Subject:
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Re: Vertices not coplanar?
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Newsgroups:
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lugnet.cad.dev
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Date:
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Fri, 18 Jul 2003 00:19:43 GMT
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Viewed:
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1088 times
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In lugnet.cad.dev, Tore Eriksson wrote:
> What troubles do these warnings cause? I see no problems with the renderings, no
> matter what renderer I use.
>
> How do I correct them? By splitting all the quads into two triangles each I
> guess? If so, why not make a tool that splits them up?
When a renderer goes to render a quad in 3D, it will almost always (perhaps
always) first split the quad into two triangles. As long as the quad forms a
planar surface, this will result in fairly equivalent results no matter which of
the two possible ways the split is performed. If, however, the points aren't
planar, then the two different splits will result in different final results.
It's hard to describe the difference, but consider a tetrahedron (a shape formed
by connecting 4 equilateral triangles, kind of like a 3-sided pyramid). Now
choose any two of the triangles. Let these represent the two triangles you
want. But just specify the four points as a quad, and the opposing two
triangles could be chosen instead. This is an extreme case, and would probably
not happen, but hopefully it illustrates what the problem is.
Correcting the problem requires you to either split the quad into the desired
two triangles, or fix the location of one or more of the points so that they are
all planar. I would hope most programs providing the warning would have a
relatively loose tolerance for what constitutes a planar quad, due to rounding
concerns.
As for splitting them, I'm sure that if one doesn't already exist, a plugin
could be easily made for LDraw Design Pad to split a quad. A really good one
would require a lot more work, since it would ideally allow you to preview the
two split options, but making one that just asks you to choose should be easy.
Note that I don't plan to make such a plugin, but I think it would be better
than a stand-alone program.
If you get the warning in LDView it will tell you the coordinates of the two
triangles that it chose. If you like the result, you could fairly easily copy
the quad line in the original file, update the 4 at the beginning of the line
and its copy to be a 3, and delete the appropriate coordinate from each copy to
produce the same results.
--Travis Cobbs
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Message is in Reply To:
 | | Vertices not coplanar?
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| What troubles do these warnings cause? I see no problems with the renderings, no matter what renderer I use. How do I correct them? By splitting all the quads into two triangles each I guess? If so, why not make a tool that splits them up? /Tore (21 years ago, 17-Jul-03, to lugnet.cad.dev)
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