[Gmsh] Points seem to move on transfinite surface after the recombination

[Christophe Geuzaine]

> On 8 Jul 2020, at 12:46, Bojan Niceno <bojan.niceno.scientist at gmail.com> wrote:
> 
> Dear all,
> 
> I have a problem when trying to mesh planar structured surfaces.  Although points seem to be uniformly placed on a transfinite surface, they move a bit when recombining the mesh to change triangles to quadrilateral cells.
> 
> Here is a simple script which demonstrates it:
> 
> // Define points
> Point(1) = {0.0,  0.0,  0.0};
> Point(2) = {0.5,  0.0,  0.0};
> Point(3) = {0.0,  0.5,  0.0};
> Point(4) = {0.7,  0.0,  0.0};
> Point(5) = {0.0,  0.7,  0.0};
> 
> // Define connectors from points ...
> Line  (1) = {2, 4};
> Circle(2) = {4, 1, 5};
> Line  (3) = {5, 3};
> Circle(4) = {3, 1, 2};
> 
> // ... and set them as transfinite
> Transfinite Curve {1, 2, 3, 4} = 11 Using Progression 1;
> 
> // Define surface ...
> Curve Loop(1) = {1, 2, 3, 4};  Plane Surface(1) = {1};
> 
> // ... and set it as transfinite too
> Transfinite Surface {1};
> 
> So, as I said, generating a 2D mesh over the surface places points properly, but command recombine moves them a bit :-/  Is there a way to prevent these movements of points?
> 
> I am also attaching a screenshot of a mesh before and after recombination, to illustrate my problem better.  
> 

Indeed, recombination a posteriori applies some smoothing. You should add

Recombine Surface{1};

after "Transfinite Surface {1};", which will directly create a quad mesh.

Christophe




>     Best regards,
> 
>     Bojan
> 
> P.S.  I am not sure if attachments go to the mailing list, I've never used a mailing list before.
> 
> <points_after_generation.png><points_after_recombination.png>_______________________________________________
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— 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science 
http://www.montefiore.ulg.ac.be/~geuzaine