[Gmsh] Identifiers of entities generated by extrusion

[Christophe Geuzaine]

Lucas Blattner Martinho wrote:
> Dear Gmsh developers and users,
> 
> 
> 
> I am using Gmsh to build the geometric model of an oil pipeline 
> subjected to the electromagnetic interference from a nearby high voltage 
> transmission line.
> 
> After making an extrusion to build a surface corresponding to a quarter 
> of the pipe cross-section with the command
> 
> /Pipeline_Quarter_Section1[] = Extrude{ {1,0,0}, 
> {0,Pipe_Center_Y,Pipe_Center_Z}, Pi/2} 
> {Line{Pipe_Inner_Radius_Line1,Pipe_Thickness_Line1,Coating_Thickness_Line1}; 
> Layers{Circunference_Discretization}; Recombine;};
> /
> 
> I would like use the indentifiers of the entities generated by this 
> extrusion (the surface and a few lines) that became stored in the list 
> Pipeline_Quarter_Section1[] in further geometric transformations.
> However, the only way I could find to tell for example that 
> Pipeline_Quarter_Section1[3] stores the identifier of a particular line 
> (and not of a different one generated by the same extrusion 
> transformation) was trial and error...
> 
> Is it possible to predict the pattern of these vectors of entities 
> generated by Gmsh in such transformations? I have found a few hints at 
> the mesh tutorial t3.geo 
> (http://www.geuz.org/gmsh/doc/texinfo/gmsh-full.html#SEC65), but it does 
> not seem to cover all possible cases...


Indeed, we forgot to document this. I've added the following in the
reference manual:

 >>>
As explained in @ref{Floating point expressions}, @var{extrude} can be
used in an expression, in which case it returns a list of identification
numbers. By default, the list contains the ``top'' of the extruded
entity at index 0 and the extruded entity at index 1, followed by the
``sides'' of the extruded entity at indices 2, 3, etc.  For example:

@example
   Point(1) = {0,0,0};
   Point(2) = {1,0,0};
   Line(1) = {1, 2};
   out[] = Extrude{0,1,0}{ Line{1}; };
   Printf("top line = %g", out[0]);
   Printf("surface = %g", out[1]);
   Printf("side lines = %g and %g", out[2], out[3]);
@end example

This behaviour can be changed with the
@code{Geometry.ExtrudeReturnLateralEntities} option (@pxref{Geometry
options list}).
<<<



> 
> Would it also be possible to generate through the extrusion separated 
> lists of entities that do not mix lines, surfaces and volumes in an 
> unpredictable sequence?
> 

Yes, but we'd have to change the syntax...


> 
> 
> Yours faithfully,
> 
> Lucas Blattner Martinho
> 
> 
> Laboratório de Eletromagnetismo Aplicado  - LMAG
> Escola Politécnica da Universidade de São Paulo
> São Paulo,  Brasil
> 
> 
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-- 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science
http://www.montefiore.ulg.ac.be/~geuzaine