CGAL users discussion list

[Sebastian Bachmann <>]

Hello!

Anyone has an idea what could cause that?
In like 95% of meshes I create I do not have that problem, but once in a 
while this turns up and is annoying...
It seems to be more often the case, when more than two labels are used 
(i.e, 0, 1 and 2 and not just 0 and 1).

Anything that I could test will definitely help me :)

Best,
Sebastian

Am 18.02.2025 um 09:06 schrieb Sebastian Bachmann ( via 
cgal-discuss Mailing List):
> Hi!
>
> Sure, here is an example: https://files.reox.at/.mesh/
>
> You may have to run it several times until such a hole appears, as no 
> optimization is present. However, you can make them appear much easier 
> by changing the line with refine_mesh_3 to:
>    CGAL::refine_mesh_3(c3t3, domain, criteria, CGAL::parameters::odt());
>
> ODT seems to be more prone than Lloyd.
> I uploaded two surface.off files as well, where this can be seen 
> (surface.off is without odt/lloyd and the second run, surface_odt.off is 
> with ODT active).
>
> Best,
> Sebastian
>
> Am 17.02.2025 um 11:34 schrieb Jane Tournois:
> > Hello,
> > can you please post some code and data so that we can try to reproduce 
> > your issue?
> > Best,
> > Jane
> >
> > Le 07/02/2025 à 16:30, Sebastian Bachmann ( via cgal- 
> > discuss Mailing List) a écrit :
> > > Hi!
> > >
> > > I send this mail to the list yesterday, but apparently it was not 
> > > posted - but I also did not get a rejection mail? I suspect that 
> > > attachments are not allowed and get silently dropped?
> > > So here it is again with a link instead, sorry if it is now posted 
> > > twice!
> > >
> > >
> > > I extended the example code from 
> > > mesh_3D_image_with_custom_initialization.cpp to mesh a multi-label 3D 
> > > image, i.e., I have for example 3 voxel values: 0 (background), 1 
> > > (some part), and 2 (some other part).
> > > This works reasonable well, but if I go down with the cell size, I 
> > > get weird artifacts in the surface of the mesh: at some random 
> > > places, "holes" appear (see screenshot here: https:// 
> > > files.reox.at/.mesh/ screenshot.png).
> > >
> > > I tried to track down the problem and it can be triggered by using 
> > > lloyd or odt optimization. When either of these are used, such holes 
> > > appear in my mesh (much much more in odt) - if I switch them off, 
> > > these holes are usually gone. But I found also images were such holes 
> > > appear in certain configurations, even without llyod or odt active.
> > > There, I can influence these holes by choosing a larger 
> > > relative_error_bound in create_labeled_image_mesh_domain (my default 
> > > is 1e-7) or by generating a new mesh, as there is some randomness in 
> > > initialize_triangulation_from_labeled_image (in my code, I specified 
> > > a seed for the PRNG and thus can reliably trigger it by choosing a 
> > > specific seed).
> > >
> > > Interestingly, it seems to appear more often when more than two 
> > > labels are used. Thus, in a binary image (only 0 and 1), this does 
> > > not happen. For example, if I take the same image with 3 labels and 
> > > replace all 2's by 1's, I get a much better mesh.
> > >
> > > Is there anything I may have missed to implement, for example during 
> > > the creation of the label domain? I can try to track down the problem 
> > > even more, but don't know at the moment where to look next and thus 
> > > would appreciate some guidance!
> > >
> > > Best,
> > > Sebastian