Physically based rendering Flashcards
Lecture 8 (18 cards)
What are the conditions that physically based rendering (PBR) must follow?
- Energy conservation
- Positivity
- Law of reflection
- Fresnel equation
- HelmHoltz reciprocity
- Everything is shiny (even dark materials have some specular)
- Dielectric reflectance is between 0.02-0.05
What does “Energy conservation” mean in PBR?
A surface cannot reflect more light than it receives. This limits how much diffuse + specular can be returned.
What is the law of reflection?
The angle of incoming light equals the angle of outgoing reflected light for specular reflection.
What is positivity in the context of PBR?
Surfaces cannot reflect a negative amount of light.
What is Helmholtz reciprocity?
If light travels a path in one direction, it behaves the same when reversed.
What does “everything is shiny” mean in PBR?
No material is completely matte - even dark materials have small amounts of specular reflection.
What is the reflectance range for dielectric materials?
Between 0.02 and 0.05 (f0 value for dielectric materials)
What does the Fresnel formula describe in PBR?
It describes how much light is reflected versus retracted, depending on the angle between the light and the surface.
How does the amount of reflected light change with angle of Fresnel?
At shallow (grazing) angles, surfaces reflect more light; at perpendicular angles, they reflect less
What are f0 and f90 in the Fresnel model?
- f0 is the reflectance when light hits the surface head-on
- f90 is the reflectance when light hits at a grazing angle
- f90 is typically 1 for dielectrics
How is Fresnel used to mix diffuse and specular light?
Fresnel is used to interpolate between diffuse and specularFinal = Diffuse * (1 - F) + Specular * F
What is a BRDF?
Bidirectional Reflectance Distribution Function:
A function that describes how much light is reflected at a surface point, depending on incoming and outgoing directions.
What does a BRDF return?
The ratio of reflected light based on viewing direction and light direction at a surface.
What are the core properties of a valid BRDF?
- Positivity
- Helmholtz reciprocity
- Energy conservation
What are the three terms in the Cook-Torrance specular model?
- D: Distribution (microfacet alignment)
- G: Geometry (shadowing/masking)
- F: Fresnel (angle-dependent reflectance)
What does the D term represent?
(D) Distribution measures how many microfacets are aligned with the half-vector.
What does the G term represent?
(G) Geometry accounts for microfacets being blocked before or after reflecting light.
What does the F term represent?
(F) Fresnel determines the ratio of light that is reflected depending on viewing angle and material.
