Download Measuring and Modeling Anisotropic Reflection

Measuring and Modeling
Anisotropic Reflection
Gregory J. Ward
Lighting Systems Research Group
Lawrence Berkeley Laboratory
Outline





Definition of the BRDF
A typical Gonioreflectometer
An Imaging Gonioretlectometer
Modeling Anisotropic Reflectance
Rendering
Definition of the BRDF
Wavelength and polarization are
contained implicitly in the function
A typical Gonioreflectometer
Designed by Murray – Coleman and Smith
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
An Imaging Gonioretlectometer
(Side view)
An Imaging Gonioretlectometer
(Front View)
Imaging Gonioretlectometer geometry
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
Measurement Limitations

Limited to measure the reflectance function
near grazing angles

Limited to measure more polished surface
with sharp specular peaks
Modeling Anisotropic Reflectance

Directly using the data or using series
approximation


Some model published for anisotropic case


impractical
may not fit measured data
Use Gaussian Model
The Isotropic Gaussian Model
The Anisotropic Gaussian Model
Data and model comparison
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
Rendering
Deterministic
without sampling
solely on stochastic
sampling
hybrid method.
Rendering
photograph
Deterministic and
isotropic Gaussian
model
Hybrid and anisotropic
Gaussian model
rendering