2nd/10_Wiki/Topics/Architecture/PBR.md

---
id: wiki-2026-0508-pbr
title: PBR (Physically Based Rendering)
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Physically Based Rendering, PBR shading, metallic-roughness]
duplicate_of: none
source_trust_level: A
confidence_score: 0.95
verification_status: applied
tags: [graphics, shading, rendering, materials]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: hlsl
  framework: unreal-engine
---

# PBR (Physically Based Rendering)

## 매 한 줄
> **"매 빛-표면 상호작용을 물리법칙(에너지 보존, microfacet, Fresnel)에 따라 모델링"**. 매 2014 Disney/Burley BRDF + Unreal/Frostbite 의 industry standardization → 매 2026 의 모든 real-time 엔진의 default. 매 metallic-roughness workflow 의 universal.

## 매 핵심

### 매 핵심 원칙
1. **에너지 보존**: 매 reflected + transmitted ≤ incoming.
2. **Fresnel**: 매 grazing angle에서 reflectance ↑.
3. **Microfacet**: 매 표면을 미세 mirror 의 분포로 모델 (NDF + G + F).
4. **Linear-space lighting**: 매 sRGB → linear → tonemap.

### 매 BRDF 구조 (Cook-Torrance)
```
f = kd * Lambert + ks * (D * G * F) / (4 * NoL * NoV)
```
- **D**: Normal Distribution Function — Trowbridge-Reitz GGX (default).
- **G**: Geometry / Shadowing — Smith / Schlick-GGX.
- **F**: Fresnel — Schlick approx.
- **kd**: diffuse weight = (1 - F) * (1 - metallic).

### 매 workflow
- **Metallic-Roughness** (Unreal, glTF, Substance default): albedo + metallic + roughness + normal + ao.
- **Specular-Glossiness** (legacy): 매 deprecated.
- **Clearcoat / Sheen / Anisotropy / Subsurface** = 매 extensions (glTF KHR\_\*).

### 매 IBL (Image-Based Lighting)
- 매 environment HDR → 매 prefiltered specular cubemap (split-sum approximation) + 매 diffuse irradiance.
- 매 BRDF LUT (NoV × roughness) precompute.

## 💻 패턴

### Core PBR PS (HLSL, UE-style)
```hlsl
float3 F_Schlick(float3 F0, float VoH) {
    return F0 + (1 - F0) * pow(1 - VoH, 5);
}

float D_GGX(float NoH, float a) {
    float a2 = a * a;
    float d = NoH * NoH * (a2 - 1) + 1;
    return a2 / (PI * d * d);
}

float V_SmithGGX(float NoV, float NoL, float a) {
    float a2 = a * a;
    float gv = NoL * sqrt(NoV * NoV * (1 - a2) + a2);
    float gl = NoV * sqrt(NoL * NoL * (1 - a2) + a2);
    return 0.5 / (gv + gl + 1e-5);
}

float3 PBR(float3 albedo, float metallic, float roughness,
           float3 N, float3 V, float3 L, float3 lightColor) {
    float3 H = normalize(V + L);
    float NoL = saturate(dot(N, L));
    float NoV = saturate(dot(N, V));
    float NoH = saturate(dot(N, H));
    float VoH = saturate(dot(V, H));

    float a = roughness * roughness;
    float3 F0 = lerp(0.04, albedo, metallic);

    float D = D_GGX(NoH, a);
    float V_ = V_SmithGGX(NoV, NoL, a);
    float3 F = F_Schlick(F0, VoH);

    float3 spec = D * V_ * F;
    float3 kd = (1 - F) * (1 - metallic);
    float3 diff = kd * albedo / PI;

    return (diff + spec) * lightColor * NoL;
}
```

