# Gmsh Mesh

## Geometry

We create the geometry directly from the box primitive provided by the OpenCascade geometry engine in Gmsh.

## 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-usr-run-pylith-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.
We use the Gmsh MeshSize options to define a uniform discretization size of 3 km.

```{code-block} console
---
caption: Run the `generate_gmsh.py` Python script to generate the mesh.
---
# Generate a mesh with hexahedral cells and save it to `mesh_hex.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 hexahedral cells and save it to the file `mesh_hex.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_hex.msh` using Gmsh.
---
gmsh -open mesh_hex.msh
```

:::{figure-md} fig:example:strikeslip:3d:gmsh:mesh
<img src="figs/gmsh-hex.*" alt="Finite-element mesh with hexahedral cells generated by Gmsh." width="100%"/>

Finite-element mesh with hexahedral cells generated by Gmsh.
:::