02941 Physically Based RenderingMicrofacet Models

Jeppe Revall Frisvad

June 2021

From smooth to rough surfaces

I Triangles can represent any surface.I Smooth: Use interpolated vertex normals.I Rough: Use triangle face normals.

I If surface features are very small:I Triangles are very small (numerical problems).I Triangles are numerous (computation is expensive).

I Microfacet BRDF models.I A BRDF can replace tiny surface features with randomly oriented microfacets.

Mesoscopic Bidirectional Reflectance Distribution Function

I In general, we have the bidirectional scattering-surface reflectance distributionfunction (BSSRDF):

S(X ; xi , ~ωi ; xo , ~ωo) =dLo(xo , ~ωo)

dΦi (xi , ~ωi ),

where X is the object boundary and Lo is the radiance reflected in the direction ~ωo

at the position xo due to the flux incident at the position xi from the direction ~ωi .

I Suppose we consider surface scattering only, or interior scattering so large that anelement of irradiance (dE = Li cos θidωi ) has an influence only for xo ∈ Ai , then∫

Ai

S(X ; xi , ~ωi ; xo , ~ωo) dAi →dLo(xo , ~ωo)

dΦi (xi , ~ωi )dA =

dLo(x , ~ωo)

dE (x , ~ωi )= fr (x , ~ωi , ~ωo)

for Ai → dAo such that xi ≈ xo = x .

I This is a special case of the BRDF (fr ) for which we may assume dAi = dAo = dA.

Microfacet surfaces

I A microfacet surface is modelled by a BRDF that scatters light in more than onedirection.

I One way to describe this is by a distribution of microfacet normals.

Figure by Pharr and Humphreys [Physically Based Rendering, Morgan Kaufmann/Elsevier, 2004]

The originsscalar diffraction by surface elements around a plane

I The scattering of electromagnetic waves from rough surfaces.I Overview by Beckmann and Spizzichino [1963].

I How to develop facet normal distribution functions.V-grooves

I Translation to geometrical optics.I Theory for off-specular reflection from roughened surfaces

by Torrance and Sparrow [1967].I Introducing the BRDF model

fr (x , ~ωi , ~ωo) =FGD

4(~n · ~ωi )(~n · ~ωo)=

FGD

4 cos θi cos θo,

whereF is the Fresnel reflectance,G is the geometrical attenuation factor,D is the facet normal distribution function.

References

- Beckmann, P., and Spizzichino, A. The Scattering of Electromagnetic Waves from Rough Surfaces. International Series of Monographs onElectromagnetic waves, Vol. 4, Pergamon Press, 1963.

- Torrance, K. E., and Sparrow, E. M. Theory of off-specular reflection from roughened surfaces. Journal of the Optical Society of America57(9), pp. 1105-1114, September 1967.

Geometrical attenuation

I Important effects to consider:

(a) Masking.(b) Shadowing.(c) Interreflections.

Figure by Pharr and Humphreys [Physically Based Rendering, Morgan Kaufmann/Elsevier, 2004]

Facet normal distributionsI Assuming perfectly specular microfacets, facets with normals

~ωh =~ωo + ~ωi

‖~ωo + ~ωi‖

contribute to the reflected radiance in the direction ~ωo .

I Subscripts denote: i - incident, o - observed, h - half.I D(~ωh) is the facet normal distribution. Then

I D(~ωh) dωh is the proportion of facets with normals in the solid angle dωh.I D(~ωh) dωh dA is the proportion of facets in the area dA with normals in dωh.I D(~ωh) dωh cos θhdA is the proportion of facets in the projected area cos θhdA with

normals in dωh, where cos θh = ~ωi · ~ωh.

I The flux incident at the projected area of the microfacets with normals in dωh istherefore (using the definition of radiance)

dΦi = LiD(~ωh) dωh cos θhdA dωi ,

where Li is radiance incident from the direction ~ωi .

Facet normal distributionsI The flux incident at the projected area of the microfacets with normals in dωh is

therefore (using the definition of radiance)

dΦi = LiD(~ωh) dωh cos θhdA dωi ,

where Li is radiance incident from the direction ~ωi .

