[Gmsh] 2D: physical volume tags

[Nico Schlömer]

Hi all,

I'm trying to define a 2D mesh with regions. To this end

=================== *snip* ===================
Function Rectangle
  // Points.
  cp1 = newp;
  Point(cp1) = {xmin,ymin,z,lcar};
  cp2 = newp;
  Point(cp2) = {xmax,ymin,z,lcar};
  cp3 = newp;
  Point(cp3) = {xmax,ymax,z,lcar};
  cp4 = newp;
  Point(cp4) = {xmin,ymax,z,lcar};

  // Lines.
  cl1 = newreg;
  Line(cl1) = {cp1,cp2};
  cl2 = newreg;
  Line(cl2) = {cp2,cp3};
  cl3 = newreg;
  Line(cl3) = {cp3,cp4};
  cl4 = newreg;
  Line(cl4) = {cp4,cp1};

  loop = newreg;
  Line Loop(loop) = {cl1,cl2,cl3,cl4};
  surf = newreg;
  Plane Surface(surf) = {loop};

  Physical Volume(Sprintf("rectangle %g", t)) = surf;

Return

// Build two rectangles.
lcar = 0.1;
z = 0;
ymin = -1.0;
ymax= 1.0;

t = 0;
xmin = -1.0;
xmax = 0.0;
Call Rectangle;

t = 1;
xmin = 0.0;
xmax = 1.0;
Call Rectangle;

Coherence;
=================== *snap* ===================

Meshing this works well, and when saving the data as MSH-file, I get

=================== *snip* ===================
$MeshFormat
2.2 0 8
$EndMeshFormat
$PhysicalNames
2
3 1 "rectangle 0"
3 2 "rectangle 1"
$EndPhysicalNames
$Nodes
504
1 -1 -1 0
2 0 -1 0
3 0 1 0
[...]
$EndNodes
$Elements
1031
1 15 2 0 1 1
[...]
1023 2 2 0 12 344 455 494
1024 2 2 0 12 355 476 464
1025 2 2 0 12 344 441 501
1026 2 2 0 12 344 494 441
[...]
=================== *snap* ===================

I'm wondering about the tags for the elements here. It seems that the tags
are "0" and "12", one of which I would expect to correspond with a
PhysicalName.
Can anyone enlighten me about the meaning of the tags here and how they
correspond with physical volumes?

Cheers,
Nico
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