ARGH!  I don't beleive that I missed this one, as well. The cross product
definition is the most inclusive. I should have seen that. We just covered it
in calculus

Josh

Ron Parker wrote:

> On 5 Jun 2000 06:01:51 -0400, Warp wrote:
> >  About a recent post in p.general:
> >
> >  When is a triangle degenerate?
> >  I think that one case is when two of the vertices or all the three are
> >actually the same point. In this case it would be just an infinitely thin
> >line or point.
>
> Or when they fall in a line, such as x, 2*x, 3*x.  The actual condition is
> "when the cross product of P1-P2 and P3-P2 is zero."  Note that this is not
> the usual "within epsilon of zero" condition; it must actually be zero.
>
> >  I faintly remember that povray also considers denerate a smooth triangle
> >with the vertex normals pointing to different sides of the triangle. Is
> >this so?
>
> The comment in triangle.c says
>
>   /* Degenerate if smooth normals are more than 90 from actual normal
>      or its inverse. */
>
> where the actual normal is the aforementioned cross product after
> normalization.  The actual test, however, makes more sense: it requires
> that the signs of the dot products of the corner normals with the actual
> normal must either be all positive or all negative.  In short, what you
> said, with the additional requirement that corner normals in the plane
> of the triangle are also degenerate.
>
> This calculation, too, ignores epsilon.
>
> --
> Ron Parker   http://www2.fwi.com/~parkerr/traces.html
> My opinions.  Mine.  Not anyone else's.

--
Josh English
eng### [at] spiritonecom
"May your hopes, dreams, and plans not be destroyed by a few zeros."

Post a reply to this message