ngan05 brdf eval

Experimental Analysis of BRDF Models : 

Experimental Analysis of BRDF Models Addy Ngan1 Frédo Durand1 Wojciech Matusik2 MIT CSAIL1 MERL2 Eurographics Symposium on Rendering 2005

Goal: 

Goal Evaluate the performance of analytical reflectance models Based on measured data

Background: 

Background Bidirectional Reflectance Distribution Function

BRDF: 

BRDF Bidirectional Reflectance Distribution Function ρ(θi ,i ; θo, o)

BRDF: 

BRDF Bidirectional Reflectance Distribution Function ρ(θi ,i ; θo, o) Isotropic material Invariant when material is rotated BRDF is 3D

Previous Measurements: 

Previous Measurements Columbia-Utrecht Reflectance and Texture Database ~60 materials, 205 measurements per BRDF Cornell’s measurements ~10 materials, 1439 measurements per BRDF Bonn BTF Database 6 materials, 6561 view/light combinations Matusik’s image-based measurements ~100 materials, ~106 measurements per BRDF Include metals, plastic, paints, fabrics.

BRDF Models: 

BRDF Models Phenomenological Phong [75] Blinn-Phong [77] Ward [92] Lafortune et al. [97] Ashikhmin et al. [00] Physical Cook-Torrance [81] He et al. [91] Lafortune [97] Cook-Torrance [81]

Outline: 

Outline Background BRDF Measurements BRDF Fitting Isotropic materials results Anisotropic materials results Conclusion

BRDF Measurements: 

BRDF Measurements Isotropic : Data from Matusik [03] 100 materials chosen Reprocessed to remove unreliable data Flare Near grazing angle Anisotropic : New acquisition

Anisotropic Measurements: 

Anisotropic Measurements Similar to Lu et al. [00]

Anisotropic Measurements: 

Anisotropic Measurements 4 materials measured (brushed aluminum, satins, velvet) Each: 18 hours acquisition time, 30GB raw data Tabulated into bins in 2° intervals (~108 bins) 10-20% bins populated

Outline: 

Outline Background BRDF Measurements BRDF Fitting Isotropic materials results Anisotropic materials results Conclusion

BRDF Fitting: 

BRDF Fitting Target models: Blinn-Phong, Cook-Torrance, He et al., Lafortune et al. , Ward, Ashikhmin-Shirley Metric: RMS of (ρmeasured– M(p)) (cos θi) Linear w.r.t. diffuse/specular intensity

BRDF Fitting: 

BRDF Fitting Other potential metrics Logarithmic remapping Arbitrary scale Highlights overly blurry Perceptual metrics Context dependent Costly to compute/fit Intensity parameters become nonlinear – optimization less stable

Outline: 

Outline Background BRDF Measurements BRDF Fitting Isotropic materials results Anisotropic materials results Conclusion

Fitting Errors: 

Fitting Errors

Dark blue paint: 

Dark blue paint Acquired data Material – Dark blue paint Environment map

Dark blue paint: 

Dark blue paint Acquired data Cook-Torrance Material – Dark blue paint

Dark blue paint: 

Dark blue paint Acquired data Blinn-Phong Material – Dark blue paint

Dark blue paint: 

Dark blue paint Acquired data Ward Material – Dark blue paint

Dark blue paint: 

Dark blue paint Acquired data Lafortune Material – Dark blue paint

Dark blue paint – error plots: 

Dark blue paint – error plots Lafortune Cook-Torrance Blinn-Phong Ward

Dark blue paint: 

Dark blue paint Material – Dark blue paint Cook-Torrance fit, incidence plane, 4 different incident angles

Dark blue paint: 

Dark blue paint Cook-Torrance Material – Dark blue paint

Dark blue paint: 

Dark blue paint Original Lafortune Ashikhmin Cook-Torrance Material – Dark blue paint

Lafortune Lobe: 

Lafortune Lobe Distorted highlights near grazing angle Acquired data – gold paint Lafortune fit

Lafortune Lobe: 

Lafortune Lobe Acquired data – nickel Lafortune fit Distorted highlights near grazing angle

Lobe Comparison: 

Lobe Comparison Half vector lobe Gradually narrower when approaching grazing Mirror lobe Always circular Half vector lobe Mirror lobe

Half vector lobe: 

Half vector lobe Consistent with what we observe in the dataset. More details in the paper Example: Plot of 'PVC' BRDF at 55° incidence

Observations - numerical: 

Observations - numerical Rough order of quality He, Cook-Torrance, Ashikhmin Lafortune Ward Blinn-Phong Poor fit Good fit

Observations - visual: 

Observations - visual Mirror-like metals, some plastics All models match well visually Glossy paints, some metals, some wood Fresnel effect Distorted shape for Lafortune highlight Near diffuse fabrics, paints Fresnel effect

Observations: 

Some materials impossible to represent with a single lobe Observations Material – Red Christmas Ball Acquired data Cook-Torrance

Adding a second lobe: 

Adding a second lobe Material – Red Christmas Ball Some materials impossible to represent with a single lobe Acquired data Cook-Torrance 2 lobes

Outline: 

Outline Background BRDF Measurements BRDF Fitting Isotropic materials results Anisotropic materials results Conclusion

Anisotropic Materials: 

Anisotropic Materials

Brushed Aluminum: 

Brushed Aluminum Acquired data Ward Reasonable qualitative fit

Yellow Satin: 

Yellow Satin Reasonable qualitative fit Acquired data Ward

Purple Satin: 

Purple Satin Split highlights ?

Outline: 

Outline Background BRDF Measurements BRDF Fitting Isotropic materials results Anisotropic materials results Estimation of microfacet distribution Conclusion

Microfacet Theory: 

Microfacet Theory [Torrance andamp; Sparrow 1967] Surface modeled by tiny mirrors Value of BRDF at # of mirrors oriented halfway between L and Modulated by Fresnel, shadowing/masking [Shirley 97]

Estimating the MF distribution: 

Estimating the MF distribution Ashikhmin’s microfacet-based BRDF generator [00]

Estimating the MF distribution: 

Estimating the MF distribution Rearranging terms:

Estimating the MF distribution: 

Estimating the MF distribution depends on the distribution Iterate to solve for Compute using current estimate Estimate given Converges quickly in practice Measurements

Purple Satin: 

Purple Satin Split specular reflection microfacet distribution

Purple Satin: 

Purple Satin Acquired data microfacet distribution fit

Brushed Aluminum: 

Brushed Aluminum Acquired data microfacet distribution fit

Brushed Aluminum: 

Brushed Aluminum microfacet distribution fit Ward fit

MF-based BRDF generator: 

MF-based BRDF generator Expressive Easy to estimate No optimization necessary Inexpensive to compute

Outline: 

Outline Background BRDF Measurements BRDF Fitting Isotropic materials results Anisotropic materials results Conclusion

Conclusion: 

Conclusion Isotropic materials He, Cook-Torrance, Ashikhmin perform well Explicit Fresnel *multiple lobes help Half-vector based lobe performs better Most materials can be well-represented Anisotropic materials Cases where analytical models cannot match qualitatively Estimation of the microfacet distribution is straightforward Ashikhmin’s MF-based BRDF generator does well

Future Work: 

Future Work Metric Generalized lobe based on half vector Efficient acquisition based on the microfacet distribution

Acknowledgement: 

Acknowledgement Eric Chan, Jan Kautz, Jaakko Lehtinen, Daniel Vlasic NSF CAREER award 0447561 NSF CISE Research Infrastructure Award (EIA9802220) Singapore-MIT Alliance

Questions?: 

Questions?