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A Simple LayeredA Simple LayeredRGB BRDF ModelRGB BRDF Model
Xavier Granier - Wolfgang HeidrichIMAGER / University of British Columbia
2 IMAGER / UBCA Simple layered RGB BRDF
MotivationsMotivations
Increase the range of possible effects For graphic content creation
Work in Color Space RBG - XYZ - LMSCurrently limited to linear reflection
Convincing and simple modelNot a full simulationEffect as realistic as possible
3 IMAGER / UBCA Simple layered RGB BRDF
MotivationsMotivations
Wavelength effectsInterferenceColour dispersion
Investigation Framework
4 IMAGER / UBCA Simple layered RGB BRDF
OverviewOverview
Previous Work General Configuration Glossy Case Diffuse Case Results Conclusion
5 IMAGER / UBCA Simple layered RGB BRDF
OverviewOverview
Previous Work General Configuration Glossy Case Diffuse Case Results Conclusion
6 IMAGER / UBCA Simple layered RGB BRDF
Uniform BRDFUniform 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 effectsWavelength effects
Diffraction [Stam99,Sun00,…]
InterferencesRecursive Ray-Tracing [Hirayama00-01,…]Full model [Icart99-00,…]
Fine Spectral representation RGB based BRDF Interferences + Colour dispersion
8 IMAGER / UBCA Simple layered RGB BRDF
OverviewOverview
Previous Work General Configuration Glossy Case Diffuse Case Results Conclusion
9 IMAGER / UBCA Simple layered RGB BRDF
Approach Approach
Semi-transparent layerInterferences effects
Local prism configurationOne refraction index by colour componentNon-parallel layer interfacesColour dispersion
RGB colour spaceCommonly used in image production
10 IMAGER / UBCA Simple layered RGB BRDF
Layer configurationLayer configuration
0
1
20n
1n
d
i
o
ir
it
tirot
Air
Layer
Support
11 IMAGER / UBCA Simple layered RGB BRDF
Interference : Phase ChangeInterference : Phase Change
0
1 , nttdr
oi
Uncorrelated layers
Resulting energy from interferences trtr IIIII 2cos2
0
1 ,2 ntdr
i
Parallel layers
12 IMAGER / UBCA Simple layered RGB BRDF
Color dispersion : AssumptionColor dispersion : Assumption
i o
o
i
i o
13 IMAGER / UBCA Simple layered RGB BRDF
BRDF BRDF general general expressionexpression
k{r,g,b} R (reflected BRDF)
3 (RGB) lobe-like models T (transmitted BRDF)
3 (RGB) lobe-like models Ex: using Phong models
koiTkoiRkoi
koiTkoiRkoi
,,,,,,2cos2
,,,,,,
14 IMAGER / UBCA Simple layered RGB BRDF
OverviewOverview
Previous Work General Configuration Glossy Case Diffuse Case Results Conclusion
15 IMAGER / UBCA Simple layered RGB BRDF
Reflected partReflected part
Phong 0,,,,e
i orkifkoiR
0n
1n
i o
ir
16 IMAGER / UBCA Simple layered RGB BRDF
Transmitted partTransmitted part
Assumption No absorption Only one reflection
1,,,,,e
otio trkttkitkoiT
Transmitted term
d
i
it
tirot
kvfkvt ,1,
i
ot
17 IMAGER / UBCA Simple layered RGB BRDF
Main ParametersMain 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 IMAGER / UBCA Simple layered RGB BRDF
OverviewOverview
Previous Work General Configuration Glossy Case Diffuse Case Results Conclusion
19 IMAGER / UBCA Simple layered RGB BRDF
AssumptionsAssumptions
kTkRkkTkRk dddddd 2cos2
Similar expression
kRTkTkRk 2cos2
Diffuse case Average along direction
Rd average reflected energy
No colour dispersion
20 IMAGER / UBCA Simple layered RGB BRDF
Diffuse componentDiffuse component
kTkTk
dkrk ddd
1
)(4cos21 1
Phase change for orthogonal incidence
)(4 1
k
dkr
Final expression
No absorption at the interface kRkT dd 1
21 IMAGER / UBCA Simple layered RGB BRDF
OverviewOverview
Previous Work General Configuration Glossy Case Diffuse Case Results Conclusion
22 IMAGER / UBCA Simple layered RGB BRDF
Diffuse componentDiffuse component
1 - 30 nm67 - 73 nm 1 - 210 nm
23 IMAGER / UBCA Simple layered RGB BRDF
Layer Size ChangeLayer Size Change
1-30 nm 79-106 nm 148-200 nm
Constant normal deviation
24 IMAGER / UBCA Simple layered RGB BRDF
Constant deviationConstant deviation
Parallel Interfaces Constant deviation
25 IMAGER / UBCA Simple layered RGB BRDF
Size / Normal correlationSize / Normal correlation
1-10 nm 1-90 nmrR = 1.5 rG = 1.7 rB = 1.8
26 IMAGER / UBCA Simple layered RGB BRDF
OverviewOverview
Previous Work General Configuration Glossy Case Diffuse Case Results Conclusion
27 IMAGER / UBCA Simple layered RGB BRDF
ConclusionConclusion
RGB ModelInterferences and colour dispersionContinuous along direction
Two modelsPhong - like for specularityDiffuse
ValidationSuch effects are possible in colour space
28 IMAGER / UBCA Simple layered RGB BRDF
Future WorkFuture Work
With current modelHardware acceleration (shader)Try to fit some measured BRDF
Investigate otherIncrease accuracy / More physicalInvestigate colour spacesKeep simplicity
Multi-layer
29 IMAGER / UBCA Simple layered RGB BRDF
AcknowledgementsAcknowledgements
IMAGER / University of British ColumbiaPost-doctoral position
PIMS Post-doctoral Fellowship
Wolfgang Heidrich & Lionel BastardUseful comments and support