threejs-lights

## When to use this skill

**ALWAYS use this skill when the user mentions:**

- Enabling `castShadow` / `receiveShadow`, shadow map size, bias, normal bias, camera frusta for shadow casters
- Physical lights: intensity, distance/decay, angle/penumbra for spots, rect area setup
- `LightProbe` for irradiance-style probes; `IESSpotLight` with IES data

**IMPORTANT: lights vs loaders vs node-tsl**

| Topic | Skill |
|-------|--------|
| Light classes and shadows | **threejs-lights** |
| Loading IES/HDR files | **threejs-loaders** |
| LightNode / TSL lighting | **threejs-node-tsl** |

**Trigger phrases include:**

- "DirectionalLight", "SpotLight", "RectAreaLight", "castShadow", "shadow map"
- "阴影", "点光源", "面光源", "LightProbe"

## How to use this skill

1. **Base recipe**: ambient/hemisphere fill + directional sun + local points/spots.
2. **Shadows**: enable on renderer, mark casters/receivers, tune map size vs performance, adjust bias to remove acne/peter-panning.
3. **RectArea**: initialize addon libs per docs page before using light type.
4. **Probes**: place probes; generate data via addon generator when applicable; relate to materials env reflections (**threejs-materials**, **threejs-textures**).
5. **IES**: load profile via loader skill, attach to `IESSpotLight` per docs.
6. **Performance**: limit shadow-casting lights; use layers (**threejs-objects**) to exclude objects.

### Example: Shadow setup with validation

```javascript
// 1. Enable shadows on renderer
renderer.shadowMap.enabled = true;

// 2. Configure light as shadow caster
const light = new THREE.DirectionalLight(0xffffff, 1);
light.castShadow = true;
light.shadow.mapSize.set(2048, 2048);
light.shadow.bias = -0.0001; // Adjust to remove shadow acne

// 3. Validate: verify shadow appears before tuning bias
// Set castShadow on meshes, receiveShadow on ground
mesh.castShadow = true;
ground.receiveShadow = true;
```

See [examples/workflow-directional-shadow.md](examples/workflow-directional-shadow.md).

## Doc map (official)

| Docs section | Representative links |
|--------------|----------------------|
| Lights | https://threejs.org/docs/Light.html |
| Directional | https://threejs.org/docs/DirectionalLight.html |
| Spot | https://threejs.org/docs/SpotLight.html |
| Rect area | https://threejs.org/docs/RectAreaLight.html |

## Scope

- **In scope:** Core lights, shadow maps, probes, listed addons for rect area and probe generation.
- **Out of scope:** CSM deep theory (see addon Csm docs if user names it); baked lightmaps in DCC.

## Common pitfalls and best practices

- Shadow map resolution must match scene scale—tiny shadows on huge worlds look blocky.
- Point light shadows are six-face expensive—use wisely.
- `RectAreaLight` without required libs yields black or wrong shading—verify init.
- Mismatched physical units (intensity vs exposure) with **threejs-renderers** tone mapping causes blown or dim scenes.
- Shadow **bias** / **normalBias** trade-offs: acne vs peter-paning—tune with helper frusta (**threejs-helpers**).

## Documentation and version

Light and shadow classes live under [Lights](https://threejs.org/docs/#Lights) in [three.js docs](https://threejs.org/docs/). `RectAreaLight` and probe addons depend on extra init from **Addons → Lights**; IES profiles require **threejs-loaders** for file fetch before the light API is usable.

## Agent response checklist

When answering under this skill, prefer responses that:

1. Link the concrete light type (`DirectionalLight`, `SpotLight`, …) and shadow pages when shadows are on.
2. Separate **IES loading** to **threejs-loaders** and **LightNode** topics to **threejs-node-tsl**.
3. Give practical shadow map size / bias guidance with **threejs-helpers** for frustum visualization.
4. Mention `renderer.shadowMap.enabled` alongside light `castShadow` (see **threejs-renderers**).
5. Note performance cost of multiple shadow-casting lights.

## References

- https://threejs.org/docs/#Lights
- https://threejs.org/docs/DirectionalLight.html
- https://threejs.org/docs/SpotLight.html
- https://threejs.org/docs/LightShadow.html

## Keywords

**English:** lights, shadows, directional, spotlight, rectarea, lightprobe, three.js

**中文：** 灯光、阴影、平行光、聚光灯、面光源、LightProbe、three.js