I Taking into account Fresnel reflectance and geometrical attenuation, the reflectedflux is

dΦo = F (cos θh)G (~ωi , ~ωo) dΦi .

I The BRDF model is then (using dE = Li cos θi dωi )

fr (x , ~ωi , ~ωo) =dLodE

=dΦo

cos θo dA dωo

/dE

=F (cos θh)G (~ωi , ~ωo) LiD(~ωh) dωh cos θhdA dωi

cos θo dA dωo Li cos θi dωi.

Facet normal distributionsI The BRDF model is then (using dEi = Li cos θi dωi )

fr (x , ~ωi , ~ωo) =dωh cos θh

dωo

F (cos θh)G (~ωi , ~ωo)D(~ωh)

cos θi cos θo.

I Expressing the solid angles in spherical coordinates with ~ωi as zenith, we have

dωh = sin θh dθh dφh ,

dωo = sin θ′o dθ′o dφ′o .

where θ′o is the angle between ~ωi and ~ωo .

I Then according to the law of reflection φ′o = φh and θ′o = 2θh.

I This means that

dωh

dωo=

sin θh dθh dφhsin(2θh) d(2θh) dφh

=sin θh

2 cos θh sin θh 2=

1

4 cos θh=

1

4(~ωi · ~ωh).

I Inserting this result gives the Torrance-Sparrow BRDF model.

Microfacet models in graphics

I Introduced by Blinn [1977].I The Torrance-Sparrow model with different microfacet distributions (D):

I The modified Phong [1975] model for D (cosine lobe distribution using half-vector).I The Torrance-Sparrow [1967] model for D (Gaussian distribution).I A model by Trowbridge and Reitz [1975] for D (microfacets as ellipsoids of

revolution).

I There are other options as well.I See Cook and Torrance [1981] and Walter et al. [2007].

References

- Blinn, J. F. Models of light reflection for computer synthesized pictures. Computer Graphics (Proceedings of ACM SIGGRAPH 77) 11(2),pp. 192-198, 1977.

- Phong, B. T. Illumination for computer generated images. Communications of the ACM 18(6), pp. 311–317, June 1975.

- Torrance, K. E., and Sparrow, E. M. Theory of off-specular reflection from roughened surfaces. Journal of the Optical Society of America57(9), pp. 1105-1114, September 1967.

- Trowbridge, T. S., and Reitz, K. P. Average irregularity representation of a roughened surface for ray reflection. Journal of the OpticalSociety of America 65(5), pp. 531-536, 1975.

- Cook, R. L., and Torrance, K. E. A reflectance model for computer graphics. Computer Graphcis (Proceedings of ACM SIGGRAPH 81)15(3), pp. 307-316, August 1981.

- Walter, B., Marschner, S. R., Li, H., and Torrance, K. E. Microfacet models for refraction through rough surfaces. In Proceedings ofEurographics Symposium on Rendering (EGSR 2007), pp. 195–206, 2007.

The Torrance-Sparrow model

Image by Pharr and Humphreys [Physically Based Rendering, Morgan Kaufmann/Elsevier, 2004]

I Using the Blinn-Phong microfacet distribution.

Newer microfacet modelsI Oren-Nayar [1994].

I Using Lambertian microfacets.

I Lafortune [1997].I Using multiple Phong lobes.

I Ashikhmin-Shirley [2000].I Two Phong lobes and Fresnel reflectance.

I Weidlich and Wilkie [2007, 2009]I Layered microfacet models.

References

- Oren, M., and Nayar, S. K. Generalization of Lambert’s reflectance model. In Proceedings of ACM SIGGRAPH 94, pp. 239-246, 1994.

- Lafortune, E. P. F., Foo, S.-C., Torrance, K. E., and Greenberg, D. P. Non-linear approximation of reflectance functions. In Proceedings ofACM SIGGRAPH 97, pp. 117-126, 1997.

