Physically based rendering Flashcards

Lecture 8 (18 cards)

1
Q

What are the conditions that physically based rendering (PBR) must follow?

A
  • 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
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2
Q

What does “Energy conservation” mean in PBR?

A

A surface cannot reflect more light than it receives. This limits how much diffuse + specular can be returned.

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3
Q

What is the law of reflection?

A

The angle of incoming light equals the angle of outgoing reflected light for specular reflection.

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4
Q

What is positivity in the context of PBR?

A

Surfaces cannot reflect a negative amount of light.

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5
Q

What is Helmholtz reciprocity?

A

If light travels a path in one direction, it behaves the same when reversed.

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6
Q

What does “everything is shiny” mean in PBR?

A

No material is completely matte - even dark materials have small amounts of specular reflection.

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7
Q

What is the reflectance range for dielectric materials?

A

Between 0.02 and 0.05 (f0 value for dielectric materials)

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8
Q

What does the Fresnel formula describe in PBR?

A

It describes how much light is reflected versus retracted, depending on the angle between the light and the surface.

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9
Q

How does the amount of reflected light change with angle of Fresnel?

A

At shallow (grazing) angles, surfaces reflect more light; at perpendicular angles, they reflect less

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10
Q

What are f0 and f90 in the Fresnel model?

A
  • 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
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11
Q

How is Fresnel used to mix diffuse and specular light?

A

Fresnel is used to interpolate between diffuse and specular
Final = Diffuse * (1 - F) + Specular * F

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12
Q

What is a BRDF?

A

Bidirectional Reflectance Distribution Function:
A function that describes how much light is reflected at a surface point, depending on incoming and outgoing directions.

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13
Q

What does a BRDF return?

A

The ratio of reflected light based on viewing direction and light direction at a surface.

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14
Q

What are the core properties of a valid BRDF?

A
  • Positivity
  • Helmholtz reciprocity
  • Energy conservation
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15
Q

What are the three terms in the Cook-Torrance specular model?

A
  • D: Distribution (microfacet alignment)
  • G: Geometry (shadowing/masking)
  • F: Fresnel (angle-dependent reflectance)
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16
Q

What does the D term represent?

A

(D) Distribution measures how many microfacets are aligned with the half-vector.

17
Q

What does the G term represent?

A

(G) Geometry accounts for microfacets being blocked before or after reflecting light.

18
Q

What does the F term represent?

A

(F) Fresnel determines the ratio of light that is reflected depending on viewing angle and material.