Subject:
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Re: concave polys and LDraw
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Newsgroups:
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lugnet.cad.dev
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Date:
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Fri, 26 Feb 1999 17:33:03 GMT
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Viewed:
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762 times
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Lars C. Hassing writes:
> It seems that LDraw first transforms the 4 vertices and then fills
> the four-sided polygon or triangle (if one vertex is in the middle of
> the others) now spanned between the transformed vertices. This also
> takes care of bow-ties and non-coplanar quads!
> The rule may be clear, but is the result desirable?
unfortunately because of our limitatiions here, i can't illustrate the
problem... but if you take a 4-pointed chevron-type shape... sort of like a "V"
and hand its points off to LDraw, it comes out as a triange... that's bad if
you need the "crotch" of the "V" to be seen through... so insead of a 4-vert
quad, i need to determine concavity and then just draw 2 adjoining triangles...
that part is easy, because my conversion routine is basically looking to "mate"
pairs of adjoined coplanar triangles and make them quads... so if it finds two
it can mate, but they create a concave poly, i just don't mate them and instead
draw them as their original separate triangles... the complicated part comes in
determining concavity... i found a math page that tells how to do it via a
matrix determinant (the same type of calc i used to determine coplanarism of
the two triangles in the first place)... so i think i can get it together...
J
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Message is in Reply To:
 | | RE: concave polys and LDraw
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| (...) There was a thread about it back in June 1998 ("LDraw and quads"), I don't think we ever decided if quads were allowed to be concave. It seems that LDraw first transforms the 4 vertices and then fills the four-sided polygon or triangle (if one (...) (25 years ago, 26-Feb-99, to lugnet.cad.dev)
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