### IBL (split-sum)
```hlsl
float3 IBL(float3 N, float3 V, float roughness, float3 albedo, float metallic,
           TextureCube prefiltered, Texture2D brdfLUT, TextureCube irradiance) {
    float NoV = saturate(dot(N, V));
    float3 R = reflect(-V, N);

    float3 F0 = lerp(0.04, albedo, metallic);
    float3 F = F_Schlick(F0, NoV);

    float mip = roughness * 6.0; // 매 mip count - 1
    float3 specIBL = prefiltered.SampleLevel(s, R, mip).rgb;
    float2 brdf = brdfLUT.Sample(s, float2(NoV, roughness)).rg;
    float3 spec = specIBL * (F * brdf.x + brdf.y);

    float3 kd = (1 - F) * (1 - metallic);
    float3 diff = kd * albedo * irradiance.Sample(s, N).rgb;

    return diff + spec;
}
```

### glTF PBR material loader (Three.js / WebGPU)
```ts
const mat = new THREE.MeshPhysicalMaterial({
  map: albedo,             // sRGB
  metalnessMap: metalRough, normalMap: normal,
  roughnessMap: metalRough, // packed: G=rough, B=metal (glTF spec)
  aoMap: ao,
  clearcoat: 0.5, clearcoatRoughness: 0.1, // KHR_materials_clearcoat
  sheen: 0.3, sheenColor: new THREE.Color(0xff8888), // KHR_materials_sheen
});
```

### Disney "Principled" BSDF param mapping
```python
# Blender Principled BSDF — 매 artist-friendly
mat.inputs["Base Color"] = albedo
mat.inputs["Metallic"] = metallic    # 0 dielectric, 1 metal
mat.inputs["Roughness"] = roughness  # 0 mirror, 1 chalk
mat.inputs["IOR"] = 1.5              # dielectric F0 = ((ior-1)/(ior+1))^2
mat.inputs["Specular IOR Level"] = 0.5 # 매 dielectric strength
mat.inputs["Coat Weight"] = 0.0
mat.inputs["Sheen Weight"] = 0.0
mat.inputs["Subsurface Weight"] = 0.0
```

### Texture authoring rules
```
albedo:    sRGB,  no lighting baked, dielectric ≥ 30 sRGB, metal ≥ 200 sRGB
metallic:  linear, binary 가까이 (0 or 1) — 매 dielectric/metal 의 mix X
roughness: linear, 0.04 floor (perfect mirror 회피)
normal:    linear, OpenGL or DirectX convention 통일
ao:        linear, multiplied with diffuse only
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Real-time game | Metallic-Roughness GGX + IBL |
| Skin / wax | + Subsurface scattering |
| Car paint / lacquer | + Clearcoat |
| Cloth / velvet | + Sheen |
| Brushed metal / hair | + Anisotropy |
| Path-traced offline | Disney Principled (super-set) |

**기본값**: 매 GGX + Smith + Schlick + metallic-roughness + IBL split-sum.

## 🔗 Graph

## 🤖 LLM 활용
**언제**: 매 photoreal real-time/offline rendering. 매 cross-engine asset (glTF). 매 artist 의 physical intuition.
**언제 X**: 매 stylized non-photoreal (NPR/toon) — 매 PBR 위에 override 또는 별도 model.

## ❌ 안티패턴
- **gamma 안 맞춤**: 매 albedo 의 linear 처리 → 매 wash-out / overbright.
- **Metallic 0.5**: 매 binary 가까이가 정답. 0.5 = 매 무의미 (전이 영역 없음).
- **Roughness 0.0**: 매 NaN / fireflies. floor 0.04 + clamp.
- **AO multiplied with specular**: 매 specular AO 별도 계산 필요.
- **F0=0.04 for everything**: 매 metal은 albedo 색이 F0.

## 🧪 검증 / 중복
- Verified (Disney 2012 Burley paper, UE5 docs, Filament PBR pipeline doc, glTF 2.0 spec, Real-Time Rendering 4th ed).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — Cook-Torrance + IBL + glTF workflow |