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Loss of scale data [message #15437] Thu, 29 July 2010 20:55 UTC Go to next message
avatar BenRaceyModelling  is currently offline BenRaceyModelling
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Registered: December 2009
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Dear all,

Very much a newbie to the shapeways community, and I'm hoping someone can help.

I've managed to crack using sketchup to design a model, understand how to get it through meshlab and save as a .stl file, and am getting used to using netfabb to clean and prepare the stl file for input to the shapeways website.

However, a niggling problem is loss of scale data along the way. Quite important as I'm trying to build scale models in the model railwy scales. The loss of scale information seems to happen somewhere in between sketchup / meshlab, as the scale data is lost before the file gets to netfabb.

I can rescale using netfabb, but surely there is a better way to keep the information in the first place? Has anyone else had a similar problem and how have you solved it!?

Many thanks in advance for any tips.

Regards

Richard.
Re: Loss of scale data [message #15440 is a reply to message #15437 ] Thu, 29 July 2010 21:19 UTC Go to previous messageGo to next message
avatar arno  is currently offline arno
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Hi Richard,

Did you take in account that the .stl file format does not store the unit inside the file? So you can get interpretation errors when you open the file. (This is also the reason why Shapeways asks the unit of your stl upload)

In what unit did you model the part, and what is the 'scale' when you read it in netfabb? Typically what happens is drawing in ' inch', storing as stl (without unit) and then reading the stl in eg mm, which causes the part to seem scaled down 25.4 times.

Hope this helps,

kind regards, Arno
Re: Loss of scale data [message #15444 is a reply to message #15440 ] Thu, 29 July 2010 21:34 UTC Go to previous messageGo to next message
avatar BenRaceyModelling  is currently offline BenRaceyModelling
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Hi ARno,
Thanks for that. I wasn't aware the .stl file format didn't save the measurement data. I'm modelling in mm.

Is there a better file format that does save measurement data and is compatible with sketchup/meshlab/netfabb/shapeways?

Thanks in advance!

Richard
Re: Loss of scale data [message #15463 is a reply to message #15437 ] Fri, 30 July 2010 01:27 UTC Go to previous messageGo to next message
avatar Drawn-SteelHero  is currently offline Drawn-SteelHero
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I'm not 100% certain, but the STL should still be fine; as Arno said, the STL still stores the measurements, just not the units. So a 70mm long model, for instance, is stored faithfully as 70 "units" long, but the STL doesn't recall which units. You can dictate to Shapeways what those units are when you upload to the site.

Andy
Re: Loss of scale data [message #15468 is a reply to message #15444 ] Fri, 30 July 2010 02:36 UTC Go to previous messageGo to next message
avatar stonysmith  is currently offline stonysmith
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Richard, I also have been working on some model train parts. Rather than using measurements to specify the size of the printed (scaled down) part, instead I've been building the models in "real world" measurements using decimal feet.

Example: For the height of a standard seven foot door, I just put 7.0

Then, as a final step in netfabb, I multiply by the appropriate scale factor and upload the STL specifying "millimeters".

By simply changing the multiplier, I've got diferent gauge models, all still proportionately accurate.

Scale Factor
HO 3.499
N 1.905
Z 1.385

Stony

[Updated on: Fri, 30 July 2010 02:37 UTC]

Re: Loss of scale data [message #15472 is a reply to message #15437 ] Fri, 30 July 2010 09:12 UTC Go to previous messageGo to next message
avatar BenRaceyModelling  is currently offline BenRaceyModelling
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Hi Stonysmith

Thanks for the tip on that, a useful technique. I was building in 4mm scale straight off so that I could see how the model would fare with things like minimum wall thickness etc.

Thanks for the tips!

Regards

Richard.
Re: Loss of scale data [message #15606 is a reply to message #15472 ] Wed, 04 August 2010 08:39 UTC Go to previous messageGo to next message
avatar bartv  is currently offline bartv
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As a side-note: although you can upload STL's with units of meters, inches and millimeters, I'd like to suggest you always use millimeters.

When using meters, the coordinates of the vertices in your STL file get *very* small and may be below our software's internal accuracy. In turn, this may lead to (false) non-manifold errors.

Inches also have this issue, but less so than meters.

Just my $0.02 Wink

Bart


Community Manager Europe | Shapeways
Re: Loss of scale data [message #15652 is a reply to message #15437 ] Thu, 05 August 2010 09:01 UTC Go to previous messageGo to next message
avatar BenRaceyModelling  is currently offline BenRaceyModelling
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Hi Bartv,
Thanks, yes I have tried to upload in mm. The issue appears to be somewhere between Sketchup/meshlab/netfabb. I've designed the model quite carefully in mm throughout in Sketchup, but when I get to fixing it in netfabb the dimensions are a lot smaller than originally designed in Sketchup.

The blog entry on using S/M/Neffabb is very helpful, I did scale the model up in netfabb but I've probably made the model too big as I didn't understand that the dimensions given are those of the bounding box, rather than the actual model itself.

Getting there, slowly!, really enjoying the process though!

Regards

Richard.
Re: Loss of scale data [message #24912 is a reply to message #15606 ] Thu, 17 March 2011 00:15 UTC Go to previous messageGo to next message
avatar stonysmith  is currently offline stonysmith
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Bart: one major question.. the units for VRML seem to "always" be meters... I'm having that very issue that the coordinates are extremely close together for my 2cm long models.


Patience, Persistance, Politeness - the 3Ps will help us get us to Perfect Printed Products
Re: Loss of scale data [message #25048 is a reply to message #24912 ] Mon, 21 March 2011 09:06 UTC Go to previous messageGo to next message
avatar bartv  is currently offline bartv
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Hey Richard,

yeah, we're aware of that. The internal accuracy of our software can cause non-manifold errors because of this. We're already discussing if we shouldn't just switch everything to mm scale..

Do you *need* VRML output? Do you use texturemaps?

If not, you might as well convert your models to STL and switch to mm.

Bart


Community Manager Europe | Shapeways
Re: Loss of scale data [message #25098 is a reply to message #24912 ] Mon, 21 March 2011 20:42 UTC Go to previous message
avatar kontor_apart  is currently offline kontor_apart
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check the guide to rescaling VRMLs without loss of precision

 
   
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