Subject:
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Re: Why quads? Re: L(EGO)Draw parts - 4th batch
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Newsgroups:
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lugnet.cad.dev.org.ldraw
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Date:
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Wed, 22 Sep 2010 03:46:39 GMT
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Viewed:
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27213 times
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In lugnet.cad.dev.org.ldraw, Travis Cobbs wrote:
> In lugnet.cad.dev.org.ldraw, Don Heyse wrote:
> > In lugnet.cad.dev.org.ldraw, Travis Cobbs wrote:
> > > Note that I consider this behavior to be a bug in LDView, and as such
> > > I will fix it when I get a chance.
> >
> > Isn't this the same problem we ran into with the twisted wedge bricks a
> > while back? Those gave us problematic conditional lines in addition to
> > the ugly zigzag shading artifacts.
> >
> > http://news.lugnet.com/cad/dat/parts/?n=5654
>
> No. This is a different problem. This problem is a bug in LDView's
> smoothing algorithm. LDView uses Type 5 lines in the LDraw file for
> smoothing, and considers any two surfaces that have a Type 5 line
> joining them to be part of a smooth curve. The problem is that two
> co-planar triangles sitting next to each other should also be
> considered to be joined together for smoothing, but they aren't,
> because there is no Type 5 line between them.
>
> Since the two triangles are co-planar, a Type 5 line between them would
> in fact be an error in the part. For some shapes, the problem can be
> gotten rid of in LDView by simply joining the two triangles into a quad.
> However, this should not be necessary, and it only works when the flat
> surface only has four points. Flat surfaces made up of more than two
> triangles cannot be fixed this way, and would require Type 5 lines
> between all their triangles in order to be smoothed properly by LDView.
I see. So what you really want to do in LDView is ignore the Type 5
lines (at least for smoothing) and smooth wherever there's no Type 2
edge lines between the triangles (and quads).
> The reason for the shading artifacts you showed was that the individual
> triangles were in fact forming a bumpy surface, and the shading was
> simply making that more obvious. Rearranging the triangles in the
> curve got rid of this problem.
Did anybody ever did actually fix the parts though? I got about
halfway through part 43721.dat before I lost focus. Meanwhile Philo
introduced the Coverer tool which may have institutionalized the
automatic generation of the problematic bumpy wedge surfaces. You can
see the ugly sawtooth shading on the wedge parts in the documentation
for the Coverer tool:
http://www.philohome.com/isecalc/coverer.htm
The problem is that the naive algorithm generates triangles where the
joins between them alternate between slightly convex and slightly
concave. I think perhaps we might need a warning for that tool to watch
out for this. Or is it possible to fix the tool to follow the convex
and concave saddle curves between the opposite corners instead? Maybe
as an option?
Anyhow I took another stab at the 43721 wedge part after re-reading
Lars' 5 year old comment about just making one octant of a twist
primitive. I used quadrants because it was easier for my feeble brain,
but I think you just scale them and you can use 8 chunks instead of 4.
http://news.lugnet.com/cad/dat/parts/?n=6571
I also didn't do any Type 5 lines yet but I think it's a bit closer
to what we really want than my previous feeble attempt.
Don
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Message has 2 Replies:
 | | Re: Why quads? Re: L(EGO)Draw parts - 4th batch
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| (...) Sorry for that ;o) But (fortunately) here are very few twisted surfaces in LEGO pieces, so I don't think my tool has created too much havoc... (...) Problem is that at this low level I have almost no information about the intended geometry - I (...) (14 years ago, 22-Sep-10, to lugnet.cad.dev.org.ldraw)
| | | Re: Why quads? Re: L(EGO)Draw parts - 4th batch
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| (...) Actually, no. That is indeed a long-term goal, but it is hampered by two things. First of all, it's a lot of work. Secondly, the current algorithm is actually useful to part authors, since it can visually highlight missing conditional lines. (...) (14 years ago, 23-Sep-10, to lugnet.cad.dev.org.ldraw)
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