# Gmsh Mesh

## Geometry

We construct the geometry for the vertical cross section using 2D primitives available in the Gmsh OpenCASCADE geometry engine.
{numref}`fig:example:magma:2d:geometry:gmsh` shows the geometry and variables names of the curves and surfaces.

:::{figure-md} fig:example:magma:2d:geometry:gmsh
<img src="figs/geometry-gmsh.*" alt="Geometry created in Gmsh for generating the mesh." width="400px"/>

Geometry created in Gmsh for generating the finite-element mesh.
We create a rectangle for the domain, an ellipse for the magma chamber, and a second rectangle for the conduit.
We join the ellipse and second rectangle to form the shape of the magma chamber, and them embed it in the domain.
:::

## Meshing using Python Script

We use the Python script `generate_gmsh.py` to create the geometry and generate the mesh.
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.
The main difference is that the geometry is slightly more complex, and we calculate the location of the points on the fault using trigonometry.

We use the Gmsh MeshSize options to define a discretization size that grows slowly at a geometric rate with distance from the main fault.

```{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:magma:2d:gmsh:mesh
<img src="figs/gmsh-tri.*" alt="Finite-element mesh with triangular cells generated by Gmsh." width="600px"/>

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