---
name: threejs-viewer
description: >
  Domain knowledge for building the hmwebviewer Three.js 3D viewer. Load when working
  on src/viewer, src/dnd, src/ui, the asset pipeline, or hydration. Covers the locked
  technical decisions, loader setup, Draco/KTX2, Blob URL lifecycle, and SSR hydration
  patterns specific to this project.
---

# hmwebviewer — Three.js 3D viewer domain guide

Source spec: `3d_viewer_architecture_spec.pdf`. Always also read `CLAUDE.md` + `PLAN.md`.

## Locked stack
- Three.js + Vite + TypeScript.
- Loaders: `GLTFLoader` + `DRACOLoader` + `KTX2Loader`.
- Compression: Draco (geometry) + KTX2/Basis Universal (textures).

## Critical patterns (do not deviate without approval)

### Single shared loader instances
Multiple simultaneous `DRACOLoader` instances crash (three.js #22445). KTX2 same risk. Build a singleton:

```ts
// src/viewer/loaders.ts
import * as THREE from 'three';
import { GLTFLoader } from 'three/examples/jsm/loaders/GLTFLoader.js';
import { DRACOLoader } from 'three/examples/jsm/loaders/DRACOLoader.js';
import { KTX2Loader } from 'three/examples/jsm/loaders/KTX2Loader.js';

let _gltf: GLTFLoader | null = null;
export function getLoaders(renderer: THREE.WebGLRenderer) {
  if (!_gltf) {
    const draco = new DRACOLoader().setDecoderPath('/draco/');
    const ktx2  = new KTX2Loader().setTranscoderPath('/basis/').detectSupport(renderer);
    _gltf = new GLTFLoader().setDRACOLoader(draco).setKTX2Loader(ktx2);
  }
  return _gltf;
}
```

Pin decoder WASM version to the installed three.js version. Mismatch → silent decode failures.

### Local file load (Blob URL lifecycle)
```ts
const url = URL.createObjectURL(file);
loader.load(url, (gltf) => {
  scene.add(gltf.scene);
  URL.revokeObjectURL(url);          // success → revoke
}, undefined, (err) => {
  URL.revokeObjectURL(url);          // error → also revoke
  throw err;
});
```
For repeat/cached loads: `FileReader` → `ArrayBuffer` → `GLTFLoader.parse()` enables IndexedDB caching and avoids URL overhead.

### Server asset (SSR + CSR + hydration)
1. SSR ships HTML/CSS skeleton + pre-rendered 360° animated WebP placeholder.
2. CSR background: `GLTFLoader.load(serverUrl, onLoad, onProgress)`.
3. Hydration: gate the fade on BOTH `webglReady` AND `modelLoaded` (Promise.all). Fade CSS `opacity 0.5s ease` → `display:none` after 500ms. Race → empty canvas flash.

### Drag & drop
```ts
dropzone.addEventListener('dragover', (e) => { e.preventDefault(); });
dropzone.addEventListener('drop', (e) => {
  e.preventDefault();
  const file = e.dataTransfer?.files?.[0];
  if (!file) return;
  if (!/\.gl[bt]f$/i.test(file.name)) return showError('GLB/GLTF only');
  loadLocalFile(file);   // viewer-core, handles createObjectURL/revoke
});
```
Also wire click→`<input type=file">` for accessibility.

### Disposal
On teardown / model swap: `geometry.dispose()`, `material.dispose()` (and material maps), `texture.dispose()`. Revoke any lingering object URLs.

## Offline asset pipeline
- Compress: `gltf-transform` CLI or programmatic functions — Draco + KTX2 in one pass.
- KTX2 encoder binary `toktx` must be on PATH (gltf-transform fetches via `@ktx2/basis-transcoder`/platform binaries — confirm available).
- Pre-render 360°: Blender headless `blender -b scene.blend -o //frame_### -f 1..N -F PNG` → ffmpeg → animated WebP. Fallback: Puppeteer + headless three.js screenshot per rotation.

## Common pitfalls
- `setDecoderPath` wrong → worker fetch 404 in console. Verify `/draco/` resolves under `public/`.
- Forgetting `e.preventDefault()` on dragover → browser opens the file.
- Decoder version mismatch → model fails to decode with no obvious error.
- Creating loaders per load → intermittent crashes + memory growth.

References in user memory `reference-threejs-resources.md`.