2nd/10_Wiki/Topics/AI_and_ML/20k skinned instances demo.md

---
id: wiki-2026-0508-20k-skinned-instances-demo
title: 20k Skinned Instances Demo (Three.js)
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [InstancedMesh2 demo, large skinned mesh, three.js performance demo, GPU skinning]
duplicate_of: none
source_trust_level: B
confidence_score: 0.85
verification_status: applied
tags: [three-js, webgl, performance, skinned-mesh, gpu-skinning, lod, frustum-culling, instancing]
raw_sources: []
last_reinforced: 2026-05-09
github_commit: pending
tech_stack:
  language: TypeScript
  framework: Three.js / WebGL
---

# 20k Skinned Instances Demo

## 📌 한 줄 통찰
> **20,000 character 의 same scene + 5 draw call only**. agargaro 의 InstancedMesh2 + frustum culling + LOD + animation throttle. **Mobile 도 3000+ instance 60fps**.

## 📖 핵심

### 매 demo 의 capability
- **Desktop**: 20,000 skinned instance @ 60 FPS.
- **Mobile**: 3,000 instance @ 60 FPS.
- **Draw calls**: 5 only (despite 20k unit).
- **GPU**: integrated 도 OK.

→ 매 traditional 의 100-1000 instance 의 limit 의 break.

### 매 optimization technique

#### 1. Frustum culling
- 매 camera 의 outside instance 의 skip.
- 매 bone update 의 only visible.

#### 2. Distance-based animation
- Near: 60 FPS bone update.
- Mid: 30 FPS.
- Far: 10 FPS.
- Very far: 0 (static pose).

→ 매 update cost 의 80% saving.

#### 3. Multi-LOD
- LOD 0: 매 detailed mesh + bone.
- LOD 1: 매 simplified mesh.
- LOD 2: 매 imposter (billboard).

→ 매 distance 의 different polygon count.

#### 4. GPU skinning
- 매 bone matrix 의 texture 저장.
- 매 vertex shader 의 calculate.
- CPU 의 free.

#### 5. Single material / atlas
- 매 instance 의 same material.
- 매 atlas texture (1 texture, multiple variation).

#### 6. Indirect rendering
- GPU 의 매 instance 의 visibility 결정.
- CPU → GPU transfer 최소화.

### 매 architecture

```
Scene
└── InstancedMesh2 (1)
    ├── Geometry: skinned mesh (1)
    ├── Material: shared (1)
    ├── BoneTexture: 매 instance 의 bone matrix (RGBA float)
    └── Per-instance:
        ├── Position
        ├── Rotation
        ├── Scale
        ├── Color (optional)
        └── Animation state (frame, speed)
```

→ 1 InstancedMesh2 = 1 draw call.

### 매 use case

#### Game (RTS / open world)
- 매 1000+ unit (StarCraft 식).
- 매 crowd (city, stadium).

#### Visualization
- 매 large dataset (data point 의 character).
- 매 scientific (molecule, particle).

#### Simulation
- 매 swarm.
- 매 evacuation.
- 매 pedestrian.

#### Web 3D
- 매 metaverse-style.
- 매 large social space.

## 💻 Code

### Setup
```typescript
import { InstancedMesh2 } from '@three.ez/instanced-mesh';
import * as THREE from 'three';

// Load skinned mesh
const loader = new GLTFLoader();
const gltf = await loader.loadAsync('character.glb');
const skinnedMesh = gltf.scene.children[0] as THREE.SkinnedMesh;

// Create InstancedMesh2
const instancedMesh = new InstancedMesh2(
  skinnedMesh.geometry,
  skinnedMesh.material,
  {
    capacity: 20000,
    createEntities: true,
    skinned: true,   // GPU skinning
  }
);

// Add instances
for (let i = 0; i < 20000; i++) {
  instancedMesh.addInstances(1, (instance) => {
    instance.position.set(
      (Math.random() - 0.5) * 1000,
      0,
      (Math.random() - 0.5) * 1000
    );
    instance.rotation.y = Math.random() * Math.PI * 2;
  });
}

scene.add(instancedMesh);
```

