I'm playing around with hair rendering these days and have implemented the shading model that is used in AMD's Tress FX sample which I believe is the kajiya-kay model.
However having done that I'm not really satisfied with the results. Especially after using a physically based GGX for my regular shading.
UE4 has recently worked and presented something that can be seen in this Video here: https://youtu.be/toLJh5nnJA8?t=2598 which looks really nice.
I've also no idea how to use my image-based-lighting with this Kind of BRDF since using my ggx convoluted probes the normal way looks very wrong.
Since the kajiya-kay diffuse term uses something like
float cosTL = saturate(dot(hairTangent, LightDirection)); // dot(T, L) instead of dot(N, L)
I was thinking maybe use the Tangent as the lookup vector for the irradiance map and something similar for the glossy env map but that also didn't seem to work out of the box.
Brian Karis told me on Twitter he was using an approximation that square-enix uses however I could not find anything about that on the Internet.
Does anyone know of a physically based hair model (marschner?) that can be used for realtime shading ?
And does somebody have an idea how to solve the IBL Problem ?
Update: After watching the Video again I found the paper his shading model is based on (not sure I'll be able to translate that into Code but I'll try)