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A Simple Layered RGB BRDF Model

This paper presents a simple yet convincing model for layered RGB BRDF, aiming to increase the range of possible effects in graphic content creation. The model works in color spaces RGB, XYZ, and LMS, and incorporates wavelength effects, interferences, and color dispersion. Results show that the model produces realistic effects, though it is not a full simulation.

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A Simple Layered RGB BRDF Model

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  1. Xavier Granier - Wolfgang Heidrich IMAGER/University of British Columbia A Simple LayeredRGB BRDF Model

  2. Motivations • Increase the range of possible effects • For graphic content creation • Work in Color Space RBG - XYZ - LMS • Currently limited to linear reflection • Convincing and simple model • Not a full simulation • Effect as realistic as possible

  3. Motivations • Wavelength effects • Interference • Colour dispersion • Investigation • Framework

  4. Overview • Previous Work • General Configuration • Glossy Case • Diffuse Case • Results • Conclusion

  5. Overview • Previous Work • General Configuration • Glossy Case • Diffuse Case • Results • Conclusion

  6. Uniform BRDF • Phong models [Phong75,Lafortune94-97,…] • Most commonly used • Simplified models [Ward92,Schlick94,…] • Faster / Better for Global illumination • Micro-facet [Torrance67,Ashikhmin00,…] • Physically based [He91,Hanrahan93,…] • No wavelength dependent effects

  7. Wavelength effects • Diffraction [Stam99,Sun00,…] • Interferences • Recursive Ray-Tracing [Hirayama00-01,…] • Full model [Icart99-00,…] • Fine Spectral representation • RGB based BRDF • Interferences + Colour dispersion

  8. Overview • Previous Work • General Configuration • Glossy Case • Diffuse Case • Results • Conclusion

  9. Approach • Semi-transparent layer • Interferences effects • Local prism configuration • One refraction index by colour component • Non-parallel layer interfaces • Colour dispersion • RGB colour space • Commonly used in image production

  10. Layer configuration 0 Air 1 Layer 2 Support

  11. Resulting energy from interferences Parallel layers Uncorrelated layers Interference : Phase Change

  12. Color dispersion : Assumption

  13. BRDF general expression • k{r,g,b} • R (reflected BRDF) • 3 (RGB) lobe-like models • T (transmitted BRDF) • 3 (RGB) lobe-like models • Ex: using Phong models

  14. Overview • Previous Work • General Configuration • Glossy Case • Diffuse Case • Results • Conclusion

  15. Reflected part • Phong

  16. Transmitted part • Assumption • No absorption • Only one reflection Transmitted term

  17. Main Parameters • Normally r0(k)1 (air/vacuum) • Local geometric configuration : layer-normal • Material properties : exponents - indices • Fully determined by 4-12 parameters • 2-6 Exponents (control transition smoothness) • 1-3 RGB refraction indices (rB  rG  rR) • 1 Layer size • 0-2 Normal variation (colour dispersion)

  18. Overview • Previous Work • General Configuration • Glossy Case • Diffuse Case • Results • Conclusion

  19. Diffuse case  Average along direction Similar expression Assumptions No colour dispersion Rd average reflected energy

  20. Phase change for orthogonal incidence No absorption at the interface Diffuse component • Final expression

  21. Overview • Previous Work • General Configuration • Glossy Case • Diffuse Case • Results • Conclusion

  22. Diffuse component 67 - 73 nm 1 - 30 nm 1 - 210 nm

  23. Layer Size Change Constant normal deviation 148-200 nm 1-30 nm 79-106 nm

  24. Constant deviation Parallel Interfaces Constant deviation

  25. Size / Normal correlation 1-90 nm 1-10 nm rR = 1.5 rG = 1.7 rB = 1.8

  26. Overview • Previous Work • General Configuration • Glossy Case • Diffuse Case • Results • Conclusion

  27. Conclusion • RGB Model • Interferences and colour dispersion • Continuous along direction • Two models • Phong - like for specularity • Diffuse • Validation • Such effects are possible in colour space

  28. Future Work • With current model • Hardware acceleration (shader) • Try to fit some measured BRDF • Investigate other • Increase accuracy / More physical • Investigate colour spaces • Keep simplicity • Multi-layer

  29. Acknowledgements • IMAGER/ University of British Columbia • Post-doctoral position • PIMS Post-doctoral Fellowship • Wolfgang Heidrich & Lionel Bastard • Useful comments and support

  30. The End

  31. Fresnel term (Schlick approximation) Reflected part • Phong reflection

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