### Frustum culling + distance animation
```typescript
function animate() {
  const cameraPos = camera.position;
  
  instancedMesh.instances.forEach((instance, i) => {
    const dist = instance.position.distanceTo(cameraPos);
    
    // Distance-based update rate
    if (dist < 50) {
      instance.updateAnimation(deltaTime);   // 60 FPS
    } else if (dist < 200) {
      if (frame % 2 === 0) instance.updateAnimation(deltaTime * 2);   // 30 FPS
    } else if (dist < 500) {
      if (frame % 6 === 0) instance.updateAnimation(deltaTime * 6);   // 10 FPS
    }
    // > 500: no animation update (static pose)
  });
  
  // Auto frustum culling
  instancedMesh.performFrustumCulling(camera);
  
  renderer.render(scene, camera);
}
```

### Multi-LOD
```typescript
const lod0 = new InstancedMesh2(highPolyGeo, mat, { capacity: 5000, skinned: true });
const lod1 = new InstancedMesh2(midPolyGeo, mat, { capacity: 10000, skinned: true });
const lod2 = new InstancedMesh2(impostorGeo, mat, { capacity: 5000 });

function updateLOD(instances) {
  instances.forEach((inst, i) => {
    const d = inst.distanceToCamera(camera);
    if (d < 50) inst.assignTo(lod0);
    else if (d < 200) inst.assignTo(lod1);
    else inst.assignTo(lod2);
  });
}
```

### GPU skinning (custom shader)
```glsl
// Vertex shader
attribute vec4 skinIndices;
attribute vec4 skinWeights;

uniform sampler2D boneTexture;   // 매 instance 의 bone matrix
uniform float boneTextureSize;

mat4 getBoneMatrix(float index, float instanceIndex) {
  float u = (index * 4.0 + 0.5) / boneTextureSize;
  float v = (instanceIndex + 0.5) / boneTextureSize;
  return mat4(
    texture2D(boneTexture, vec2(u, v)),
    texture2D(boneTexture, vec2(u + 1.0/boneTextureSize, v)),
    texture2D(boneTexture, vec2(u + 2.0/boneTextureSize, v)),
    texture2D(boneTexture, vec2(u + 3.0/boneTextureSize, v))
  );
}

void main() {
  mat4 boneMat = 
    getBoneMatrix(skinIndices.x, gl_InstanceID) * skinWeights.x +
    getBoneMatrix(skinIndices.y, gl_InstanceID) * skinWeights.y +
    getBoneMatrix(skinIndices.z, gl_InstanceID) * skinWeights.z +
    getBoneMatrix(skinIndices.w, gl_InstanceID) * skinWeights.w;
  
  vec4 transformed = boneMat * vec4(position, 1.0);
  gl_Position = projectionMatrix * modelViewMatrix * transformed;
}
```

→ 매 vertex 의 GPU 가 calculate.

### Performance metric
```typescript
const stats = new Stats();
document.body.appendChild(stats.dom);

function animate() {
  stats.begin();
  
  // ... render
  
  stats.end();
  requestAnimationFrame(animate);
}

console.log({
  draws: renderer.info.render.calls,
  triangles: renderer.info.render.triangles,
  geometries: renderer.info.memory.geometries,
});
// Goal: draws < 10, FPS = 60
```

## 🤔 결정 기준

| Instance count | Approach |
|---|---|
| < 100 | Native skinned mesh (each its own) |
| 100-1000 | InstancedMesh2 + frustum cull |
| 1000-10000 | + LOD + distance animation |
| 10000+ | + GPU skinning + impostor |
| Mobile | 3000 max + heavy LOD |

**기본값**: InstancedMesh2 + 5 optimization (frustum, LOD, animation throttle, GPU skin, atlas).

## 🔗 Graph
- 부모: [[Three-js-Performance]] · [[SkinnedMesh]] · [[Instancing]]
- 변형: [[BatchedMesh]]
- 응용: [[Crowd-Simulation]]
- 기술: [[GPU-Skinning]] · [[Frustum Culling]] · [[Level-of-Detail]]

## 🤖 LLM 활용
**언제**: 매 large character scene 의 design. 매 mobile / web 3D 의 performance.
**언제 X**: 매 small scene. 매 specific Unity / Unreal (different).

## ❌ 안티패턴
- **Native skinned + 1000 instance**: 30 FPS.
- **No LOD + variable distance**: GPU waste.
- **Bone update 60 FPS 매 instance**: CPU 의 bottleneck.
- **No frustum cull**: hidden update.
- **Multiple material per instance**: 매 draw call 의 multiply.

## 🧪 검증 / 중복
- Applied (agargaro 의 demo).
- 신뢰도 B (GitHub repo, real demo).
- Related: [[agargaro-libraries]] · [[Three-js-Performance]].

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-09 | Manual cleanup — 5 optimization + Three.js code + GPU skinning shader |