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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Tue, 21 Sep 2010 16:14:31 GMT
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Viewed:
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24953 times
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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 thought the solution was to orient the triangles orthogonal to the curve
> of the surface rather than zigzag across it. You don't have to use quads,
> but you end up orienting the triangles as if they were 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.
> Can you really fix this in software without using a replacement part with
> the triangles oriented better? I'd like to hear more about how that works,
> especially if it can be adapted for my favorite low end flat shading
> renderer :-)
Not as far as I know.
--Travis
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Message has 1 Reply:
 | | Re: Why quads? Re: L(EGO)Draw parts - 4th batch
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| (...) 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). (...) Did anybody ever did actually fix the parts though? I (...) (14 years ago, 22-Sep-10, to lugnet.cad.dev.org.ldraw)
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Message is in Reply To:
| | Re: Why quads? Re: L(EGO)Draw parts - 4th batch
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| (...) 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. (URL) thought the solution was to orient the triangles orthogonal (...) (14 years ago, 18-Sep-10, to lugnet.cad.dev.org.ldraw)
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