|Ground glass using the Beckman microfacet distribution.|
And finally, here's an image showing ground glass using the GGX distribution:
|Ground glass using the GGX microfacet distribution.|
I do like the look of the GGX distribution, but it has a few issues. It looks very good overall, but it's somewhat dark, because the GGX distribution has more shadowing and masking at this high roughness.
Another issue is that there are some fireflies (really bright spots), which are only occuring with GGX, but not Beckman. I have a feeling the fireflies could be due to very large sample weights that occur in rare cases, but I haven't looked into it deeply yet.
The paper ("Microfacet Models for Refraction through Rough Surfaces") mentions that sample weights can get huge at grazing angles (when i is close to perpendicular to n). They suggest widening the distribution width at grazing angles and provide math that works well for the Beckman distribution, which I'm currently also using for GGX, but I might need to tweak that math for GGX.
It also looks like the sample weight would get very large if the sampled microsurface normal is close to perpendicular to the macrosurface normal, and that seems to be much more likely with the GGX distribution because of its wider tails. So I should look into that as well.