I tried to implement some BRDFs, but kind of failed.
I tried really hard to implement the Ashikmin-Shirley anistropic brdf model, but it was producing all sorts of strange result I can only conclude that there are probably some nasty bugs in the rendering engine… I spent a week trying to make it work.
I also tried to implement refraction through rough surfaces a paper by Walter et al. with mixed success. My implementation doesnt seem to play well with the GGX or Beckmann distribution so theres definitely a bug somewhere.
I rendered a similar image to the one in the paper; a glass sphere with the world map etched in.