STL vs OBJ

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STL vs OBJ

jon_bondy
I created a fairly simple object to use as input to a sculpting program
(Sculptris).  Since Sculptris only accepts OBJ files as input, I
converted from STL to OBJ in NetFabb.  The result is puzzling.  Much of
the surface is as one would expect, but there are visual defects
(surfaces with unexpected divots and bumps), especially at the bottom.

Not sure what to make of it.  Perhaps all of the defects were introduced
by NetFabb, and this has nothing to do with OpenSCAD.

Any comments appreciated.

Jon

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Reflecting Surface Blank - OpenSCAD_Model.stl (468K) Download Attachment
Reflecting Surface Blank.obj (685K) Download Attachment
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Re: STL vs OBJ

fred_dot_u
I was able to load both objects into Meshmixer. Neither object raised any
indication of problem with the Analysis/Inspector. I zoomed into the surface
in question and found a flat surface. When I performed a test by making an
Edit/Plane cut as closely to the bottom as possible, the resultant item was
a flat surface, no divots or bumps or problems.

I was able to load the object into Simplify3D and when running the slicing
preview, it was also flat at 0.100 layer height.

I'm inclined to agree that NetFabb is generating a problem, but not
necessarily a real one so much as a virtual one.

One curious aspect of the model is that the flat undersurface is a series of
very long thin triangles, the bases of which become the circumference of the
circle, while the other two legs are chords of the circle. I usually see
circular shapes in wireframe mode as a series of smaller triangles evenly
distributed over that surface.

If you have troubles with NetFabb in the future (and the future is NOW),
Meshmixer will export as OBJ easily enough.



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Re: STL vs OBJ

jon_bondy
Thank you for your thoughtful analysis.  When I take the STL and convert
it into an OBJ and then mess with it in Sculptris, the shape (which
seems fairly uniform if you look at the triangle patterns) deforms in a
variety of ways, as if the surface was inconsistent.  Perhaps this is an
issue with Sculptris.  I will try another sculpting program.

:)


On 3/7/2018 5:17 PM, fred_dot_u wrote:

> I was able to load both objects into Meshmixer. Neither object raised any
> indication of problem with the Analysis/Inspector. I zoomed into the surface
> in question and found a flat surface. When I performed a test by making an
> Edit/Plane cut as closely to the bottom as possible, the resultant item was
> a flat surface, no divots or bumps or problems.
>
> I was able to load the object into Simplify3D and when running the slicing
> preview, it was also flat at 0.100 layer height.
>
> I'm inclined to agree that NetFabb is generating a problem, but not
> necessarily a real one so much as a virtual one.
>
> One curious aspect of the model is that the flat undersurface is a series of
> very long thin triangles, the bases of which become the circumference of the
> circle, while the other two legs are chords of the circle. I usually see
> circular shapes in wireframe mode as a series of smaller triangles evenly
> distributed over that surface.
>
> If you have troubles with NetFabb in the future (and the future is NOW),
> Meshmixer will export as OBJ easily enough.
>
>
>
> --
> Sent from: http://forum.openscad.org/
>
> _______________________________________________
> OpenSCAD mailing list
> [hidden email]
> http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org
>
>

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Re: STL vs OBJ

fred_dot_u
Reflecting_Surface_Blank_02.obj
<http://forum.openscad.org/file/t824/Reflecting_Surface_Blank_02.obj>  

I've attached your OBJ file, but with a slight modification. I used
Meshmixer to create face groups, then selected only the bottom surface.

Using the remesh feature, I performed two iterations and used a very small
target edge length for the parameters.

Now what used to be long slender triangles has been changed to a series of
tiny ones more uniformly scattered over the surface.

It's possible that your program will look more favorably upon such a
surface, as it lends itself better to organic modeling such as Sculptris



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Re: STL vs OBJ

Neon22
In reply to this post by jon_bondy
Jon the long thin triangles are the problem. You need to remesh the
object. not sure if sculptris can do this but ZBrush can.

You need a more even distribution of less sharp triangles.



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Re: STL vs OBJ

jon_bondy
Thank you.  I guess the question is "how/why did OpenSCAD produce these
triangles?"


