Appearance Models for Graphics

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Appearance Models for Graphics

• COMS 6998-3, Lecture 2
• BRDFs and Radiometry

Many slides courtesy Pat Hanrahan
http//graphics.stanford.edu/courses/cs348b-02/lec
tures/lecture4/illumination.pdf
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Radiometry

• Physical measurement of electromagnetic energy
• We consider light field
• Transport theory
• Radiance, Irradiance
• Reflection functions BRDF
• Examples, Properties
• Simple BRDF models

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Transport Theory

• Flow of stuff. In this case, stuff photons of
  light
• Consider particle flow through small area CW18

density
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Radiance

• Power per unit projected area perpendicular to
  the ray per unit solid angle in the direction of
  the ray
• Symbol L(x,?) (W/m2 sr)
• Flux given by
  dF L(x,?) cos ? d?
  dA

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Radiance properties

• Radiance is constant as it propagates along ray
• Derived from conservation of flux
• Fundamental in Light Transport.

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Radiance properties

• Sensor response proportional to surface radiance
  (constant of proportionality is
  throughput)
• Far away surface See more, but subtends smaller
  angle
• Wall is equally bright across range of viewing
  distances
• Consequences
• Radiance associated with rays in a ray tracer
• All other radiometric quantities derived from
  radiance

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Irradiance, Radiosity

• Irradiance E is the radiant power per unit area
• Integrate incoming radiance over hemisphere
• Projected solid angle (cos ? d?)
• Uniform illumination
  Irradiance p CW
  24,25
• Units W/m2
• Radiosity
• Power per unit area leaving
  surface (like irradiance)

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BRDF

• Reflected Radiance proportional to Irradiance
• Constant proportionality BRDF CW pp 28,29
• Bidirectional Reflection Distribution Function
• (4 Vars)
• Reflectance Equation CW pp 30

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Mirror
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Lambertian
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Reflectance/Energy Conservation
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Retroreflection
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Brdf Viewer plots
Diffuse
Torrance-Sparrow
Anisotropic
bv written by Szymon Rusinkiewicz
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Representations
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Reparameterizations