[Insight-users] Mesh and FEM

[James Gee]

To follow up on Luis's response, there were considerable design
discussions and coding iterations regarding the integration of itkMesh
into the FEM framework.  The reasons for the current separation of the
two mesh descriptions are highly technical but they boil down to the
loss of certain numerical efficiencies that are key to FEM if we had
adopted the existing itkMesh implementation.  We are developing
additional documentation outlining the issues and rationale for the
current design, which we hope will serve as a basis for future
refactoring that may reconcile the needs of the two mesh
representations.

About your immediate question about how to input custom meshes, keep in
mind that ITK does not provide any stand-alone mesh generation
facilities (the itkFEMRegistrationFilter does automatically generate
regular meshes to span the spatial domain of the images to be
registered), although we will be providing some functionality towards
this end as part of another project unrelated to ITK.  Meanwhile, you
will need to use your favorite mesh generator, and convert the format of
the produced mesh for your problem into one compatible with the FEM
registration filter.  We have helper routines to do this but these
aren't in the repository yet; however, we'll be happy to forward the
programs to you and guide you in their use for your application.

Finally, the current FEM framework implements a variety of element
types, and if these cover your needs, then you're set to go and there
are several code examples that will guide you in the use of multiple
element types.  If, on the other hand, the elements in which you're
interested are not available in the current library, there are the usual
two options in ITK: we will consider implementing the new elements if
they are of general interest; or, better, you can contribute these to
the library and we'll assist you in their development.

Please let us know how we can be of more specific help to you,
Jim


> -----Original Message-----
> From: insight-users-admin@public.kitware.com 
> [mailto:insight-users-admin@public.kitware.com] On Behalf Of 
> Luis Ibanez
> Sent: Wednesday, April 09, 2003 11:22 PM
> To: Lara Vigneron
> Cc: ITK; Aljaz Noe; Tessa A Sundaram
> Subject: Re: [Insight-users] Mesh and FEM
> 
> 
> 
> Hi Lara,
> 
> Your observation is correct,
> 
> The FEM framework do not use the
> itkMesh for its internal computation.
> 
> There have been attemts to integrate
> the itk::Mesh into the FEM structure
> but they have not proved a definite
> advantage in doing so.
> 
> The itk::Mesh is used mainly by the
> deformable models and the cellular
> agreggates methods for segmentation.
> 
> Aljaz and Tessa are the main developers
> of the FEM framwork. They could probably
> be able to give you some guidance in
> its use.
> 
> 
> Regards,
> 
> 
>     Luis
> 
> 
> -----------------------
> 
> Lara Vigneron wrote:
> > Dear all,
> > 
> > I have several questions about itkMesh and the FEM framework.
> > 
> > I saw that the "representation" of a mesh by itkMesh is made with 
> > points
> > and cells.
> > 
> > On the other hand, I want to use the 
> itkFEMRegistrationFilter like in
> > the DeformableRegistration1.cxx example. For the moment, I 
> would like to 
> > create a mesh out of ITK and push into via the  CreateMesh 
> method of the 
> > itkFEMRegistrationFilter. However, I think the 
> "representation"  of a 
> > mesh in the FEM framework is made via arrays representing elements, 
> > nodes and materials,  and I didn't find any link with the 
> representation 
> > of the mesh with points and cells (itkMesh). Is it correct?
> > 
> > And in this case, how can I use for instance the BoundaryFeature 
> > concept
> > of itkMesh with FEM?
> > 
> > Also, I saw that to enter a mesh with the CreateMesh 
> method, you have 
> > to
> > inject a file characterizing the mesh via the parameter file (it is 
> > asked for the MeshFileName near the end of the file) used in the 
> > itkFEMRegistrationFilter ( an example of the parameter file 
> is given 
> > page 208 of the SoftwareGuide). Is there any example of a MeshFile 
> > somewhere to see the synthax to use?
> > 
> > Lastly, I want to use several elements types with different material
> > properties. Is it sufficient if these different elements 
> are noticed in 
> > the MeshFile (see just above) and not in the parameter file 
> where, for 
> > instance, only one E value is given?
> > 
> > Thanks a lot for any help...
> > 
> > Lara
> >  
> > 
> > --
> > Lara VIGNERON
> > 
> > Signal Processing Group
> > 
> > University of Liege
> > Institut Montefiore
> > Sart-Tilman, Batiment B28
> > B-4000 Liege
> > BELGIUM
> > 
> > Tel: +32-(0)4-366.26.42
> > Fax: +32-(0)4-366.29.84
> > Lara.Vigneron@ulg.ac.be
> > 
> >  
> 
> 
> 
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