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Measuring and Modeling Anisotropic Reflection

Explore the definition and measurement of Bidirectional Reflectance Distribution Function (BRDF) using gonioreflectometers and modeling anisotropic reflectance. Learn about limitations and strategies in rendering with isotropic and anisotropic Gaussian models.

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Measuring and Modeling Anisotropic Reflection

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  1. Measuring and Modeling Anisotropic Reflection Gregory J. Ward Lighting Systems Research Group Lawrence Berkeley Laboratory

  2. Outline • Definition of the BRDF • A typical Gonioreflectometer • An Imaging Gonioretlectometer • Modeling Anisotropic Reflectance • Rendering

  3. Definition of the BRDF Wavelength and polarization are contained implicitly in the function

  4. A typical Gonioreflectometer Designed by Murray – Coleman and Smith

  5. An Imaging Gonioretlectometer • Key elements • A half–silvered hemisphere • A sample target holder • A CCD camera with fisheye lens • Measuring step • Light reflect off the sample surface • The mirror collect it • Then reflect back into the lens and CCD array

  6. An Imaging Gonioretlectometer (Side view)

  7. An Imaging Gonioretlectometer (Front View)

  8. Imaging Gonioretlectometer geometry

  9. Recovering the reflected angles • Two step • First Determine mapping function from pixel location to lens incident direction • Second compute reflection angles from camera incident angles • A captured image

  10. Measurement Limitations • Limited to measure the reflectance function near grazing angles • Limited to measure more polished surface with sharp specular peaks

  11. Modeling Anisotropic Reflectance • Directly using the data or using series approximation • impractical • Some model published for anisotropic case • may not fit measured data • Use Gaussian Model

  12. The Isotropic Gaussian Model

  13. The Anisotropic Gaussian Model

  14. Data and model comparison

  15. Rendering • An unbiased and low variance approximation of rendering equation required but • Either unbiased and high variance • Or low variance and biased • Hybrid can be used! • deterministic solution for source contributions • stochastic sampling for indirect contributions

  16. Rendering Deterministic without sampling solely on stochastic sampling hybrid method.

  17. Rendering photograph Deterministic and isotropic Gaussian model Hybrid and anisotropic Gaussian model

  18. rendering

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