Add white furnace testing section (#192)

Addresses #146.

index.html

@@ -1365,6 +1365,31 @@
 A more accurate implementation should in principle account for these effects.
 
 
+White furnace testing
+-------------------------------------
+
+The ground truth appearance should perfectly preserve energy in various configurations, in which case the material will satisfy a _white furnace test_ where the object should "disappear" (i.e. merge into the background) when illuminated by uniform background light, and rendered in the limit of a large number of bounces. We list here for convenience the configurations in which such a white furnace test should pass, all of which would be useful to check as part of a unit testing framework:
+
+   - A fully metallic base (**`base_metalness`**=1) with white **`base_color`** and white **`specular_color`**.
+   - A fully dielectric base (**`base_metalness`**=0), with white **`specular_color`**, including:
+       - diffuse base with white **`base_color`**.
+       - subsurface (**`subsurface_weight`**=1) with white **`subsurface_color`**.
+       - translucent base (**`transmission_weight`**=1) with either:
+            - white **`transmission_color`** if **`transmission_depth`**=0, or
+            - white **`transmission_scatter`** if **`transmission_depth`** > 0.
+   - Any mix of the above.
+   - All of the above, with a coat (**`coat_weight`** > 0) and white **`coat_color`**.
+   - All of the above, with fuzz (**`fuzz_weight`** > 0) and white **`fuzz_color`**.
+
+In all of the above cases, the white furnace test should also continue to pass regardless of:
+
+   - the presence of thin-film (via **`thin_film_weight`**) as the film is non-absorptive.
+   - NDF roughness or anisotropy (**`specular_roughness`**, **`specular_roughness_anisotropy`**, and **`coat_roughness`**, **`coat_roughness_anisotropy`**).
+   - volume anisotropy (**`subsurface_scatter_anisotropy`**, **`transmission_scatter_anisotropy`**).
+   - the presence of bump/normal mapped base or coat (**`geometry_normal`**, **`geometry_coat_normal`**)
+   - opacity (**`geometry_opacity`**).
+
+
 MaterialX reference implementation
 ----------------------------------