# Gmsh Mesh
## Geometry
We construct the geometry by first creating points, then connecting the points into curves, and finally the curves into a surface.
{numref}`fig:example:crustal:strikeslip:2d:geometry:gmsh` shows the geometry and variables names of the vertices and curves.
:::{figure-md} fig:example:crustal:strikeslip:2d:geometry:gmsh
Geometry created in Gmsh for generating the finite-element mesh.
We construct curves from points (`pSW`, ..., `pNW`) and surfaces from the curves (for example, `c_north`).
The arrows indicate the direction (orientation) of the curves.
:::
:::{important}
Each curve in Gmsh has a direction (orientation).
The direction is from the starting point to the ending point.
When connecting curves into surfaces, you must connect the curves in a consistent direction.
We connect the curves in a counter-clockwise direction.
To reverse the direction of a curve, use the negative tag.
:::
## Meshing using Python Script
We use the Python script `generate_gmsh.py` to create the geometry and generate the mesh.
The script makes use of the `gmsh_utils.GenerateMesh` class (discussed in {ref}`sec-user-meshing-gmsh-utils`), which provides the command line arguments and boilerplate methods.
In our `generate_gmsh.py` Python script, we create a class `App` that implements the functionality missing in `gmsh_utils.GenerateMesh`.
We must implement the `create_geometry()`, `mark()`, and `generate_mesh()` methods that are abstract in the `GenerateMesh` base class.
:::{important}
In order for Gmsh to create cells with edges that align with the fault traces, we must embed the curves into the surface.
Additionally, we must split the main fault trace into two so that there is a common point among all three fault traces where they intersect.
:::
We use the Gmsh MeshSize options to define a discretization size that grows slowly at a geometric rate with distance from the fault.
See [6.3.1 Specifying mesh element sizes in the Gmsh documentation](https://gmsh.info/doc/texinfo/gmsh.html#Specifying-mesh-element-sizes) for more information.
```{code-block} console
---
caption: Run the `generate_gmsh.py` Python script to generate the mesh.
---
# Generate a mesh with triangular cells and save it to `mesh_tri.msh` (default filename).
$ ./generate_gmsh.py --write
# Save as above but start the Gmsh graphical interface after saving the mesh.
$ ./generate_gmsh.py --write --gui
# Create only the geometry and start the Gmsh graphical interface.
$ ./generate_gmsh.py --geometry --gui
# Show available command line arguments.
$ ./generate_gmsh.py --help
```
By default the Python script will generate a finite-element mesh with triangular cells and save it to the file `mesh_tri.msh`.
You can view the mesh using Gmsh either by using the `--gui` command line argument when you generate the mesh or running Gmsh from the command line and opening the file.
```{code-block} console
---
caption: View the Gmsh mesh file `mesh_tri.msh` using Gmsh.
---
gmsh -open mesh_tri.msh
```
:::{figure-md} fig:example:crustal:strikeslip:2d:gmsh:mesh
Finite-element mesh with triangular cells generated by Gmsh.
:::