Physically-Based Rendering (PBR) and Global Illumination

(work in progress…)

The global illumination is linked to the Light Transport Equation, aka. the famous rendering equation/ reflectance equation. In a virtual scene, we can separate the illumination models into direct (DI: light travels directly from a light source to a visible surface. However, in real-time GI, the backward light transport is often used. Similar to the ancient Greek believe that light ray emits from the eye. Nonetheless, instead of the eye, here light rays are travel directly from the virtual camera mode. In Forward rendering, it is directly light travels from the source to the visible surface (see More)) and indirect categories. The indirect illumination often named as global illumination (GI) that includes light reflects, refracts, scatters, or bounces from other surface before reacing the camera. GI is a genre of algorithms, e.g., radiosity, path tracing, etc. The conventional ray tracing sometimes skip from this genre. However, the Ray tracing with recursive reflection/refraction might be called as GI. Some of the common GI effects include:

• diffuse interreflection
• color bleeding
• soft indirect shadows
• caustics
• glossy indirect reflection
• multiple light bounces
• ambient occlusion-like contact darkening
• participating media scattering, in volumetric GI

There are hundreds of GI algorithms available in the literature. The classical algortithms include radiosity, path tracing, bidirectional path tracing, Metropolis light transport, photon mapping, instant radiosity (virtual point lights), irradiance caching, light propagation volumes, voxel cone tracing, screen space GI, radiance cahching etc. Each of the GI has their own advantages and limitations, for example, some are good in caustic effect. All the GI algorithms have more or less the convergence challenge. However, some GI algorithms are better suitable for real-time applications. For instance,

• DDGI (Dynamic Diffuse Global Illumination), probe-based
• RTXGI, probe-based (NVidia)
• ReSTIR GI, reservior-based (NVidia)
• SVOGI (Voxel-based Global Illumination), voxel-based, Amazon
• Screen Space Global Illumination, Unity
• Lumen, Unreal Engine

PBR

Now, I think, the Physically-based rendering (PBR) has not well defined in the literature or it is hard to define a boundary, because it might mean many things. PBR is a philosophy that makes (tries to) the rendering process obey physical principles, e.g., light transport, material reflection, energy conservation, camera response, etc. According to the Learn OpenGL, a rendering process should called PBR when it obeys three principles, (i) microfacet surface model, (ii) energy conservesion, and (iii) physically-based BRDF. The third parameter is core idea for PBR and still an active research field. Unlike the traditional models (Phong, Blinn-Phong), the PBR models looks much plausible.

A complete PBR system usually includes:

• physically plausible material models
• energy conservation
• Fresnel reflection
• microfacet BRDFs
• real-world light units or radiometric quantities
• linear color-space rendering
• physically meaningful camera/exposure models, etc.

A common PBR material model uses (as buffer/ texture map)

• base color/ albedo/ diffuse map (true color without any light or shadow information)
• roughness/ glossiness map
• metalness/ specular map
• normal/ bump map

Other than these four, other maps include

• ambient occlusion (AO)
• Emission
• height/ displacement map
• opacity/ transparency map

Some of the popular Physically based BRDF (material) models are Lambertian, Oren-Nayer, Cook-torrance, GGX / Trowbridge-Reitz distribution, Beckmann microfacet model, Disney Principled BRDF, Fresnel-Schlick approximation, Smith masking-shadowing model, Burley diffuse model, BSSRD, etc.

In summary, both GI and PBR are closely related, however not the same thing. GI can be a part of PBR, but PBR is much broader. And, all GI cannot be called PBR. For instance, the earlier radiosity algorithm limited to the diffuse surface only. In this sense, it can be called GI but not PBR. In other example, suppose we need to render a metallic sphere in a room like Cornell Box. The sphere has a PBR properties as metallic = 1.0, roughness = 0.25, Fresnel reflection, and Cook-Torrance BRDF. However, it would look plausible under direct lighting (illumination). However, the room’s walls may also bounce lights. Without a proper GI algorithm we will miss that color bleeding effect.

GI ⊂ PBR 

PBR makes materials behave physically, while GI makes lighting behave physically. Together, they achieve realistic plausible results.