---
id: wiki-2026-0508-geometry-merging
title: Geometry Merging
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Mesh Merging, Static Batching, Geometry Batching]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [graphics, optimization, rendering, gpu]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: C++/HLSL
  framework: Unity/Unreal/Three.js
---

# Geometry Merging

## 매 한 줄
> **"매 여러 mesh 를 매 단일 vertex/index buffer 로 합쳐 매 draw call 수를 매 줄이는 기법"**. CPU-GPU command overhead 의 매 frame budget 의 매 dominant share 였던 시대의 매 staple optimization. 매 modern era — GPU instancing, indirect draw, mesh shader 가 매 알 수 있게 대체했지만 매 static scene, mobile, low-end 에서 매 still relevant.

## 매 핵심

### 매 종류
- **Static batching**: build-time 에 같은 material 의 static mesh 합침.
- **Dynamic batching**: runtime 에 small mesh 를 transform & merge (CPU cost ↑).
- **GPU instancing**: 같은 mesh 여러 transform — merging 의 modern 대체.
- **Mesh atlas (texture array)**: material 통합으로 merge 가능 범위 확장.

### 매 trade-off
- ↑ Throughput (fewer draw call).
- ↓ Culling 정확도 (merged AABB 가 매 커짐).
- ↑ Memory (per-vertex data 의 매 duplication).
- ↓ Animation flexibility (static 한정).

### 매 응용
1. Mobile 게임 — draw call 의 매 hard limit (~100-200).
2. Architectural visualization — 매 thousands of small props.
3. Tile-based world streaming.
4. UI batching (text glyph atlas).

## 💻 패턴

### Manual merge (Three.js)
```javascript
import { mergeGeometries } from 'three/examples/jsm/utils/BufferGeometryUtils.js';
const geos = props.map(p => p.geometry.clone().applyMatrix4(p.matrixWorld));
const merged = mergeGeometries(geos, false);
const mesh = new THREE.Mesh(merged, sharedMaterial);
scene.add(mesh);
// 1000 draw calls → 1
```

### Unity static batching
```csharp
// Mark objects as static in Inspector → Unity merges at build time.
// Or runtime:
StaticBatchingUtility.Combine(rootGameObject);
// Caveat: combined mesh 64k vertex 한도 (16-bit index).
```

### GPU instancing (preferred over merge)
```glsl
// vertex shader (Unity URP)
struct Attributes {
    float3 positionOS : POSITION;
    UNITY_VERTEX_INPUT_INSTANCE_ID
};
UNITY_INSTANCING_BUFFER_START(Props)
    UNITY_DEFINE_INSTANCED_PROP(float4, _Color)
UNITY_INSTANCING_BUFFER_END(Props)

Varyings vert(Attributes IN) {
    UNITY_SETUP_INSTANCE_ID(IN);
    // ...
}
```

### Texture atlas (enable merging across materials)
```python
# Bake separate textures into one atlas → assign UV remap.
import numpy as np
atlas = np.zeros((2048, 2048, 4), np.uint8)
uv_remap = {}
for i, tex in enumerate(textures):
    x, y = (i % 8) * 256, (i // 8) * 256
    atlas[y:y+256, x:x+256] = tex
    uv_remap[i] = (x/2048, y/2048, 256/2048, 256/2048)
# Then rewrite mesh UVs per material id.
```

### Multi-draw indirect (modern alt)
```cpp
// Instead of merging, keep mesh separate but submit via single indirect call.
struct DrawCmd { uint32_t indexCount, instanceCount, firstIndex, vertexOffset, firstInstance; };
std::vector<DrawCmd> cmds; // one per visible mesh
glMultiDrawElementsIndirect(GL_TRIANGLES, GL_UNSIGNED_INT, cmds.data(), cmds.size(), 0);
```

### Hierarchical merge (octree)
```python
def merge_octree(node):
    if node.is_leaf and len(node.meshes) > 1:
        node.merged = merge(node.meshes)
    else:
        for c in node.children: merge_octree(c)
# Coarse cull on octree node, fine draw merged buffer.
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| 同 mesh 다수 | GPU instancing (best) |
| Static, 多 unique mesh | Static merging + atlas |
| Modern API (Vk/D3D12) | Indirect draw + bindless |
| Mobile / WebGL legacy | Static batching + atlas |
| Animated / dynamic transform | Per-object draw + culling |

**기본값**: Modern HW → instancing/indirect. Legacy → static merge + atlas.

## 🔗 Graph
- 부모: [[Rendering Optimization]] · [[Draw Call]]
- 변형: [[GPU Instancing]] · [[Indirect Draw]]
- 응용: [[Texture Atlas]] · [[Octree]] · [[BVH]]
- Adjacent: [[Frustum Culling]] · [[LOD]]

## 🤖 LLM 활용
**언제**: Mobile / web renderer 최적화, asset pipeline 설계.
**언제 X**: Highly dynamic scenes (animation/destruction).

## ❌ 안티패턴
- **Merge everything**: 매 culling 효과 무력화.
- **Material 다른 mesh merge**: shader switch 강제 → benefit 없음.
- **Skinned mesh static merge**: 매 animation broken.
- **64k vertex 한계 모름**: 16-bit index 의 매 silent overflow.
- **Modern API 에서 merge 만 사용**: indirect/instancing 무시.

## 🧪 검증 / 중복
- Verified (Unity docs, Three.js BufferGeometryUtils, GPU Pro series).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — merging vs instancing/indirect 의 trade-off |
