObject(model), scale uniformly so the largest dimension hits a known target, re-center the pivot, and never assume -Z is "forward" — verify orientation explicitly. Helper in references/model-normalization.md.

Camera & controls — bounded and damped

With OrbitControls, set enableDamping = true and call controls.update() each frame. Clamp minDistance/maxDistance and maxPolarAngle (so it can't go under the floor). Frame the subject: derive camera distance from the target's bounding size, don't hardcode position.set(0,0,5). Set sane near/far for the scene scale — a huge far/near ratio causes z-fighting.

Renderer & resize — the silent omissions

AI code habitually skips both, and the scene distorts on the first window resize:

renderer.setPixelRatio(Math.min(window.devicePixelRatio, 2)); // don't melt phones
addEventListener('resize', () => {
  camera.aspect = innerWidth / innerHeight;
  camera.updateProjectionMatrix();
  renderer.setSize(innerWidth, innerHeight);
});

Lighting — coherent, not a void or a blowout

Baseline: soft AmbientLight/HemisphereLight (fill) + one DirectionalLight (key, with shadows if needed) + an environment map (scene.environment) so PBR materials don't read flat/dark. Enable renderer.toneMapping = THREE.ACESFilmicToneMapping with sensible exposure. For shadows, size the directional light's shadow camera to the scene — the default frustum clips shadows in scenes larger than a few units.

Teardown

Dispose geometries, materials, and textures when a scene unmounts, or you leak GPU memory. In r3f, drei's useGLTF handles caching; manual new THREE.* resources still need disposal.

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Pre-flight checklist (before returning any scene)

• Models normalized (scale + centered + orientation checked)
• Objects on the ground, intentional layout, finite count (see world-coherence)
• World bounds defined; colliders present if things move (see collisions)
• Camera framed to subject; damping on; distance/angle clamped
• Resize handler + pixel ratio clamp present
• Ambient + key light + environment; tone mapping on
• Input frame-rate independent; diagonals normalized (see input-controls)
• Characters don't slide/float (see locomotion)
• Geometries/materials/textures disposed on teardown

Version awareness

Three.js changes fast and tutorials rot. Known breakpoints: outputEncoding became outputColorSpace in r152; physically-correct lighting became the default in r155 (useLegacyLights removed in r165), so light intensity numbers from older tutorials are wrong; import paths moved from three/examples/jsm/ to three/addons/. Follow the target version when known; otherwise write for the current API and flag anything version-sensitive instead of copying old numbers.