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

Steffen Becker steffenbecker87 at yahoo.de
Wed Mar 12 10:17:47 CET 2014


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
> *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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