Special:
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[DAT] (requires LDraw-compatible viewer)
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Subject:
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Re: Line in the Sand
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Newsgroups:
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lugnet.cad.dev
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Date:
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Mon, 20 Dec 1999 23:12:04 GMT
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Viewed:
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2797 times
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Jacob Sparre Andersen wrote...
> [ Still discussing
> http://www.geocities.com/SouthBeach/Lagoon/8641/bfcspecv4.txt ]
>
> Steve:
(snip)
> > Second question: why did we decide that matrix-inversions were evil? I remember
> > being thick-headed about seeing the problem, but now I don't remember what the
> > problem is.
>
> One problem is that it can be difficult to see if a matrix
> has negative determinant.
Why? Are you thinking about
1 16 -4 0 0 0 0 0 0 0 4 1-4disc.dat
The determinant is zero, which BTW causes POVRay to halt
("singular matrix"). L3P has to fix these matrices or POVRay
would not render many parts at all!
As part of certifying a DAT file I believe this kind of subfile
reference should be fixed as well, like into
1 16 -4 0 0 0 1 0 0 0 4 1-4disc.dat
"L3P -check -w3" will help spot the problems.
> Another one is that some of us are lazy enough to mirror our
> subfiles in stead of writing two copies.
Mirroring is not a problem. It causes a "negative determinant matrix"
which can be detected and adjusted for (by toggling winding).
So matrix-inversions are not evil.
/Lars
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Message has 1 Reply:
 | | Re: Line in the Sand
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| (...) Matrix-inversions are not evil. But for some reason, using them to actually invert subfiles is evil (as opposed to using INVERTNEXT to invert subfiles). If I reallly needed an answer to this, I'd go read past messages. But I *do* remember: a) (...) (24 years ago, 21-Dec-99, to lugnet.cad.dev)
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Message is in Reply To:
| | Re: Line in the Sand
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| [ Still discussing (URL) ] Steve: [...] (...) Correct. (...) J subfile command line, and it only influences the immediately J following subfile command. I don't think the "and nowhere else" is that important. You already have written "only". (...) (...) (24 years ago, 16-Dec-99, to lugnet.cad.dev)
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