On Sun, 25 Jul 1999 09:48:00 -0400, Dave Kreskowiak wrote:
>In my experience, coincident surfaces tend to render a jittered imaged of
>both textures, not a black spot. 

Black spots (actually transparent spots) are fairly common.  They usually
happen when you have an intersection involving coincident surfaces.  POV
will find one or the other surface, then determine that that surface is
"outside" the other surface and ignore that intersection.  It then goes
on to find another intersection, but because it always starts looking at
a distance of EPSILON along the ray from the last intersection, it misses
the second surface at that point entirely, leaving a transparent spot 
that could show up as black under the right circumstances.

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