Dear all,

I am an inexperienced OpenSCAD user. I would like to prepare an STL file
for a 2D simulation in OpenFOAM using cfMesh's cartesian2DMesh generator
(if you can suggest a workflow I would appreciate it). Whats is needed is a
ribbon surface that represents the out-of-plane extrude. This means that if
my 2D simulation has the Z-axis as the out-of-plane axis, then I need to
have the X-axis direction on all surfaces. My geometry is simple I create a
union of a couple of cubes. How can I then remove the X-Y plane surfaces of
the union. I would like to avoid inputting the points with their
coordinates directly to reduce overhead calculations and maintain things
parametric. So if you know the magic trick, I would really appreciate it!

Best regards,
Hisham

Hi guys,

Thanks a lot for your quick response. I am sorry for struggling to explain
myself. If you consider page 4 of this document
http://cfmesh.com/wp-content/uploads/2015/09/User_Guide-cfMesh_v1.1.pdf I
would like to turn a volume enclosed shape like Fig. 5 into a hollow shape
like Fig.4. Because all side surfaces are X-Y plane surfaces, I thought it
might be a useful condition.

I think projection may not be the optimum solution but it is nice to know
this capability.

I produce a union of several cubes:

And would like to export only the outer surface and delete the front and
back surfaces!

Best regards,
Hisham

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Hi Hisham, that makes sense.

So you're trying to remove the top and bottom face of an STL, leaving the
sides intact, deliberately producing a non-manifold object.

If you're trying to do this as a one-off, the quickest way is to download
'meshmixer', open the exported STL file from OpenSCAD, select the triangles
on the surfaces you don't want and hit delete.

If you need to do this for a batch of files and make a workflow, you could
use this:  https://www.thingiverse.com/thing:62666

It's an STL file to OpenSCAD converter - takes an STL mesh and turns it into
an OpenSCAD code polygon.  You could then edit that polygon to remove all
the faces where all points are on the same Z height (IE they are flat on the
XY plane).  Not sure if it will produce an error if you try to render the
STL as it will certainly be non-manifold, but if it works it's highly
reproducible...

Cheers,

Alex Gibson

admg consulting

edumaker limited

. Project management
. Operations & Process improvement
. 3D Printing

-----Original Message-----
From: Discuss [mailto:discuss-bounces@lists.openscad.org] On Behalf Of
hsafti
Sent: 08 January 2020 12:06
To: discuss@lists.openscad.org
Subject: Re: [OpenSCAD] Removing all surfaces in the X-Y Plane

Hi guys,

Thanks a lot for your quick response. I am sorry for struggling to explain
myself. If you consider page 4 of this document
http://cfmesh.com/wp-content/uploads/2015/09/User_Guide-cfMesh_v1.1.pdf I
would like to turn a volume enclosed shape like Fig. 5 into a hollow shape
like Fig.4. Because all side surfaces are X-Y plane surfaces, I thought it
might be a useful condition.

I think projection may not be the optimum solution but it is nice to know
this capability.

I produce a union of several cubes:

And would like to export only the outer surface and delete the front and
back surfaces!

Best regards,
Hisham

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Another option that might be even easier is to use the Inkscape (free, multi-platform) svg editor with the Path to OpenSCAD extension installed.
SVG to OpenSCAD Bezier - InkScape extension by gaellafond

It has a somewhat documented feature in that an open path generates a user-specified wall thickness line rather than an enclosed polygon/path. This means that if you create a path such as in figure 4 but don't close the end, merely overlap the lines, then use the extension to create an OpenSCAD output, it will be identical to figure 4. Because of the user-specified wall thickness, it's still a 3-dimensional object.
The creator of the extension states that he will not be supporting/developing the extension, as it is native in the newest version of Inkscape. This would require the process to be changed within Inkscape to offset the original drawing to create the "thin-wall" extrusion.
Of course if a thin wall is unacceptable, the real answer remains Meshmixer.
Another work flow option in Meshmixer is to use the Edit/Plane Cut feature and select No Fill, then slice away the top and bottom of the envelope. It's a good bit faster for those shapes which permit entire planes to be vaporized.

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SVG to OpenSCAD Bezier - InkScape extension by gaellafond

[UPDATE] OpenSCAD 2019-05 has added native SVG support, so I won't maintain this extension anymore.https://en.wi...
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The suggestion for Meshmixer was one that popped into my alleged mind too, as the methodology described is spot on.

On Wednesday, January 8, 2020, 7:27:50 AM EST, Alex Gibson <alex@alexgibson.net> wrote:  

Hi Hisham, that makes sense.

So you're trying to remove the top and bottom face of an STL, leaving the
sides intact, deliberately producing a non-manifold object.

If you're trying to do this as a one-off, the quickest way is to download
'meshmixer', open the exported STL file from OpenSCAD, select the triangles
on the surfaces you don't want and hit delete.

If you need to do this for a batch of files and make a workflow, you could
use this:  https://www.thingiverse.com/thing:62666

It's an STL file to OpenSCAD converter - takes an STL mesh and turns it into
an OpenSCAD code polygon.  You could then edit that polygon to remove all
the faces where all points are on the same Z height (IE they are flat on the
XY plane).  Not sure if it will produce an error if you try to render the
STL as it will certainly be non-manifold, but if it works it's highly
reproducible...