$fn = 200;

module torus()
     translate([0, 0, 20])
         rotate_extrude(convexity = 10)
             translate([40, 0, 0])
                 circle(d = 20);
module Main() {
     hull() {
         cylinder(h = 1, d = 80);
         torus();
         }

     hull() {
         torus();
         translate([0, 0, 120]) sphere(30);
         }
     }

difference() {
     Main();
     for (a = [0:90:359]) {
         rotate([0, 0, a     ]) translate([135, 0,  90]) sphere(100);
         rotate([0, 0, a     ]) translate([130, 0, 130]) sphere(100);
         rotate([0, 0, a + 10]) translate([145, 0,  45]) sphere(100);
         rotate([0, 0, a + 50]) translate([132, 0, 110]) sphere(100);
         rotate([0, 0, a + 35]) translate([141, 0,  70]) sphere(100);
         rotate([0, 0, a + 63]) translate([143, 0,  60]) sphere(100);
         }
     }


On 3/8/2018 3:33 AM, Mark Schafer wrote:

> Jon the long thin triangles are the problem. You need to remesh the
> object. not sure if sculptris can do this but ZBrush can.
>
> You need a more even distribution of less sharp triangles.
>
>
>
> _______________________________________________
> OpenSCAD mailing list
> [hidden email]
> http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org
>
>

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nor do I receive texts on my main phone number
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Re: STL vs OBJ

jon_bondy
In reply to this post by fred_dot_u
Thank you.  It is interesting that Sculptris shows your repaired object
as having inconsistent (not pretty) surfaces.


On 3/7/2018 7:58 PM, fred_dot_u wrote:

> Reflecting_Surface_Blank_02.obj
> <http://forum.openscad.org/file/t824/Reflecting_Surface_Blank_02.obj>
>
> I've attached your OBJ file, but with a slight modification. I used
> Meshmixer to create face groups, then selected only the bottom surface.
>
> Using the remesh feature, I performed two iterations and used a very small
> target edge length for the parameters.
>
> Now what used to be long slender triangles has been changed to a series of
> tiny ones more uniformly scattered over the surface.
>
> It's possible that your program will look more favorably upon such a
> surface, as it lends itself better to organic modeling such as Sculptris
>
>
>
> --
> Sent from: http://forum.openscad.org/
>
> _______________________________________________
> OpenSCAD mailing list
> [hidden email]
> http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org
>
>

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Re: STL vs OBJ

jon_bondy
In reply to this post by Neon22
Interesting that a planar object can appear differently depending on how
the triangles are arranged on that planar surface.  It is still a plane,
after all...


On 3/8/2018 3:33 AM, Mark Schafer wrote:

> Jon the long thin triangles are the problem. You need to remesh the
> object. not sure if sculptris can do this but ZBrush can.
>
> You need a more even distribution of less sharp triangles.
>
>
>
> _______________________________________________
> OpenSCAD mailing list
> [hidden email]
> http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org
>
>

--
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I do not receive emails while away from my desk,
nor do I receive texts on my main phone number
(which is a land line).
If you know that I am on the road, please text me.
If you know that I am home, please email me.


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Re: STL vs OBJ

fred_dot_u
I downloaded and installed Sculptris to see how things developed. The first
item of note is that the import of the OBJ file is not accurate. The
perimeter of the base of the model shows in wireframe in Meshmixer as a
clean circle. In Sculptris, that feature is destroyed. The triangles are
arrayed almost randomly at the interface between the circular base and the
curved portion moving upward.

I also noted that some of the sides are irregular as well.

I think the blame falls on the internal works of Sculptris and not within
any particular slicer or other modeling program, especially not within
OpenSCAD.



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Re: STL vs OBJ

jon_bondy
I think I agree.  So much for a 4 year old free program!

Thank you all!


On 3/8/2018 9:42 AM, fred_dot_u wrote:

> I downloaded and installed Sculptris to see how things developed. The first
> item of note is that the import of the OBJ file is not accurate. The
> perimeter of the base of the model shows in wireframe in Meshmixer as a
> clean circle. In Sculptris, that feature is destroyed. The triangles are
> arrayed almost randomly at the interface between the circular base and the
> curved portion moving upward.
>
> I also noted that some of the sides are irregular as well.
>
> I think the blame falls on the internal works of Sculptris and not within
> any particular slicer or other modeling program, especially not within
> OpenSCAD.
>
>
>
> --
> Sent from: http://forum.openscad.org/
>
> _______________________________________________
> OpenSCAD mailing list
> [hidden email]
> http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org
>
>

--
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I do not receive emails while away from my desk,
nor do I receive texts on my main phone number
(which is a land line).
If you know that I am on the road, please text me.
If you know that I am home, please email me.


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