• Learning Resources

→ physically based rendering

→ aims to mimic light in a physically plausible way, it generally looks more realistic compared to our original lighting algorithms like Phong and Blinn-Phong

→ author surface materials based on physical parameters without having to resort to cheap hacks and tweaks to make the lighting look right

→ One of the bigger advantages of authoring materials based on physical parameters is that these materials will look correct regardless of lighting conditions

→ PBR lighting model to be considered physically based, it has to satisfy the following 3 conditions

1. Be based on the microfacet surface model.
2. Be energy conserving.
3. Use a physically based BRDF.

Microfacet model

<aside> 💡 All the PBR techniques are based on the theory of microfacets. The theory describes that any surface at a microscopic scale can be described by tiny little perfectly reflective mirrors called microfacets.

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specifically talking about specular lighting/reflection:

• rough surface: incoming light rays are more likely to scatter along completely different directions

• smooth surface: light rays are more likely to reflect in roughly the same direction, giving us smaller and sharper reflections

  

  Roughness

  → parameter used to statistically approximate the surface's microfacet roughness

  → Based on the roughness of a surface, we can calculate the ratio of microfacets roughly aligned to some vector h. This vector h is the halfway vector that sits halfway between the light l and view v vector.

  <aside> 💡 * Large Roughness → Rough Surface: larger specular reflection shape (less reflective & less shinny)

  • Low Roughness → Smooth Surface: smaller and sharper specular reflection shape (more reflective and shinny)

  

  </aside>

Energy Conservation

<aside> 💡 The microfacet approximation employs a form of energy conservation: outgoing light energy should never exceed the incoming light energy (excluding emissive surfaces).

Looking at the above image we see the specular reflection area increase (bigger roughness), but also its brightness decrease at increasing roughness levels.

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Reflection v.s. Refraction:

For energy conservation to hold, we need to make a clear distinction between diffuse and specular light. The moment a light ray hits a surface, it gets split in both a refraction part and a reflection part.

• Reflection is light that directly gets reflected and doesn't enter the surface, known as specular lighting
• Refraction is the remaining light that enters the surface and gets absorbed; known as diffuse lighting

→ In PBR, we assume that all refracted light gets absorbed and thus ignored

→ Subsurface Scattering technique takes this into account: significantly improve the visual quality on materials like skin, marble, or wax, but come at the price of performance.