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Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

2026-05-20 23:52:15 +09:00

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title

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

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

💻 패턴

Manual merge (Three.js)

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

// 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)

// 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)

# 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)

// 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)

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

🤖 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

4.9 KiB Raw Blame History