Nurbs & meshing.

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 From:  chrisd (CHRIS_DORDONI)
5600.1 
Hi,

I wanted to break this part of the discussion out from an earlier thread http://moi3d.com/forum/index.php?webtag=MOI

Michael responded to my "Can Moi export an OBJ with quads?" with this:

> It can export an OBJ with n-gons - it will be quads in internal areas of the surface and n-gons in areas where the surface has been trimmed - it basically starts out with quads on the base surface and then n-gons are formed where the surface has trimming information. This n-gon structure gives it a clean wireframe look but it is not the same as "all quads" output.

MoI is generally focused on producing an output mesh that will be used for rendering - if you're worrying about all quads that might mean that you're thinking of applying sub-d smoothing on to the mesh? If so then no - MoI's output is not intended to be used for sub-d smoothing, that really requires hand tuned topology and you will usually need to create a sub-d cage by hand in a sub-d modeling program and not as some automatic generated output. You can use the MoI output as a starting point for a retopology process though.

See here for some previous discussion:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4923.1

But if you want to generate a rendering, there is no need to have "all quads" for that anyway.
>

What I am looking for is a way to get mesh files that capture the detail where it is needed, but are economical in terms of overall file size. So when I mentioned quads, I was thinking that it could be more efficient than triangles. I wish it were possible to work exclusively in polys or nurbs, but there are advantages and limitations to both. So what happens is that at some point I end up transferring large meshes between applications, which is time consuming. Ultimately, the work is 3d printed.

I use SolidWorks to do some of the work, and the meshing is poor, with very limited user control. So if there were something that does a better job and it were within my budget, I would consider it.

Maybe if the AMF standard catches on, there will be a better way to deal with meshes http://amf.wikispaces.com/AMF+Editor
Until that happens, I have to work with what out there now.
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 From:  Michael Gibson
5600.2 In reply to 5600.1 
Hi Chris,

> What I am looking for is a way to get mesh files that capture the detail where it is needed, but
> are economical in terms of overall file size.

MoI's mesher uses some various parameters to help with this, when you export a mesh from MoI a dialog pops up that has various controls on it that you can adjust, there is an overview of the meshing controls here:
http://moi3d.com/2.0/docs/moi_command_reference11.htm#meshdialog


The slider controls the angle parameter, the way that works is that it breaks down the mesh until the surface normals have less than that given angle. That makes for a less dense mesh in flatter areas of the model, and more polygons created in areas that have more curvature to them.

There's also another parameter "Avoid smaller than", that you can use to make a coarser mesh on small sized areas - you enter in a distance there and anything smaller than that distance gets a much coarser mesh on it. This can help to optimize file size by putting fewer polygons on little tiny fillets and things like that. See here for a demonstration on how that works:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3869.70


> So when I mentioned quads, I was thinking that it could be more efficient than triangles.

As far as optimizing file size goes, usually with CAD models using n-gons is more efficient than either quads or triangles. Here's a simple example model being exported using n-gon output:




So note there that when generating n-gons the top face is just one big n-gon. If it were triangles it would have the same number of vertices but a whole lot more polygon faces, twice as many for all the side wall parts, and a whole lot more on the top. If it were "all quads", it would be worse yet because the top face would need to have numerous additional vertices implanted in it throughout in order to try and make a sequence of somewhat regular sized quads that tiled that same area.


> So if there were something that does a better job and it were within my budget, I would consider it.

MoI will likely do a better job, I'd say that its mesher is currently the "state of the art" in NURBS to polygon conversion, it is custom developed and has features like n-gon generation that you won't find anywhere else. And if SolidWorks was in your budget, MoI at less than one tenth the price of SolidWorks should be well within it as well I would think. So it's a good fit for what you are describing here, I would recommend trying out the trial version to see if it is useful for your particular work, there is a 30 day trial version available from here: http://moi3d.com/download.htm .

The best way to answer your question about whether MoI will be useful for you is to test it out.



EDIT: - now that I read your question again I see that your final output destination is 3D printing - usually that will mean exporting to STL format which can only contain triangles within it, so generating either quads or n-gons is not actually feasible for that particular use case. Normally OBJ format (which can contain quads or n-gons, not just triangles) is not used very often for 3D printing.

Usually when people ask about generating quads, they are trying to build an all quad topology to be used for subdivision smoothing in a sub-d polygon modeler, so that's what I thought you were after originally when you asked about that. MoI's mesher still has some nice qualities to its overall mesh generation aside from n-gon generation so it's still worth checking out even if you are not going to be able to use quad or n-gon output since you're doing 3D printing.


- Michael

EDITED: 14 Dec 2012 by MICHAEL GIBSON

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