Check out this interesting article from FxGuide on Pixar's short film, Piper. For those of us who are very used to using raytracers most of their comments will not come at all as a surprise, however the work on the sand is impressive.
https://www.fxguide.com/featured/the-tech-of-pixar-part-1-piper-daring-to-be-different/#disqus_thread
