Antigravity Agent
151 lines
5.0 KiB
Markdown
151 lines
5.0 KiB
Markdown
---
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id: wiki-2026-0508-geometry-merging
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title: Geometry Merging
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category: 10_Wiki/Topics
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status: verified
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canonical_id: self
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aliases: [Mesh Merging, Static Batching, Geometry Batching]
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duplicate_of: none
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source_trust_level: A
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confidence_score: 0.9
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verification_status: applied
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tags: [graphics, optimization, rendering, gpu]
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raw_sources: []
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last_reinforced: 2026-05-10
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github_commit: pending
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tech_stack:
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language: C++/HLSL
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framework: Unity/Unreal/Three.js
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---
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# Geometry Merging
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## 매 한 줄
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> **"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.
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## 매 핵심
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### 매 종류
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- **Static batching**: build-time 에 같은 material 의 static mesh 합침.
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- **Dynamic batching**: runtime 에 small mesh 를 transform & merge (CPU cost ↑).
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- **GPU instancing**: 같은 mesh 여러 transform — merging 의 modern 대체.
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- **Mesh atlas (texture array)**: material 통합으로 merge 가능 범위 확장.
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### 매 trade-off
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- ↑ Throughput (fewer draw call).
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- ↓ Culling 정확도 (merged AABB 가 매 커짐).
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- ↑ Memory (per-vertex data 의 매 duplication).
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- ↓ Animation flexibility (static 한정).
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### 매 응용
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1. Mobile 게임 — draw call 의 매 hard limit (~100-200).
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2. Architectural visualization — 매 thousands of small props.
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3. Tile-based world streaming.
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4. UI batching (text glyph atlas).
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## 💻 패턴
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### Manual merge (Three.js)
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```javascript
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import { mergeGeometries } from 'three/examples/jsm/utils/BufferGeometryUtils.js';
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const geos = props.map(p => p.geometry.clone().applyMatrix4(p.matrixWorld));
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const merged = mergeGeometries(geos, false);
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const mesh = new THREE.Mesh(merged, sharedMaterial);
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scene.add(mesh);
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// 1000 draw calls → 1
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```
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### Unity static batching
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```csharp
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// Mark objects as static in Inspector → Unity merges at build time.
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// Or runtime:
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StaticBatchingUtility.Combine(rootGameObject);
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// Caveat: combined mesh 64k vertex 한도 (16-bit index).
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```
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### GPU instancing (preferred over merge)
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```glsl
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// vertex shader (Unity URP)
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struct Attributes {
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float3 positionOS : POSITION;
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UNITY_VERTEX_INPUT_INSTANCE_ID
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};
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UNITY_INSTANCING_BUFFER_START(Props)
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UNITY_DEFINE_INSTANCED_PROP(float4, _Color)
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UNITY_INSTANCING_BUFFER_END(Props)
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Varyings vert(Attributes IN) {
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UNITY_SETUP_INSTANCE_ID(IN);
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// ...
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}
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```
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### Texture atlas (enable merging across materials)
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```python
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# Bake separate textures into one atlas → assign UV remap.
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import numpy as np
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atlas = np.zeros((2048, 2048, 4), np.uint8)
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uv_remap = {}
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for i, tex in enumerate(textures):
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x, y = (i % 8) * 256, (i // 8) * 256
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atlas[y:y+256, x:x+256] = tex
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uv_remap[i] = (x/2048, y/2048, 256/2048, 256/2048)
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# Then rewrite mesh UVs per material id.
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```
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### Multi-draw indirect (modern alt)
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```cpp
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// Instead of merging, keep mesh separate but submit via single indirect call.
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struct DrawCmd { uint32_t indexCount, instanceCount, firstIndex, vertexOffset, firstInstance; };
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std::vector<DrawCmd> cmds; // one per visible mesh
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glMultiDrawElementsIndirect(GL_TRIANGLES, GL_UNSIGNED_INT, cmds.data(), cmds.size(), 0);
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```
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### Hierarchical merge (octree)
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```python
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def merge_octree(node):
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if node.is_leaf and len(node.meshes) > 1:
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node.merged = merge(node.meshes)
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else:
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for c in node.children: merge_octree(c)
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# Coarse cull on octree node, fine draw merged buffer.
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```
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## 매 결정 기준
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| 상황 | Approach |
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|---|---|
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| 同 mesh 다수 | GPU instancing (best) |
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| Static, 多 unique mesh | Static merging + atlas |
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| Modern API (Vk/D3D12) | Indirect draw + bindless |
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| Mobile / WebGL legacy | Static batching + atlas |
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| Animated / dynamic transform | Per-object draw + culling |
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**기본값**: Modern HW → instancing/indirect. Legacy → static merge + atlas.
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## 🔗 Graph
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- 부모: [[Rendering Optimization]] · [[Draw Call]]
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- 변형: [[GPU Instancing]] · [[Mesh Shader]] · [[Indirect Draw]]
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- 응용: [[Texture Atlas]] · [[Octree]] · [[BVH]]
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- Adjacent: [[Frustum Culling]] · [[LOD]] · [[Vertex Buffer]]
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## 🤖 LLM 활용
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**언제**: Mobile / web renderer 최적화, asset pipeline 설계.
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**언제 X**: Highly dynamic scenes (animation/destruction).
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## ❌ 안티패턴
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- **Merge everything**: 매 culling 효과 무력화.
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- **Material 다른 mesh merge**: shader switch 강제 → benefit 없음.
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- **Skinned mesh static merge**: 매 animation broken.
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- **64k vertex 한계 모름**: 16-bit index 의 매 silent overflow.
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- **Modern API 에서 merge 만 사용**: indirect/instancing 무시.
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## 🧪 검증 / 중복
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- Verified (Unity docs, Three.js BufferGeometryUtils, GPU Pro series).
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- 신뢰도 A.
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## 🕓 Changelog
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| 날짜 | 변경 |
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|---|---|
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| 2026-05-08 | Phase 1 |
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| 2026-05-10 | Manual cleanup — merging vs instancing/indirect 의 trade-off |
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