On Wed, 28 Oct 1998 19:35:26 -0600, Dan Connelly <djc### [at] flashnet>
wrote:

>Ron Parker wrote:
>Further, I am not sure what you mean by tangent to a
>vertex, as a vertex is just a single point, and thus all
>intersections are equivalent.

Ah, this is why I put "tangent" in quotes.  There is, of
course, no tangent to a discontinuity, but all intersections
are not equivalent.  (warning: ugly ascii art ahead)

  \             \   /
   \             \ /
- - X - - - - - - Y - - - -
   /
  /   

Pretend the horizontal line is a ray.  The intersection on the left
should be counted, the one on the right not.  Determining which is
which in 3D is left as an exercise for the reader.

>Furthermore, open objects don't have a well-defined interior,
>so care is needed to make sure the object is a "solid" object....
>if it is formed by nondegenerate CSG, this is of course
>guaranteed.

Actually, there are closed surfaces that don't have well-defined
interiors, as well, though they self-intersect so they are detectable
(painfully.)

We're allowed to use open cylinders and prisms in CSG, despite the
fact that they don't behave.  Why should determining the suitability
of a mesh fall on the POV programmers?  Why not on the people making
the scene?

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