Cheers,

Alex Gibson

I wrote a program to read and write STL files that works inside AutoCAD.  The
program colors the triangles based on the Z value of the normal vector of
each triangle.  For your purpose (I think) that you want to eliminate all
triangles that have non zero Z values for the normal vector.  Doing so will
produce a model that looks like the attached picture.  If you are somewhat
experienced in programming you could write a program that reads the STL file
and eliminates the triangles that have non zero Z values in their normal
vectors.
If you are not into programming you can open the STL file ( as long as it is
an ASCII file) in a text editor and erase all the triangles that do not have
“0” (zero) as the last number in the line that begins with “facet normal”
For example, here are several lines from the STL file produced by your
OpenSCAD code:
facet normal -1 -0 0
outer loop
vertex -6 0.0999994 1.005
vertex -6 -0.0999994 -0.00499916
vertex -6 -0.0999994 1.005
endloop
endfacet
facet normal 0 0 1
outer loop
vertex 5 13.1 1.005
vertex 0.2 3.1 1.005
vertex 5 3.1 1.005
endloop
endfacet

For your purpose erase the line that reads “facet normal 0 0 1” and the
following 6 lines.  This will erase a single triangle.  Continue editing the
entire file and erase all triangles that do not have a “0” as the last digit
in the facet normal line.
I cannot seem to get the Insert Image function to work, so here is a link to
the modified STL file and image.
http://www.carols62.com/test3.stl
http://www.carols62.com/test3.jpg

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Dear all,

Thanks a lot for all your help and support and congratulations on this
really active community spirit. It is in fact the motivation behind using
OpenSCAD to use it inside Python scripts to automate preprocessing. I even
use openpyscad which to my experience does not support the projection method
(not bad, I can still make and call an openScad script if needed). Using an
external tool manually will kill the purpose of the whole thing. I am
currently considering planning to use Python's meshio library to
conviniently read the stl file, filter points with z>0 and consider only
surfaces with verticies only from the remaining points, rewrite as stl or
some other format. Theoretically this should work but you all know how these
things go south in reality :D

So thanks again to all!

Best regards,
Hisham

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Sorry .. Just came tired from work! Of course I will remove surfaces based on
surface normal or use a utility from cfMesh that produces the surface I need
from an X-Y planar sketch ... This is maybe why both ideas got mixed in my
head!

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So a manual edit in Meshmixer would be out of the picture.

You are already aware of automating OpenSCAD - do you use the command line
instance of OpenSCAD?

So this can get you a folder-full of STL files.

'All' you need to do is in Python or something else (I have used GREP or
even MS Excel before, have very patchy programming skills!) parse the STL
files in an automation script.
You would only need to identify and delete each triangle where ALL 3 points
have the same Z value.

Good luck!

Alex Gibson

admg consulting

edumaker limited

. Project management
. Operations & Process improvement
. 3D Printing

-----Original Message-----
From: Discuss [mailto:discuss-bounces@lists.openscad.org] On Behalf Of
hsafti
Sent: 09 January 2020 18:21
To: discuss@lists.openscad.org
Subject: Re: [OpenSCAD] Removing all surfaces in the X-Y Plane

Dear all,

Thanks a lot for all your help and support and congratulations on this
really active community spirit. It is in fact the motivation behind using
OpenSCAD to use it inside Python scripts to automate preprocessing. I even
use openpyscad which to my experience does not support the projection method
(not bad, I can still make and call an openScad script if needed). Using an
external tool manually will kill the purpose of the whole thing. I am
currently considering planning to use Python's meshio library to
conviniently read the stl file, filter points with z>0 and consider only
surfaces with verticies only from the remaining points, rewrite as stl or
some other format. Theoretically this should work but you all know how these
things go south in reality :D

So thanks again to all!

Best regards,
Hisham

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Hi Alex and all,

Thanks again for your helpful tips. Python saves the day once more. Using
the numpy-stl library, one can do this short code:

import numpy
from stl import mesh

mesh3D = mesh.Mesh.from_file('mesh3D.stl')

indecies=[]
for i in range(len(mesh3D.normals)):
if mesh3D.normals[i][2] ==0:
indecies.append(i)

verticeCount = len(indecies)
data = numpy.zeros(verticeCount, dtype=mesh.Mesh.dtype)

mesh2D = mesh.Mesh(data, remove_empty_areas=False)

for k in range(verticeCount):
mesh2D.v0[k] = mesh3D.v0[indecies[k]]
mesh2D.v1[k] = mesh3D.v1[indecies[k]]
mesh2D.v2[k] = mesh3D.v2[indecies[k]]

mesh2D.save('mesh2D.stl')

Thanks again and until next time ;-)

http://forum.openscad.org/file/t2716/result.png

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Far more times than I care to admit, I’ve mistakenly click on the render button right after a render is finished. I do that just out of unsupervised muscle memory or something… I don’t know… but I suspect I’m not the only one who does this. Often this is no big deal, but when the render itself took 15 or more minutes to complete its frustrating to have to sit through the whole process again. I MEANT to save too STL… but oh no… have to wait for it to render again and hope I don’t make the same mistake again.

We already have a somewhat related warning… when you click to save an STL file OpenSCAD will warn you if you’ve made any changes to the source file to alert you to the possibility that you may not be saving the file that you think you are.

So while I’m sitting here waiting for that 2nd unnecessary render to finish I thought I’d pop this off to the list as I’m thinking that with a preferences adjustable value, openSCAD could come up with a dialog that read something like “Are you sure you want to render? The last render of this object took 48 minutes”.

Is this doable? Worthwhile? Easy or hard?

What are your Edit/Preferences/Advance/CGAL cache sizes?
That will affect re-render performance if to small.
I have both at 2048MB.

Admin - email* me if you need anything,  or if I've done something stupid...

• click on my MichaelAtOz label, there is a link to email me.

Unless specifically shown otherwise above, my contribution is in the Public Domain; to the extent possible under law, I have waived all copyright and related or neighbouring rights to this work. Obviously inclusion of works of previous authors is not included in the above.

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