Lindsay Kay

BioDigital Tech Talk

March 10, 2017

Introduction

Physically-Based Rendering

to

An

Old School: Phong Reflection Model

• ambient - amount of reflected ambient light
• diffuse - amount of reflected diffuse light
• specular - amount of reflected specular light
• shininess - tightness of specular highlights

Phong material properties:

Old School: Phong Reflection Model

• Synthetic
• Easy to create unrealistic materials
• Needs tweaking to look right
• Does not enforce energy conservation
• shininess is a poor approximation for roughness 
• shininess can't be textured, so roughness map not possible
• Best part: efficient on low-end GPUs

New School: Physically-Based Rendering (PBR)

• A family of techniques
• Math uses real measured values for color, roughness, etc.
• Means that materials can more easily be reused from libraries
• Enforces constraints
• Robust for different lighting
• Models surface roughness realistically

• Diffuse - light scattered by roughness, with some absorbed
• Specular - light reflected straight off surface 

PBR: Diffusion and Reflection

• Specular & diffusion are mutually exclusive
• Shader enforces (specular + diffuse) <= 1.0
• "Nanny physicist"
• Keeps consistency under different lighting conditions
• Best part: prevents artwork from bending rules

PBR: Energy Conservation

• AKA gloss, smoothness, roughness, microfacet
• Tiny imperfections - micro scratches, grooves, cracks
• Modeled statistically in shader, eg. using a scattering factor
• Best part: texturable across surface

PBR: Microsurface

PBR: Roughness texturing

• RGB texture where R channel contains roughness factor

• Use a reflection cube map with MIP detail levels
• At each point on surface, roughness selects which detail level to sample

PBR: Microsurface - shader

• All materials perfectly reflective at grazing angles
• Gradient (see below) does not vary much between materials
• Divergence can be accounted for analytically
• Best part: PBR shaders can handle Fresnel automatically

PBR: Fresnel

How specular reflection varies at different angles.

With PBR:

PBR: Common workflows

• base color (RGB)
• specular reflection factor (RGB)
• glossiness [0..1]

• base color RGB
• metalness [0..1] - (usually just 1 for metals, 0 for insulators)
• roughness [0..1]

1. Specular-glossiness

2. Metal-roughness

Metal-roughness is the most expressive of these two.

PBR: WebGL metal-roughness example

• Get a feel for it using an example
• http://bit.ly/2mIQzIb
• Toggle textures on/off in the menu
• Has cubemaps for lights and environmental reflection
• Roughness selects MIP level in light cubemap