Subject:
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Re: CW/CCW, vertex sequence, co-planar, convex, (115kB)
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Newsgroups:
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lugnet.cad.dev
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Date:
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Thu, 30 Sep 1999 14:11:38 GMT
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Viewed:
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607 times
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In lugnet.cad.dev, Lars C. Hassing writes:
> John VanZwieten wrote in message ...
> >
> > Leonardo Zide <leonardo@centroin.com.br> wrote
> > > "Lars C. Hassing" wrote:
> > > >
> > > > Here are 50 messages regarding the above subjects.
> > > > The first from January 1998 where Leonardo Zide suggested CW/CCW.
> > >
> > > I miss that kind of discussion, those were very interesting subjects.
>
> Me too!
>
> > > Also, where's Jeff Findley ?
> > >
> > > > I support the CW/CCW idea (isn't it annoying knowing almost 50% of all
> > > > faces are drawn to waste), though it will cost a considerable effort
> > > > fixing old parts. The process can however be automated: Leonardo already
> > > > has a program showing faces as red or green and Gary suggested a
> > > > ray-algorithm for automatically determining CW/CCW-ness.
> > >
> > > I've already thought about a ray intersection algorithm but I've never
> > > tried to implement it, I might try do it now. Some problems that I
> > > remember is that LDraw parts are not closed volumes (that's not the best
> > > description), for example: trace a ray from the top of a 1x1 brick and
> > > it will intersect 3 surfaces, and the program can't decide what's the
> > > correct orientation of the face in the middle.
> >
> >
> > What if when a ray intersected 3 surfaces, it made the first CCW, the last • CW,
> > and left the middle surface unmarked. If you traced enough rays from
> > different angles and starting positions, I think you would eventually be able
> > to mark all surfaces. In your 1x1 brick example, when you traced a ray that
> > didn't go through the top stud, you would have an even number of surfaces and
> > so could correctly mark them.
>
> "enough rays" - exactly what stroke me too when I read Leonardo's posting!
> With the fast computers nowadays we could shoot millions of rays at the
> part from all possible viewing angles (this might even disclose some
> invisible faces! - hm, are these needed when a part is drawn transparently?)
>
> > About the only time I see this not working is if a single quad is used both • as
> > an outer and an inner surface, which wouldn't exactly be great part design
> > form.
>
> Right.
>
> In addition to Leonardo's summary:
> Because the "what is inside/outside" of e.g. 4-4cyli.dat depends on the
> usage, and we don't want two sets of primitives, I think we need to
> decide on a "0 CCW" or "0 FACE CCW" or ?? keyword.
> (see the discussion I started on Feb 4th 1999)
What I was concerned about are situations where a single quad is used as the
outside of a section of a part and the inside of a section of a part. A
theoritical example would be a minifig head. You could use a single 4-
4cyli.dat to model the outside of the top stud and the inside of the hole in
the bottom, since they are the same diameter. Of course that would be bad
form.
-John Van
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Message has 1 Reply:
 | | Re: CW/CCW, vertex sequence, co-planar, convex, (115kB)
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| John VanZwieten wrote in message ... (...) the (...) in (...) What if the part that referenced such a primitive had a '0 invert' comment on the line before the primitive reference, if that instance of the primitive was intended to appear inside-out? (...) (25 years ago, 30-Sep-99, to lugnet.cad.dev)
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Message is in Reply To:
| | Re: CW/CCW, vertex sequence, co-planar, convex, (115kB)
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| John VanZwieten wrote in message ... (...) Me too! (...) "enough rays" - exactly what stroke me too when I read Leonardo's posting! With the fast computers nowadays we could shoot millions of rays at the part from all possible viewing angles (this (...) (25 years ago, 30-Sep-99, to lugnet.cad.dev)
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