I'm a little confused by part of the implementation of the shading model
used in POV-Ray.  There are parts that have changed substantially over
the last couple of versions and I don't quite see why.

1) When transmit is used, the shading model is now using the refracted
ray to determine the color.  That wasn't how (or why) I added transmit -
it's supposed to be an unrefracted ray (unaffected by IOR).  You might
as well not bother with transmit - in it's current form it just provides
a way to add white to the basic filter color.

2) The ambient and diffuse contributions are modified by (1 -
transparency).  Why?  And why just the ambient/diffuse and not specular
or reflection?

3) The shadow rays through a (partially) transparent surface aren't
modified the same way as eye rays.  You end up with colored shadows that
are either brighter or darker than the surface itself.  Results are even
worse with distance based attenuation (extinction) in an interior.

Don't know that I'd call any of these bugs - POV-Ray works the way it
works.  I just wonder if anyone knows why these choices were made.

Xander

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