CGAL users discussion list

[Stephane Tayeb <>]

Hi,


 wrote:
> hi!
> i have some questions about 3D mesh generation. not programming problems, but
> regarding parameters, underlying algorithms, etc.
>
> so far i've had some pretty good results (everything installed and compiles in
> visual studio 2008), but there are some limitations/issues.
>
> i'm trying to achieve a  SMOOTHLY GRADED mesh from a segmented volumetric 
> image
> (using liver.inr for now). 
> FYI, i've got CGAL all compiled and running in VS2008. examples work! :)
>
> 2 questions are numbered as follows:
>
> 1. i cannot understand this from the manual: is it required/necessary/better 
> to
> "refine" a coarse mesh in 'steps' or can you just assign facet and cell
> parameters from the onset to achieve accurate surface and mesh integrity?
> the manual implies (fig. 47.2) that you should start with a course mesh, and
> then refine in steps 'from the previous mesh', first by surface accuracy, and
> then by resolution. is this necessary to achieve a well configured (i.e. 
> 'good'
> tets) mesh??

You don't have to proceed by 'steps'. You should use as parameters those 
which fulfill your needs directly.
In fig 47.2, all meshes are created from scratch, but you're right the 
explanations are far from clear. Actually, the idea is to show how 
parameters change the output. So we just change one parameter from one 
mesh to the next one (but we always create a new mesh).
Thanks for reporting, this will be fixed.

> if so, fine, i just need to know from a programming standpoint.
>
> 2. (more important) is there any way to control the mesh density directly. for
> example, in 'tetgen' (http://tetgen.berlios.de/) you can control mesh density
> via an auxiliary volumetric density map. this is crucial to achieving a well
> prescribed mesh.

There is no such thing in the current release.

Nevertheless such a behavior is achievable. You'll have to create your 
own set of criteria.
For instance, you may write your own sizing criterion for tets, using a 
function f(Point_3) instead of a fixed size. You could start at this 
page 
(http://www.cgal.org/Manual/last/doc_html/cgal_manual/Mesh_3_ref/Concept_MeshCriteria_3.html#Cross_link_anchor_1421) 
of documentation and have a look at file Mesh_criteria_3.h and in its 
dependencies.

> thanksinadvance,
> todd
> drexel university

Best,
Stéphane.

--
Stephane Tayeb
Software engineer - INRIA Sophia Antipolis
Geometrica Project-Team