A Simple Layered RGB BRDF Model

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A Simple LayeredRGB BRDF Model
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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

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Motivations

• Wavelength effects
• Interference
• Colour dispersion
• Investigation
• Framework

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Overview

• Previous Work
• General Configuration
• Glossy Case
• Diffuse Case
• Results
• Conclusion

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Overview

• Previous Work
• General Configuration
• Glossy Case
• Diffuse Case
• Results
• Conclusion

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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

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Wavelength effects

• Diffraction Stam99,Sun00,

• Interferences
• Recursive Ray-Tracing Hirayama00-01,
• Full model Icart99-00,
• Fine Spectral representation
• RGB based BRDF
• Interferences Colour dispersion

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Overview

• Previous Work
• General Configuration
• Glossy Case
• Diffuse Case
• Results
• Conclusion

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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

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Layer configuration
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Air
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Layer
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Support
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Interference Phase Change
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Color dispersion Assumption
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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

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Overview

• Previous Work
• General Configuration
• Glossy Case
• Diffuse Case
• Results
• Conclusion

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Reflected part

• Phong

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Transmitted part

• Assumption
• No absorption
• Only one reflection

• Transmitted term

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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)

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Overview

• Previous Work
• General Configuration
• Glossy Case
• Diffuse Case
• Results
• Conclusion

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Assumptions

• No colour dispersion

• Rd ? average reflected energy

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Diffuse component

• Final expression

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Overview

• Previous Work
• General Configuration
• Glossy Case
• Diffuse Case
• Results
• Conclusion

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Diffuse component
1 - 30 nm
67 - 73 nm
1 - 210 nm
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Layer Size Change
Constant normal deviation
1-30 nm
79-106 nm
148-200 nm
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Constant deviation
Parallel Interfaces
Constant deviation
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Size / Normal correlation
1-10 nm
1-90 nm
rR 1.5 rG 1.7 rB 1.8
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Overview

• Previous Work
• General Configuration
• Glossy Case
• Diffuse Case
• Results
• Conclusion

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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

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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

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Acknowledgements

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

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The End
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Reflected part

• Phong reflection