Mention anisotropic reflection approximations

Author: virtualzavie

approximation.md.html

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 The specular BRDF described in the specification is the classic GGX-Smith model, which has been common in both offline and real-time rendering for a decade. The so called "split-sum" approximation introduced by [#Karis2013] allows efficient rendering of the isotropic version of this BRDF with image-based lighting.
 
 
+### Anisotropic case
+
+Several solutions are possible for the anisotropic case:
+
+- [#Cocco2024] recently proposed an image-based lighting approximation of anisotropic specular reflection consisting in several samples along the main BRDF axis.
+
+- [#McAuley15] describes a simpler and cheaper method based on [#Revie2011], consisting in modifying normals and offering a very crude approximation of anisotropic reflection.
+
+- Finally, the crudest approximation would be to ignore entirely the `specular_roughness_anisotropy` for the base layer specular lobe (respectively `coat_roughness_anisotropy` for the coat specular lobe), as the anisotropy parametrization in designed to preserve the average roughness regardless of the anisotropy (see the Microfacet model section for details).
+
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index.html

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 [#Burley2018]: Brent Burley, David Adler, Matt Jen-Yuan Chiang, Hank Driskill, Ralf Habel, Patrick Kelly, Peter Kutz, Yining Karl Li, Daniel Teece. *The Design and Evolution of Disney’s Hyperion Renderer*, ACM TOG (2018).
 
+[#Cocco2024]: Giovanni Cocco, Cédric Zanni, and Xavier Chermain. *Anisotropic Specular Image-Based Lighting Based on BRDF Major Axis Sampling*, Eurographics (2024).
+
 [#Dupuy2023]: Jonathan Dupuy, Anis Benyoub. *Sampling Visible GGX Normals with Spherical Caps*, High-Performance Graphics (2023).
 
 [#Elias2001]: Mady Elias, Lionel Simonot, and Michel Menu. “Bidirectional reflectance of a diffuse background covered by a partly absorbing layer”. In: Optics Communications 191.1 (2001).
@@ -1556,6 +1558,8 @@
 
 [#Langlands2014]: Anders Langlands. *Physically Based Shader Design in Arnold*, ACM SIGGRAPH Talks (2014).
 
+[#McAuley15]: Stephen McAuley, *Rendering the World of Far Cry 4*, GDC (2015).
+
 [#McDermott2018]: Wes McDermott. [*The PBR Guide*](https://substance3d.adobe.com/tutorials/courses/the-pbr-guide-part-1), Allegorithmic white paper (2018).
 
 [#Neubelt13]: David Neubelt, Matt Pettineo. *Crafting a Next-Gen Material Pipeline for The Order: 1886*, ACM SIGGRAPH Courses: Physically Based Shading in Theory and Practice (2013).
@@ -1568,6 +1572,8 @@
 
 [#Polyanskiy2023]: Mikhail N. Polyanskiy. [*Refractive Index Database*](https://refractiveindex.info/) (2023).
 
+[#Revie2011]: Donald Revie, *Implementing Fur Using Deferred Shading*, GPU Pro 2 (2011).
+
 [#Schott2023]: SCHOTT. [*Interactive Abbe Diagram*](https://www.schott.com/en-gb/interactive-abbe-diagram) (2023).
 
 [#Smythe2016]: Doug Smythe and Jonathan Stone. [*MaterialX: An Open Standard for Network-Based CG Object Looks*](https://materialx.org)