realistic hair shading SI-MR

[Matt Lind]

@Peter

MR hair geometry is flat ribbon, but the normal is computed from a virtual 
cylinder like a normal map with full control over shaft radius and taper 
along the length.  For most practical purposes it should create the desired 
look.  Using the utility shaders (math/lookup nodes) available in the 
rendertree, you can read hair UV coords and surface normal yourself if you 
want to shade them differently than the provided hair shaders.  Mental ray 
stores hair information in the state which can be accessed by any shader 
which cares to dig into that information.

The reason for flat ribbon is to allow hair to be represented as physical 
geometry which can be styled and groomed, respond to dynamics, and use 
material/texture shaders for shading while keeping memory consumption 
low/reasonable (representing hair as geometry is hugely expensive).  Mental 
ray does offer true cylinder hair, but to get cylinder hair would require 
the hair shader to be implemented as a volume shader (which would have it's 
own set of issues).  I believe that was how the original softimage hair 
shader from 10 years ago was implemented.  It rendered hair convincingly for 
the basics such as highlights, but most people didn't like it because as a 
volume shader it did not permit styling and grooming (or at least not much 
control over it), and rendered quite slow with frequent crashing as mental 
ray volumes can be a bit finicky.

I experimented with hair rendering in mental ray for Barnyard all those 
years ago.  From a purely technical point of view, writing shaders for hair 
isn't too hard - it's just standard material/texture shader with additional 
metadata for the hair.  The key to getting results is being smart with your 
render settings.  You must be stringent on ray depth, recursion, shadow 
type, memory limits, and so on.  Setting them too generously will make your 
render times go through the roof as you're telling mental ray to wander and 
find things to do which probably aren't necessary.  the default render 
settings in Softimage are probably too generous for hair.


Matt