CGAL users discussion list

[Nicholas Mario Wardhana <>]

Hi all,

I have a set of voxels associated to boolean values (either 0 or 1),
and I wish to generate a mesh that approximates the voxels with value
1. The '1' voxels are the result of voxelising an input surface, so
these voxels are relatively smooth (not scattered) but thin. I am able
to generate the INR file which can be properly loaded by CGAL's
surface mesher, but what confuses me is the configuration. So I would
like to ask the questions below, referencing the corresponding manual:

http://www.cgal.org/Manual/latest/doc_html/cgal_manual/Surface_mesher/Chapter_main.html

especially Section 46.3.

1) What is the coordinate system of the gray image? I notice that I am
required to determine its centre which is inside the surface, such
that the value of sign( f(centre) - isovalue ) = -1. Let isovalue =
0.5. I choose the centre to be the centre of an arbitrary voxel whose
value is 0. I suppose I can get sign(0 - 0.5) = -1, but instead I got
a 1. Am I right to assume that the mesher's coordinate system is
different from mine? I also notice that the INR file lacks of notion
of origin or anything related, so does it have to do with that?

2) Suppose I know the size of the whole voxelisation. What are the
good range estimates for the radius and centre-centre distance upper
bounds relative to the voxelisation size? I made some trial-and-error
efforts and found that the result rarely changes with different
parameters, but when it does, the change is very abrupt. For example,
there is no face generated (only point set) if I use the bounds 0.602
and 0.602, no face is generated, but if I use 0.601 and 0.601, the
algorithm produces so many faces and does not stop after a few
minutes.

3) Still related to the previous point, even if I use the same
configuration, the results might differ. Sometimes the algorithm stops
after a few seconds, sometimes it does not stop after a few minutes. I
notice somewhere (sorry I forget) that the algorithm has a random
element, but how is it possible that the results are distinctly
different?

Please let me know if more information is needed. Thank you.

Best regards,
Nicholas