Subject:
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Re: Dat format question - Quad line order
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Newsgroups:
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lugnet.cad.dev
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Date:
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Wed, 22 Dec 1999 13:38:37 GMT
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Viewed:
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331 times
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In lugnet.cad.dev, Johannes Keukelaar writes:
> Hi all,
>
> I've read the LDraw FAQ relating to the .dat file
> format. Specifically:
>
> --BEGIN-FAQ--
> Quadrilateral command.
> Draws a four-sided, filled shape between four points.
> Line format:
> 4 colour x1 y1 z1 x2 y2 z2 x3 y3 z3 x4 y4 z4
> where
> colour is a colour code: 0-15, 16, 24, 32-47, 256-511
> x1, y1, z1 is the position of the first point
> x2, y2, z2 is the position of the second point
> x3, y3, z3 is the position of the third point
> x4, y4, z4 is the position of the fourth point
> --END-FAQ--
>
> However, there are many different quads you can draw with four
> points. I assume that if the four points (p1, p2, p3, p4) have
> coordinates like this:
>
> p1 p2
>
>
>
>
> p4 p3
>
>
> The quad intended to be filled is this one:
>
> p1--------p2
> | |
> | |
> | |
> | |
> p4--------p3
>
> Is this correct?
>
> I seem to have found some places, though, where the quad thus
> constructed is not simple, i.e. has intersecting edges. An example may
> be:
>
> p1-----p2
> \ /
> \ /
> \/
> /\
> / \
> / \
> p3-----p4
>
> I have noticed this specifically in part 2850 (but also in other
> places), in the side walls of the partial studs on the top of this
> part.
>
> Is this a bug in my rendering code (and a similar bug in my strange
> quad detection code) that is only triggered for certain rare cases?
> (I've tried rendering everything with p3 and p4 swapped; then these
> few quads look normal, but all the other ones are now strange.) Or is
> this really the way the data in the dat file is ordered?
>
> If this is really what the dat file says, surely it can't be intended
> that way? How do other dat renderers deal with this? Even if this
> could be patched up in code (i.e. swap, in this case, p3 and p4),
> surely this is something that must be fixed in the dat files when we
> get around to implementing back face culling?
>
> Confused regards,
>
> Johannes.
> --
> To err is human. To grr is canine.
>
> Virtually researching virtual behaviour in a virtual world.
You are correct that the co-ordinates _should_ define a simple loop without
crossing. The rendering algorithm in the original LDraw program was not
sensitive to this, so errors crept into the parts files. There was a move some
time back to correct the existing parts, and Lars Hassing's L3P program
(http://www.netby.net/Nord/Mandelvej/Hassing/l3p.html) has a useful -check
option to detect these errors. In recent times, I believe parts with such
errors have not passed the voting. When submitting new parts, running L3P
-check is one of my final QC checks.
Maybe we should renew our efforts to clean up the existing errors, when the
parts update process gets restarted? I'm happy to help share this work.
Chris
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Message is in Reply To:
 | | Dat format question - Quad line order
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| Hi all, I've read the LDraw FAQ relating to the .dat file format. Specifically: --BEGIN-FAQ-- Quadrilateral command. Draws a four-sided, filled shape between four points. Line format: 4 colour x1 y1 z1 x2 y2 z2 x3 y3 z3 x4 y4 z4 where colour is a (...) (25 years ago, 22-Dec-99, to lugnet.cad.dev)
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