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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Thu, 16 Dec 1999 21:40:02 GMT
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Viewed:
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3213 times
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[ Still discussing
http://www.geocities.com/SouthBeach/Lagoon/8641/bfcspecv4.txt ]
Steve:
[...]
> I like your explanation better, but I think we're both missing the point: this
> paragraph is supposed to be about INVERTNEXT, not inversion. Fewer details are
> appropriate at this point. More details should go in (1) the explanation of
> inversion in the "Language Extension Functionality" and (2) pseudo-code for the
> BFC() function.
Correct.
> The documentation for INVERTNEXT should be focused the specifics of INVERTNEXT,
> not on the definition/affect of inversion:
>
> S INVERTNEXT
> S This option inverts a subfile. It may only be used immediately before a
> S subfile command line, and nowhere else.
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".
> S
> S Example:
> S 0 BFC INVERTNEXT
> S 1 16 0 0 0 1 0 0 0 1 0 0 0 1 somefile.dat
> S 1 16 0 0 0 1 0 0 0 1 0 0 0 1 another.dat
> S
> S In this example, somefile.dat would be rendered as inverted. Another.dat
> S would not be inverted.
> S
> S For further information, see "Inversion" in the Language Extension
> S Functionality section.
Good.
> For now, I'm not going to change the INVERTNEXT paragraph. It does need to be
> clarified, and I'll add that to the issues list. But before making changes, I'd
> like to hear back from you (and anyone else) on my comments and suggestions.
Try the formulation above, and insert something like my
previous INVERTNEXT text in the "Inversion" section.
> Question: should there be a definition-entry for matrix-inversion?
Shouldn't be necessary. Or is it?
> 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.
Another one is that some of us are lazy enough to mirror our
subfiles in stead of writing two copies.
> > > Try this:
> > > Matrix Inversions. Rendering engines will need to correct for transformation
> > > 1 matrices which inadvertently invert a subfile. They will also need to
> > > 1 allow for situations in which the matrix is deliberately given a negative
> > > 9 determinant, in order to mirror the subfile.
> > > 9
> > > 9 The typical method of determining that the file has been manually inverted is
> > > 9 to calculate the determinate of the transformation matrix. If the determinate
> > > 9 is negative, then the subfile has been inverted. This document does not
> > > 9 attempt to address the details of 3D graphics algorithms and issues.
> >
> > # %s/determinate/determinant/g ?
>
> Er, yes.
>
> BTW, should I change "transformation matrix" to "orientation matrix" when
> discussing the matrix being used to map the current (sub)file onto the screen?
Don't know.
> > > 9 The typical way to adjust for matrix inversions is to switch the winding of
> > > 9 the polygon vertices. That is, if the DAT specifies the winding as CW, and
> > > 9 the current subfile has been inverted, the rendering program would proceed as
> > > 9 if the winding is CCW.
> >
> > # I am not sure this actually clarifies things more. See if
> > # the text with the INVERTNEXT is enough.
[ My head is not clear enough for matrix operations at the
moment - I'll be back later with comments to the rest. ]
Play well,
Jacob
------------------------------------------------------------
-- E-mail: sparre@cats.nbi.dk --
-- Web...: <URL: http://hugin.ldraw.org/LEGO/Biler/ > --
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Message has 2 Replies:
 | | Re: Line in the Sand [DAT]
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| Jacob Sparre Andersen wrote... (...) (snip) (...) 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 (...) (25 years ago, 20-Dec-99, to lugnet.cad.dev)
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Message is in Reply To:
| | Re: Line in the Sand
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| (...) I like your explanation better, but I think we're both missing the point: this paragraph is supposed to be about INVERTNEXT, not inversion. Fewer details are appropriate at this point. More details should go in (1) the explanation of inversion (...) (25 years ago, 15-Dec-99, to lugnet.cad.dev)
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