Spherical Harmonic Lighting of Wavelengthdependent Phenomena

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Spherical Harmonic Lighting of Wavelength-dependen
t Phenomena

• Clifford Lindsay, Emmanuel Agu
• Worcester Polytechnic Institute
• (USA)

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Introduction

• Modeling of Light Interference As A BRDF

• Compactly Store Surface Response and Hemispheric
  Light Contribution In Terms Of Spherical Harmonic
  Coefficients

• Evaluation of Light And Reflection Is Done on GPU

• Working With Spectral Color Space Instead of RGB

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Spherical Harmonics
Spherical harmonics is a set of basis functions
defined in spherical coordinates
We can use spherical harmonics to approximate the
original, where ci is a vector of SH coefficient
We can then use these coefficients to reconstruct
an approximation to the original signal
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Interference

• Factors that Affect Light Interference
• Refractive index and thickness of the thin film
• Refractive indices of the media 1 2
• Incident Angle ? and incident SPD (Spectral Power
  Distribution)

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Spectral BRDF

• Just Like Regular BRDFs (but different)
• Rendering equation
• Function of 4 angles (incident, reflection)
• Conservative

• Whats Different?
• Different Color Interaction
• Different Material Interaction
• Different Viewer Interaction (non-reciprocal)

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Previous Work in S.H.

• Diffuse Reflection Maps, Hanrahan, Ramamoorthi,
  2001

• Physically Based BRDFs, Westin 1992

• Isotropic BRDFs with SH Maps, Hanrahan,
  Ramamoorthi, 2002

• Arbitrary BRDFs, Kautz et al, 2002

• Radiance Transfer, Glossy Surfaces, Self-shadow,
  and Interreflection, Sloan et al, 2002

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Our Approach
Two Stage Approach
1. Pre-computation

2. Render

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Pre-computation
Total Light Contribution

• Set of coefficients approximating total light
  contribution

BRDF Response

• Texture map based all possible view vectors

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GPU
Rendering

• Index Texture Map Based on view V

• Summing the coefficients for final contribution

• Add ambient, diffuse, Fresnel for additional
  realism

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Conclusion

• Real-time rendering of complex materials
  previously available to offline renderers (ray
  tracers)

• Data and Data structures that are GPU friendly

• Evaluation of Spherical Harmonics is a linear
  operation

• Increased realism with Image Based Lighting and
  accurate BRDFs with GPU acceleration

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Results