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 13:13:41 GMT
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Viewed:
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1825 times
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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)
Also, the part's primitives should be loaded into the test-program.
/Lars
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Message has 2 Replies:
 | | Re: CW/CCW, vertex sequence, co-planar, convex, (115kB)
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| (...) CW, (...) as (...) 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- (...) (27 years ago, 30-Sep-99, to lugnet.cad.dev)
| | | Re: CW/CCW, vertex sequence, co-planar, convex, (115kB)
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| (...) When you draw a part with a pattern transparently, I guess it's not a good idea to remove the backfaces or you might not see the pattern from some angles. (...) About that discussion, IMO it would be better to have *everything* CCW instead of (...) (27 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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| Leonardo Zide <leonardo@centroin.com.br> wrote in message news:37F24ACE.49636F....com.br... (...) 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 (...) (27 years ago, 29-Sep-99, to lugnet.cad.dev)
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