Modeling Anisotropic Surface Reflectance with Example-Based - PowerPoint PPT Presentation

Modeling Anisotropic Surface Reflectance with Example-Based Microfacet Synthesis Sean Bell and Zhiyuan Teo Cornell University CS 6630 2012 April 12 Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 1 / 43

Motivation Quest for ultimate realism. Reflectance models based on real physical data. Capture reduces work of the artist. Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 2 / 43

To rephrase our desires in graphics We want it simple . We want it realistic . We want it fast . Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 3 / 43

The BRDF ρ ( x , i , o ) = dL ( x , o ) dL ( x , o ) dE ( x , i ) = L ( x , i ) cos θ i d i Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 4 / 43

BRDF Models Phong (1975) : isotropic specular ( o · r ) α ρ ( i , o ) = k s n · i Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 5 / 43

BRDF Models Measuring and modeling anisotropic reflection , Ward, 1992 Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 6 / 43

BRDF Models Ward (1992) : anisotropic specular � cos2 φ h + sin2 φ h � − tan 2 θ h k s α 2 α 2 ρ ( i , o ) = √ cos θ i cos θ o e x y 4 πα x α y Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 7 / 43

BRDF Models Lafortune (1997) : anisotropic specular ρ ( i , o ) = k s ( C x i x o x + C y i y o y + C z i z o z ) n Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 8 / 43

BRDF Models Microfacet : general anisotropic Cook & Torrance (1982), Ashikhmin and Shirley (2000): D ( h ) S ( i ) S ( o ) F ( i , o ) ρ s ( x , i , s o ) = k s 4( i · n )( o · n ) Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 9 / 43

BRDF Models He et al (1991) : general anisotropic Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 10 / 43

BRDF Models: Comparison (2005) Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 11 / 43

A (sparse) sample of BRDF capture technologies: 1992 Flat object; photodetector and light source moveable. Ward, 1992. Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 12 / 43

A (sparse) sample of BRDF capture technologies: 1992 Gonioreflectometer. Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 13 / 43

A (sparse) sample of BRDF capture technologies Flat object; object rotatable, photodetector moveable. Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 14 / 43

A (sparse) sample of BRDF capture technologies: 1999 Curved object; camera moveable. Marschner et al., 1999. Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 15 / 43

A (sparse) sample of BRDF capture technologies: 2003 Image-based reconstruction of spatial appearance and geometric detail. Lensch et al, 2003. Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 16 / 43

A (sparse) sample of BRDF capture technologies: 2003 Linear light source reflectometry. Gardner et al, 2003. Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 17 / 43

A (sparse) sample of BRDF capture technologies: 2005 Ngan et al, 2005. Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 18 / 43

A (sparse) sample of BRDF capture technologies: 2005 Hawkins et al, 2005. Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 19 / 43

A (sparse) sample of BRDF capture technologies: 2008 Wang et al, 2008. Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 20 / 43

A review of the microfacet model Assume surface is composed of many small, flat micro mirrors (facets). Each surface reflects light according to its microfacet normal ( h ). A microfacet’s contribution only counts if it is visible to both i and o . Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 21 / 43

Microfacet BRDF The BRDF for the microfacet model is: ρ ( x , i , o ) = ρ d ( x , i , o ) + k s ( x ) ρ s ( x , i , o ) � �� � � �� � diffuse specular Diffuse term ρ d ( x , i , o ): constant and accounts for light that bounces multiple times Specular term ρ s ( x , i , o ): view-dependent Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 22 / 43

Microfacet specular term The specular includes anisotropy: 1 ρ s ( x , i , o ) = 4( i · n )( o · n ) D ( x , h ) G ( x , o , i ) F ( x , o , i ) � �� � � �� � � �� � NDF shadow term Fresnel term D : distribution of the microsurface normals (“NDF”) G : Smith shadowing term, which describes the visibility of the microsurface normal. F : Fresnel term, which describes reflection from each specular microsurface. Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 23 / 43

Experimental setup Linear light source is placed on a gantry, about 4cm above the sample. Idea: we know the position of the gantry and the camera, so we can get i and o . Move the light source to vary i , then sample the intensity at each point on the material. Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 24 / 43

Experimental setup Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 25 / 43

More specifically... Linear light source is an array of 40 point LED light sources. Switch on the LEDs one by one and sample the material. This ensures that the luminaire is a point source. Move the gantry in each step and repeat, to create a grid of point lights over the entire sample. Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 26 / 43

Data capture At the end of the run, each point on the material has been lit by LEDs from many angles ( i ). These are the raw image data samples I q ( x ). Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 27 / 43

Pre-processing Our experimental setup directly provides us with the BRDF: ρ ( x , i , o ) = dL ( x , o ) dE ( x , i ) Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 28 / 43

Pre-processing Our experimental setup directly provides us with the BRDF: ρ ( x , i , o ) = dL ( x , o ) dE ( x , i ) dL ( x , o ) = ( n · i q ( x )) L ( x , i ) d i Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 28 / 43

Pre-processing Our experimental setup directly provides us with the BRDF: ρ ( x , i , o ) = dL ( x , o ) dE ( x , i ) dL ( x , o ) = ( n · i q ( x )) L ( x , i ) d i I q ( x ) = ( n · i q ( x )) L q || P q − x || 2 Approximate the LED as a point source Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 28 / 43

Pre-processing Our experimental setup directly provides us with the BRDF: ρ ( x , i , o ) = dL ( x , o ) dE ( x , i ) dL ( x , o ) = ( n · i q ( x )) L ( x , i ) d i I q ( x ) = ( n · i q ( x )) L q || P q − x || 2 We could tabulate ρ ( x , i , o ) at this point Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 28 / 43

Pre-processing Our experimental setup directly provides us with the BRDF: ρ ( x , i , o ) = dL ( x , o ) dE ( x , i ) dL ( x , o ) = ( n · i q ( x )) L ( x , i ) d i I q ( x ) = ( n · i q ( x )) L q || P q − x || 2 We could tabulate ρ ( x , i , o ) at this point Large storage requirements Difficult to edit Gaps in the ( i , o ) domain Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 28 / 43

Pre-processing Dichromatic BRDF model ρ ( x , i , o ) = ρ d ( x , i , o ) + k s ( x ) ρ s ( x , i , o ) � �� � � �� � diffuse specular Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 29 / 43

Pre-processing Dichromatic BRDF model ρ ( x , i , o ) = ρ d ( x , i , o ) + k s ( x ) ρ s ( x , i , o ) � �� � � �� � diffuse specular Assumption: There is some ( i , o ) for which ρ s ( x , i , o ) ≈ 0 = ⇒ ρ d ( x ) = min ρ ( x , i q , o ) q I q ( x ) = min ( n · i q ( x )) L q || P q − x || 2 q Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 29 / 43

Pre-processing Subtracting away the diffuse component leaves the specular component: ρ s ( x , i q ( x ) , o ) = ρ ( x , i q ( x ) , o ) − ρ d ( x ) � �� � raw data Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 30 / 43

Pre-processing Subtracting away the diffuse component leaves the specular component: ρ s ( x , i q ( x ) , o ) = ρ ( x , i q ( x ) , o ) − ρ d ( x ) � �� � raw data But there’s another problem: Use push-pull (Gortler, 1996) to interpolate measurements. Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 30 / 43

Analysis of the data Microfacet Model ρ s ( x , i , o ) = k s ( x ) D ( x , h ) S ( x , i ) S ( x , o ) F ( x , i , o ) 4( i · n )( o · n ) Sean Bell and Zhiyuan Teo (Cornell) Fitting Anisotropic BRDFs 2012 April 12 31 / 43