CGAL users discussion list

[Andreas Fabri <>]

Hello,

Our meshing experts don't work today.   My guess is that it has to do 
with the initialization
of the mesh generation process.   From a point P inside the volume we 
shoot rays and
the intersection points of the rays with the surface are inserted.   
When the point P
is close to the surface you may get two intersection points close on the 
surface.

Andreas


On 5/24/2023 2:51 PM, Sebastian Bachmann ( via cgal-discuss 
Mailing List) wrote:
> Hi!
>
> This is an image of a set of 20 and I can see this behaviour on 4 of 
> them. On 420.inr it does not happen and the mesh looks good.
> The code was compiled against 5.4 (1050401000).
>
> If I look at the imges, I can not see any voxels standing out at the 
> position - at least not more than on other places, where it works fine.
> At least from the image, I cannot make out any particular feature that 
> would cause that.
>
> Is there any way to debug this and see what's going on?
>
> Best,
> Sebastian
>
> On 24.05.2023 14:34, Sebastien Loriot ( via 
> cgal-discuss Mailing List) wrote:
> > Do you observe this behavior on all images or only a specific one?
> > For example, when using 420.inr do you have such an issue?
> >
> > What could case such an over refinement (provided you are not using 
> > master version of CGAL) is if the mesher is trying to respect some
> > topological criteria (a very tiny handle for example, but that would 
> > be surprising in an image).
> >
> > Best,
> >
> > Sebastien.
> >
> > On 5/24/23 14:14, Sebastian Bachmann ( via cgal-discuss 
> > Mailing List) wrote:
> > > Hi everyone,
> > >
> > > I have an image from which I want to create a coarse surface mesh.
> > > The code I use for that is based upon the example 
> > > mesh_3D_image_with_features.cpp, however it is adopted to then 
> > > extract the surface and write an OFF file: 
> > > https://gist.github.com/reox/0df1f446f0ff9d46e0688b60e8e858b8
> > >
> > > As you can see, the Mesh_criteria is set to a really coarse mesh. 
> > > The input image (attached) I use has a voxel size of around 0.5mm 
> > > and represents a proximal femur.
> > > Now, for some reason, I get extremely fine triangle elements in 
> > > certain places (see attached image).
> > >
> > > I find that peculiar, as the facet_distance is already quite large, 
> > > and I thought that this might be the main issue here. I remembered, 
> > > that also the relative_error_bound in the 
> > > create_labeled_image_mesh_domain can have an influence, but reducing 
> > > this rather creates a "spiky" mesh than removing those small 
> > > elements. (Eventually when it is set to 0.1, the small elements 
> > > vanish, but then the vertices are all over the place and not on the 
> > > isosurface anymore...)
> > >
> > > Is there anything I missed here, maybe did I use wrong parameters? 
> > > Or is there another way how I could produce such coarse meshes?
> > >
> > > Best,
> > > Sebastian
--
Andreas Fabri, PhD
Chief Officer, GeometryFactory
Editor, The CGAL Project