[Gmsh] Error : Self intersecting surface mesh, computing intersections (this could take a while)

Wed Mar 12 11:31:24 CET 2014

Hi Steffen,

you also need unique edges (sorry, I forgot to mention that explicitly) and, in case of adjacent volumes, unique interfaces.
However, since you provide the 2D mesh already, duplicated triangle sides should not be a problem unless they get  subdivided in the meshing process.
What you can do in this case is to write  the 2D mesh in gmsh format (e.g. as outerface.msh) and merge that one instead of the myfile.ply2 file, leaving the other lines in your .geo file as they are. If you then do a 3D mesh, gmsh will take the 2D mesh as defined in your file. You can then refine the mesh if you wish.

HTH,

Matthias

Von: Steffen Becker [mailto:steffenbecker87 at yahoo.de]
Gesendet: Mittwoch, 12. März 2014 10:18
An: Zenker, Dr. Matthias
Cc: 'Roman Fratczak'; gmsh at geuz.org
Betreff: Re: [Gmsh] Error : Self intersecting surface mesh, computing intersections (this could take a while)

Hi Matthias & Roman,

I create a nodes- and elements-list like this:
# nodes:
N1 0,0,0
N2 0,1,0
N3 1,0,0
N4 0,0,1
# elements, which describe triangles:
E1 1,2,3
E2 2,3,4

So I do have double edges (here: between node 2+3). Didn't now that this is a problem for gmsh, sorry.

I will try Romans solution asap and give you a feedback.
Thank you both!

Steffen

Am 12.03.2014 09:59, schrieb Zenker, Dr. Matthias:
Hi Steffen,

I am not totally sure from what you wrote that there are really no duplicated nodes. In gmsh, you need to define the surfaces in a way that neighbouring surfaces/facets with a common border use the _same_ nodes to define the border. I could not see from your file if there are duplicated nodes, and would propose that you make sure that there are really no such duplications.

If the meshing still fails, then this is a question for the real gurus (Christophe?).

HTH,

Matthias

Von: Roman Fratczak [mailto:fratczakroman at o2.pl]
Gesendet: Mittwoch, 12. März 2014 09:51
An: Steffen Becker
Cc: gmsh at geuz.org<mailto:gmsh at geuz.org>
Betreff: Re: [Gmsh] Error : Self intersecting surface mesh, computing intersections (this could take a while)

Hi Steffen,

No it is more clear. If you are working with cloud of point, then you may use Meshlab for surface regeneration. There are many software that are able to handle in a resonable way the cloud of points (like rhino3D + resurf, other). But these are rather commercial.

>From the other side - try to do the simplest thing:

Adjust your python script for surface creation in order to save not as PLY but as GMSH "geo" file using points and lines

Point(1)={0,0,0} ;
Point(2)={0,1,0} ;
...

Line(1) = {1,2};
..
and.. Line Loop & Surface -> Volume ..

Then you will get a volume defined by geometric entities. In this case is shoul be easier to create a better mesh on surface and then on whole volume.

Regards
Roman

W dniu 11.03.2014 17:59, Steffen Becker pisze:
Hi,

@ Roman:
some more details:
I have a point cloud given and generate the surface mesh by myself (with python). I don't know a good algorithm to create a volume mesh out of this surface mesh - that's why I use gmsh for this task.
I also checked my surface mesh for double vertexes but I didn't find any. What do you mean with "double egdes"? Isn't there always a "double edge", because two neighbour facets always have one same edge?!

I will send you a .step or .iges file as soon as I figured out how to handle the "No OpenCASCADE model found" error.

I'm appending the other mailing history (with Matthias) to this mail - sorry for that "double mailing", I just forgot one time to "reply to all" -.-
Anyone else has an idea why the meshing fails?

Regards and thank you a lot,
Steffen

Hi Steffen,

I don't see either why the meshing fails, sorry.

Matthias

-----Ursprüngliche Nachricht-----

Von: Steffen Becker [mailto:steffenbecker87 at yahoo.de]

Gesendet: Dienstag, 11. März 2014 14:42

An: Zenker, Dr. Matthias

Cc: gmsh at geuz.org<mailto:gmsh at geuz.org>

Betreff: Re: [Gmsh] Error : Self intersecting surface mesh, computing

intersections (this could take a while)

Oh I'm really sorry, here is the file.

In addition I sent you also the intersect.pos. I don't understand why exactly

these surfaces are problematic and the "neighbour surfaces"

aren't - although they have the same geometry =/

Am 11.03.2014 12:07, schrieb Zenker, Dr. Matthias:

Hi Steffen,

if you don't provide the myfile.ply2, we cannot see your geometry...

 You can visualize intersect.pos by merging it to your geometry.

HTH,

Matthias

Am 11.03.2014 14:33, schrieb Roman Fratczak:
Hello,

It is possible, that you will get your geometry in other format?
In your "ply" there is already a surface mesh given. Try maybe use STEP, IGES,...  as input- then it is easer to recalculate the surface mesh.

In case of need of usage of "a dead mesh" - maybe try first to remesh it (i.e. in blender with modification Remesh) and/or clean it (removing double vertexes and edges).

regards
Roman

W dniu 11.03.2014 12:13, Steffen Becker pisze:
Hi,

in the appendix you find my model.
Thanks for  help!!

Regards,
Steffen

Am 11.03.2014 11:47, schrieb Roman Fratczak:
Hello Steffen,

Yes. You can try to the first Optimize quality of tetra, or all of them (just try).

I don't know how your model looks like. If the elements on the surface are too big, then the solid elements (probably tetragonal) may be with intersecting edges or faces. Therefore the reducing size of element may help (especially on the surfaces).

Or- just share your model (ply) -in order to be more precise.

regards
Roman

W dniu 10.03.2014 18:06, Steffen Becker pisze:
Hi Roman,
when you say "enable option: optimize", you mean" Tools -> Options -> Mesh -> Advanced -> Optimize quality of tetrahedra, Optimize quality of tetrahedra with Netgen, Optimize high order meshes (experimental)" ?
And why is reducing element size important? In my case it is important that the object has such "90 degree corners".

Regards,
Steffen

Am 10.03.2014 16:55, schrieb Roman Fratczak:
Hi Steffen,

If your volume is OK (is really a closed one) - try to:

1. Reduce element size (especially in a curved shapes)
2. Enable option "Optimize" - in mesh options

Regards
Roman

W dniu 10.03.2014 16:41, Steffen Becker pisze:
Hello everyone,

I'm a gmsh beginner and I want to create a 3D volume mesh out of a given surface-mesh but I got the error message as seen in the subject. You find my .geo file in the appendix.
Can anyone tell me what I have to do to create a 3D mesh? I even can't figure out where the intersection are located (don't know how to read the intersect.pos).
Sorry for my poor english.

Regards,
Steffen

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