- Ashikhmin, M., and Shirley, P. An anisotropic Phong BRDF model. Journal of Graphics Tools 5(2), pp. 25-32, 2000.

- Weidlich, A., and Wilkie, A. Arbitrarily layered micro-facet surfaces. In Proceedings of GRAPHITE 2007, pp. 171–178, ACM, 2007.

- Weidlich, A., and Wilkie, A. Exploring the potential of layered BRDF models. ACM SIGGRAPH Asia 2009 Course Notes, ACM Press, 2009.

The Oren-Nayar model

Lambertian Oren-NayarImages by Pharr and Humphreys [Physically Based Rendering, Morgan Kaufmann/Elsevier, 2004]

The Lafortune model

Image by Pharr and Humphreys [Physically Based Rendering, Morgan Kaufmann/Elsevier, 2004]

The Ashikhmin-Shirley model

Image by Pharr and Humphreys [Physically Based Rendering, Morgan Kaufmann/Elsevier, 2004]

The Weidlich-Wilkie model

Image by Weidlich and Wilkie [2007]

Importance samplingI The rendering equation:

Lo(x , ~ωo) = Le(x , ~ωo) +

∫2π

fr (x , ~ωi , ~ωo)Li (x , ~ωi ) cos θi dωi .

I The Monte Carlo estimator:

LN(x , ~ωo) = Le(x , ~ωo) +1

N

N∑j=1

fr (x , ~ωij , ~ωo)Li (x , ~ωij ) cos θi

pdf(~ωij ).

I Make the pdf cancel out the BRDF or part of it.I The Torrance-Sparrow BRDF:

fr (x , ~ωi , ~ωo) =FGD

4(~n · ~ωi )(~n · ~ωo)=

FGD

4 cos θi cos θo.

I The geometry term (χ+(a) is one if a > 0, zero otherwise):

G (~ωo , ~ωi ) = χ+

(cos θhcos θi

)χ+

(cos θhcos θo

)min

(1,

2(~n · ~ωh) cos θocos θh

,2(~n · ~ωh) cos θi

cos θh

).

Sampling the Blinn microfacet distributionI The Blinn microfacet normal distribution (s is shininess):

D(~ωh) = χ+(~n · ~ωh)s + 2

2π(~n · ~ωh)s .

I Cosine lobe:

pdf(~ωij ) =s + 2

2π

1

4 cos θh(~n · ~ωhj )

s+1 .

I Sampling technique: ~ωij = 2(~ωo · ~ωhj )~ωhj − ~ωo with

~ωhj = (θ, φ) =(

cos−1(s+2√ξ1

), 2πξ2

),

where θ, φ are spherical coordinates in the tangent space of the surface normal ~n,while ξ1, ξ2 ∈ [0, 1] are uniform random variables.

I The estimator

Lr ,N(x , ~ω) =1

N

N∑j=1

F (~ωij · ~ωhj )~ωo · ~ωhj

(cos θo)(~n · ~ωhj )G (~ωo , ~ωij )Li (x , ~ωij ) .

Exercises

I Choose a microfacet normal distribution function (Blinn or Beckmann or GGX) inthe paper by Walter et al. [2007].

I Implement sampling of the chosen normal distribution function.

I Implement shading of a glossy surface using a microfacet model.I Suggested algorithm:

- Retrieve the normal of the intersected macrosurface (~n ).- Sample a microfacet normal ( ~m = ~ωh) in the hemisphere around ~n.- Perform the same operation as in shading of a transparent object, but use the

sampled microfacet normal ~m instead of ~n.- Multiply the result by the geometric attenuation factor G and the ratio of cosine

terms in the estimator ~ωo ·~m(~ωo ·~n)(~n·~m) .

I Same approach can be used for metals, but then the Fresnel factor becomes anRGB vector and everything refracted is absorbed.

References

- Walter, B., Marschner, S. R., Li, H., and Torrance, K. E. Microfacet models for refraction through rough surfaces. In Proceedings ofEurographics Symposium on Rendering (EGSR 2007), pp. 195–206, 2007.