Study the nature of f things to imaging to -An overvie iew of of - PowerPoint PPT Presentation

Study the nature of f things to imaging to -An overvie iew of of physics-based renderin ing Wu u Min Minjie ie Art Art Technic ical l Di Director, Ubi bisoft ft Mo Montreal l Stud Studio io

The origin of PBR In the game industry

Status of PBR

Content 1.What t is P PBR 2.The influenc nce of P PBR

Background knowledge Seeing is believing?

Seeing is believing? Visible light Visible light range: 400 nm range: 400 nm – 700 nm

What is P PBR Differen ences es between en PBR and t traditi tion onal l renderin ing

Essence ce: subject ctiv ive and objecti tive PBR: Based on the physical properties of the things in the objective world Traditional rendering: Based on the viewer's subjective image 传统的渲染 PBR Diffuse: 黑色 Albedo : 0.04 闪亮高光 IOR : 1.635 …… ……

Style: "process-oriented" vs "result-oriented" PBR: Decompose a complex phenomenon into a series of formulas and parameters associated Traditional rendering: focus on final result PBR PBR Traditional renderin ing (Domino effect) (Ic (Iceberg Theory ry)

Function: Function: “ all all-weather" weather" vs vs "single "single angle" angle" PBR: it can always adapt to the environment. Traditional rendering: from a certain perspective, unable to take the overall situation into account PBR PBR Traditional renderin ing

Details ls of PBR PBR 1. 1. Physicall lly Base ased Lig Lightin ing 2. . Physic icall lly Bas ased Shadin ing 3. . Physic icall lly Base ased Sensit itisin ing

Physicall lly Base ased Lig Lightin ing Three elements of lighting Three elements of lighting (to discuss PBL from (to discuss PBL from the perspective of artist): the perspective of artist): 1. Intensity 2. Color 3. Type

Lightin ing g intensit ity Three common physical units 1.Candela 2.Lumen 3.Lux

Steradian, symbol: Steradian, symbol: sr sr 1. . Unit of solid angle 2. 2. Any closed sphere ’s solid angle is 4π

Candela, symbol: Candela, symbol: cd cd 1. Unit of visible light intensity � 1/683W/sr � 2. 2. A common candle emits light with a luminous intensity of roughly one candela.

Lumen, symbol: Lumen, symbol: lm lm 1. Unit of luminous flux 2. 1 lumen (lm)= 1 cd · sr 3. The luminous flux of a common candle is about lumens (220v)

lx Lux, symbol: Lux, symbol: lx 1. Unit of luminous flux � Illuminance � 2. 1 lux= 1 lumen/square meter

Attenua uati tion Inverse-square law: Light intensity is inversely proportional to the square of distance and attenuates (energy conservation)

Scatter erin ing Light is forced to deviate from a straight trajectory by one or more paths due to localized non-uniformities in the medium through which it passes.

Mie scatteri ring Condition o: particle radius >= wavelength of the incident light

Mie scatteri ring g is a as f follows ws: 1. Most of the incident lights will scatter along the forward direction 2. particle radius will change the model of Mie scattering Particle Particle ticle radius ius is close close ticle radius ius to the wavelen elength gth of of the the increas reases es incident ident light ht

Impact on outdoor natural light intensity weather Transmissivity Sky light Sunny About 0.85 10000 lux Cloudy About 0.55 1000 lux

IES LIGHT P hotometr hotometric ic profile profile IES ： illuminating engineering society IES Light = Maximum intensity (candela) X IES P hotometric profile

Lightin ing g color color temperature symbol:K

Rayleig igh h scatteri ring Condition: Particle radius <= One tenth of the wavelength of incident light

Rayleig igh h scatteri ring Scattering cattering intensity: ensity: inversely ersely proportional portional to the fourth rth power er of the wavelength elength

Lightin ing g type 1.directional light 2.Punctual light 3.area light

Punctua ual l light VS Area light Specular shadow instance Punctual ctual ht light Area a light ht

brie rief summary ry of of PBL BL Lighting mathematical models physically physical unit and methods measured value sun light ， sky light ， cloud ， dust 。。。。。。 physical laws

Physicall lly Base ased Sh Shadin ing Shading Shading ： material material response response to to lighting lighting Function Function ： BRDF BRDF

