I was looking at using the Dakota Red Granite macro from Thomas de Groot in a
scene that I am working on and found that it generates a material{}.  I had been
under the mistaken notion that you couldn't layer materials, until I read the
details on what they are, which made me happy.

But I started wondering how all of the components of a layered material actually
work and couldn't find a concise explanation.

The pigment part of the texture in a material makes sense to me, and I
understand that the filter/transmit values determine how to mix the texture
starting from the bottom to the top layer.  The other parts are not so clear to
me.

Normals:  Do upper layers add or replace the normal in the lower layer(s)?  It
doesn't seem that pigment transparency would have any effect on mixing the
normals from the various layers.

Finish: Same question, does it add or replace the finish from the lower layers?
If it replaces, does it only replace the elements of the finish not declared in
lower layers, or does it replace the entire finish?  I am pretty sure pigment
transparency doesn't affect this either, or else the standard layering tactic to
make the top layer a completely transparent pigment with the finish you want
wouldn't work.

Interior: This is what just recently occurred to me, and I have no idea what
happens here.  Again, do subsequent interior statements replace prior ones, or
just add what wasn't previously declared?

For most of my interiors I am only concerned with ior, so replacement makes
sense, but what about interior fading and media?

-- Chris R

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