update spec

Author: jim-ec

extensions/2.0/Khronos/KHR_materials_anisotropy/README.md

@@ -14,13 +14,22 @@ Written against the glTF 2.0 spec.
 
 ## Overview
 
+This extension defines the anisotropic property of a material as observable with brushed metals for instance.
+An assymetric specular lobe model is introduced to allow for such phenomenon.
+The visually distinct feature of that lobe is the elongated appearance of the specular reflection.
+For a single punctual light source, the specular reflection will eventually degenerate into a zero width line in the limit,
+that is where the material is fully anisotropic, as opposed to be fully isotropic in which case the specular reflecation is radially symmetric.
+
+## Extending Materials
 
 ```json
 {
     "materials": [
         {
             "extensions": {
                 "KHR_materials_anisotropy": {
+                    "anisotropy": 0.6,
+                    "anisotropyDirection": [0.0, 1.0, 0.0]
                 }
             }
         }
@@ -30,7 +39,21 @@ Written against the glTF 2.0 spec.
 
 ### Definition
 
+## Anisotropy
+
+Two new material properties are introduced: an explicit anisotropy parameter and the direction in which the specular reflection elongates relative to the surface tangents.
+The anisotropy parameter is a dimensionaless number in `[-1, 1]` and forms a bijection to the roughness distribution along two orthogonal directions,
+one of which is the direction parameter and the other the result of the crossing the direction and the geometric normal.
+
+A anisotropy of `1` means that the specular reflection will elongate along the given direction,
+while a value of `-1` will elongate it along the computed orthogonal direction.
+
+|                 | -0.5 | 0.0 | 0.5 |
+| [1.0, 0.0, 0.0] | ![Fig. 1](figures/fig1.jpg) | ![Fig. 2](figures/fig2.jpg) | ![Fig. 3](figures/fig3.jpg)
+| [1.0, 1.0, 0.0] | ![Fig. 4](figures/fig4.jpg) | ![Fig. 5](figures/fig5.jpg) | ![Fig. 6](figures/fig6.jpg)
+
 ### Reference
 
-[Google Filament - Anisotropic model](https://google.github.io/filament/Filament.md.html#materialsystem/anisotropicmodel)  
+[Google Filament - Anisotropic model](https://google.github.io/filament/Filament.md.html#materialsystem/anisotropicmodel)
+[Google Filament Materials Guide - Anisotropic model](https://google.github.io/filament/Materials.md.html#materialmodels/litmodel/anisotropy)
 [Blender Principled BSDF](https://docs.blender.org/manual/en/latest/render/shader_nodes/shader/principled.html)