BRDF BRDF 是什么 B idirectiona l R eflectance eflectance D istribution istribution F unction idirectional unction 1. 1. Bidirectional 2. Reflectance 3. Distribution Function tgoing radiance (to lens or eye)

B idirectional idirectional ��向� 1. The direction from sampling point to camera (eyes) 2. The direction from sampling point to point light source tgoing radiance (to lens or eye)

R eflectance eflectance ��射率� Reflectance = Radiance / irradiance Irradiance (power/area) ： the power of the light received by current point Radiance (power/(area x solid angle)) ： the power of the light emitted by current point tgoing radiance (to lens or eye)

D istribution istribution F unction unction tgoing radiance (to lens or eye) Picture from ： Naty Hoffman,Background: Physics and Math of Shading

Three different Three different types of BRDF types of BRDF Empirical model Physically based model Data-driven mode tgoing radiance (to lens or eye)

Basic principles of physics-based model 1. Reciprocal 2. Conservation of energy 3. Constant positive (Positivity)

Generic Shader Generic Shader Ci = Ka*ambient() Ci = Ka*ambient() + Kd*diffusion() + Kd*diffusion() + Ks*specular() + Ks*specular() 1. 1. Ambient mbient 2. 2. Diffusion iffusion 3. 3. Specular pecular SURFACE PRO 3 SURFACE PRO 3 Picture from ： Naty Hoffman,Background: Physics and Math of Shading

Diffusion iffusion Th ion ： The pr process of of dif diffusio 1. Refracting into a material 1. 2. Scattering in the material 3. Scattering out from the material Picture from ： Naty Hoffman,Background: Physics and Math of Shading

Dif Diffusion model 1. Lambert ： Base Based on on smo mooth ma materia ial l sur urface. Lambert Lambert model model characterist characteristics ics: : ： Isotropic (camera view) Light intensity distribution is in line with the cosine law (light angle) SURFACE PRO 3 SURFACE PRO 3        cos F E F  lambert o i  lambert

2. Oren-Nayar derived rived from m Lambert bert model el extended tended to the rough gh surface face controlled ntrolled by roughness ghness (0 (0-1.0) 光滑表面 粗燥表面

Theoret etic ical l basis of O Oren-Na Naya yar model Based on microfacet model theory Composed of many microfacets Every facet can be seen as a Lambertreflection plane

Oren-Nayar formula σ ： roughness 当 σ = 0 ： A =1 ， B=0 ，粗燥因子 = 1

what is ρ ρ = Albedo = Albedo

Two kinds of Albedo texture acquisition color checker Cross -polarized lighting with spectralon SURFACE PRO 3 SURFACE PRO 3

Specular Specular: Cook Cook – Tor orra ranc nce reflect ctio ion model ：

microfac acet t theory: y: 1 . The surface face is composed posed microfacet rofacets, , every ry facet et only y does s specular cular reflection lection 2 . 2 2 . Based ed on microfacetnormal rofacetnormal M, every ry facet et only y reflects lects the light ht of single gle direction ection

数学公式：   () () () D G F  F  Cook Torrance 4(l·n)(v·n ) Molecule: Molecule: D �� ： Distribution Distribution function function G �� ： Geometr Geometry attenuation attenuation SURFACE PRO 3 SURFACE PRO 3 function function F �� ： Fresnel Fresnel function function Denominator: Denominator: 4 � n.l n.l �� v.n v.n �： Correction factor for conversion Correction factor for conversion between micro between micro mirror surface and mirror surface and the overall surface the overall surface

Influen ence ces from microst stru ructu ture re of t the material al surface ce: Roughness: Value range between 0-1 Square root of slope of facet Half Vector: The he angle le between ween the hvector ctor bisecting ecting the incident ident light ht I and observation ervation direction ection v. Only when h coincides with the facet’s normal M, the microfacetwill be "activated." Picture from ： Naty Hoffman,Background: Physics and Math of Shading

Distrib ibut ution on functio ionD nD ��： Normal distribution probability of activated facets The performance of distribution function with different roughness (GGX)

Geometr try attenuat ation on functio ion n G ��： Distribution probability of facet blocking incident light and reflected light Incident light blocked Reflected light blocked Multiple bounce not be considered Picture from ： Naty Hoffman,Background: Physics and Math of Shading Physical Physical meaning meaning The performance of geometry attenuation function with different roughness (GGX)