A Simple Layered RGB BRDF Model Xavier Granier - Wolfgang Heidrich IMAGER / University of British Columbia

2 IMAGER / UBCA Simple layered RGB BRDF 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 IMAGER / UBCA Simple layered RGB BRDF Motivations  Wavelength effects  Interference  Colour dispersion  Investigation  Framework

4 IMAGER / UBCA Simple layered RGB BRDF Overview  Previous Work  General Configuration  Glossy Case  Diffuse Case  Results  Conclusion

5 IMAGER / UBCA Simple layered RGB BRDF Overview  Previous Work  General Configuration  Glossy Case  Diffuse Case  Results  Conclusion

6 IMAGER / UBCA Simple layered RGB BRDF 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 IMAGER / UBCA Simple layered RGB BRDF 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 IMAGER / UBCA Simple layered RGB BRDF Overview  Previous Work  General Configuration  Glossy Case  Diffuse Case  Results  Conclusion

9 IMAGER / UBCA Simple layered RGB BRDF 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 IMAGER / UBCA Simple layered RGB BRDF Layer configuration Air Layer Support

11 IMAGER / UBCA Simple layered RGB BRDF Interference : Phase Change  Uncorrelated layers  Resulting energy from interferences  Parallel layers

12 IMAGER / UBCA Simple layered RGB BRDF Color dispersion : Assumption

13 IMAGER / UBCA Simple layered RGB BRDF 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 IMAGER / UBCA Simple layered RGB BRDF Overview  Previous Work  General Configuration  Glossy Case  Diffuse Case  Results  Conclusion

15 IMAGER / UBCA Simple layered RGB BRDF Reflected part  Phong

16 IMAGER / UBCA Simple layered RGB BRDF Transmitted part  Assumption  No absorption  Only one reflection  Transmitted term

17 IMAGER / UBCA Simple layered RGB BRDF Main Parameters  Normally r 0 (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 ( r B  r G  r R )  1 Layer size  0-2 Normal variation (colour dispersion)

18 IMAGER / UBCA Simple layered RGB BRDF Overview  Previous Work  General Configuration  Glossy Case  Diffuse Case  Results  Conclusion

19 IMAGER / UBCA Simple layered RGB BRDF Assumptions  Similar expression  Diffuse case  Average along direction  R d  average reflected energy  No colour dispersion

20 IMAGER / UBCA Simple layered RGB BRDF Diffuse component  Phase change for orthogonal incidence  Final expression  No absorption at the interface

21 IMAGER / UBCA Simple layered RGB BRDF Overview  Previous Work  General Configuration  Glossy Case  Diffuse Case  Results  Conclusion

22 IMAGER / UBCA Simple layered RGB BRDF Diffuse component nm nm nm

23 IMAGER / UBCA Simple layered RGB BRDF Layer Size Change 1-30 nm nm nm Constant normal deviation

24 IMAGER / UBCA Simple layered RGB BRDF Constant deviation Parallel InterfacesConstant deviation

25 IMAGER / UBCA Simple layered RGB BRDF Size / Normal correlation 1-10 nm 1-90 nm r R = 1.5 r G = 1.7 r B = 1.8

26 IMAGER / UBCA Simple layered RGB BRDF Overview  Previous Work  General Configuration  Glossy Case  Diffuse Case  Results  Conclusion

27 IMAGER / UBCA Simple layered RGB BRDF 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 IMAGER / UBCA Simple layered RGB BRDF 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 IMAGER / UBCA Simple layered RGB BRDF Acknowledgements  IMAGER / University of British Columbia  Post-doctoral position  PIMS Post-doctoral Fellowship  Wolfgang Heidrich & Lionel Bastard  Useful comments and support

30 IMAGER / UBCA Simple layered RGB BRDF The End

31 IMAGER / UBCA Simple layered RGB BRDF Reflected part  Phong reflection  Fresnel term (Schlick approximation)