# [Gmsh] Role of CreateGeometry in Creating Geometry from Mesh

Christophe Geuzaine cgeuzaine at uliege.be
Mon Dec 7 10:21:42 CET 2020

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> On 7 Dec 2020, at 07:31, Prani Nalluri <pan4 at rice.edu> wrote:
>
> Hello,
>
> I would like to ask a question with regard to the CreateGeometry function. Our task is to create a geometry based off the mesh test1.msh. This geometry will be used to remesh the domain. In the script below titled remesh.geo, the line CreateGeometry{Surface{:};} appears to recognize points and edges of and within the domain. Thus, the CreateGeometry function is able to produce an accurate geometry that is the same as the geometry of the original mesh that test1.msh was based off. However, we have been unable to determine why the  CreateGeometry function with the specific inputs given to it in our script works. Would you be able to elaborate on this? Thank you for your help. Attached below are the test1.msh and remesh.geo files.
>

CreateGeometry computes a parametrization of all the discrete 1D and 2D model entities (i.e. all the curves and surfaces that are represented solely by a mesh). For surfaces this computation is carried out by solving sets of partial differential equations on the existing mesh, to compute the new coordinates. Equipped with these parametrizations, all the meshing algorithms of Gmsh can then be used on the discrete entities, as if they were classical CAD entities.

If you call

CreateGeometry{Surface{:};}

CreateGeometry

then a parametrization is only computed for the surfaces, leaving the curves as-is. The curves will then not be remeshed, and only the surfaces will. This might (or not) be what you want... For example, if you want to generate an overall coarser grid, you should simply use "CreateGeometry" : the 1D mesh will then be redone with the specified mesh size constraints.

Christophe

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—
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science
http://people.montefiore.ulg.ac.be/geuzaine

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