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An Analytic Model for Full Spectral Skydome Radiance

An Analytic Model for Full Spectral Skydome Radiance. Alexander Wilkie Charles University. Lukáš Hošek Charles University. Motivation: Skydome models. outdoor scenes prediction of material appearance architecture, outdoor scenes alternative: skyboxes. The Preetham Model.

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An Analytic Model for Full Spectral Skydome Radiance

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  1. An Analytic Model for Full Spectral Skydome Radiance Alexander Wilkie Charles University LukášHošek Charles University

  2. Motivation: Skydome models • outdoor scenes • prediction of material appearance • architecture, outdoor scenes • alternative: skyboxes

  3. The Preetham Model • De facto standard analytic skydome model • Simple closed form formulas • Outputs tristimulus data, convertible to spectral • Perez formula, fitted to the outputs of Nishita model • Fitting results publicly available

  4. The Preetham Model

  5. Issues With the Preetham Model • Simple functions for calculating zenith luminance and luma distribution parameters • Only a small subset of possible atmospheric conditions can be captured • Lack of localised aureole at higher turbidities • Spectral data are obtained by conversion from trisitmulus data

  6. Issues with the Preetham Model

  7. Goals for the New Model • spectral (including UV) • larger range of turbidities • eliminate artifacts • ground albedo (came later & almost for free)

  8. Obtaining Reference Data • Brute force Monte Carlo path tracer that incorporates • Mie scattering • Rayleigh scattering • Albedo & colour of Earth surface • Delta scattering • Approaches the simulation as a transport problem, the same way physicists simulate e.g. gamma rays passing through containment shells of nuclear reactors

  9. Obtaining Reference Data

  10. Result: Raw Data

  11. Result: Comparison

  12. Modified formulas for a better fit

  13. Model flowchart

  14. How the Fitting was Done • Levenberg-Marquardt nonlinear optimization • Yields a mean radiance and radiance distribution array • These are then used as control points for Bezier interpolation • Albedo 0-1Turbidity 1-10Solar elevation 0-90°320nm – 720nm bands, 40nm increment

  15. Results

  16. Sky Colour Patterns (sunset)

  17. UV & Fluorescence

  18. Ground Albedo black white water blue forest green

  19. Conclusion • Reference implementation and data available for download • Version 1.1 out! • http://cgg.mff.cuni.cz/projects/SkylightModelling/

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