Given the following code, how can I perform a mesh generation around the line, to make a 2D form in GMSH API? Extrude or Mirror functions? I want to add a mesh around the line, for example, 1cm to left and 1cm to right.

#include <gmsh.h>

int main(int argc, char **argv)

  gmsh::option::setNumber("General.Terminal", 1);


  double lc = 1e-2;
  gmsh::model::geo::addPoint(0, 0, 0, lc, 1);
  gmsh::model::geo::addPoint(.1, 0,  0, lc, 2);

  gmsh::model::geo::addLine(1, 2, 1);




  return 0;

You have several options.

Option A. Go for the gmsh::model::geo::extrude

The API call specification is given, as follows:

GMSH_API void extrude(const gmsh::vectorpair & dimTags,
                        const double dx,
                        const double dy,
                        const double dz,
                        gmsh::vectorpair & outDimTags,
                        const std::vector<int> & numElements = std::vector<int>(),
                        const std::vector<double> & heights = std::vector<double>(),
                        const bool recombine = false);

therefore, in your case, it is much easier if you are drawing not a central line of the planned surface, but a left/right border.

std::vector<std::pair<int, int>> line; // this will store a pair {dimension, line_id}
line.first = 1; // line is a 1-D entity
line.second = 1; // and as per your code the line to extrude has ID1
std::vector<std::pair<int, int>> surface; //this will store the result of the extrusion {dimension, surface_id}
gmsh::model::geo::extrude(line, 0, 0.02, 0, surface);

The code above assumes that your line with ID 1 is a border, not a center line going along the X-axis, and you intend to have a width along the Y-axis.

The second GMSH tutorial (available in GEO and C++ API versions) is helpful here.

Option B. Draw the surface yourself

Sometimes, it is just easier to draw the surface yourself without using any extrusions if it is simple enough. I think the code for your case is relatively straightforward.

Option C. Use OpenCASCADE factory.

Sample: tutorial 16 using C++ API.

Useful, since Rectangles are available in GMSH with OpenCASCADE kernel.

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