# Meshing

To represent a geometry in the domain, such as a body, Nassu needs to process its geometry into the solver mesh format, a process called meshing.
This is done by transforming the body's representation, given by a `.lnas` file, into a cloud of points in the domain.

```{note}
The format for Nassu geometry is a `.lnas` file, Lagrangian Nassu. 
It’s the representation of a series of triangles, similar to the `.stl` format, and support the specifications of surfaces for it.

You can know more about `.lnas` and how to generate them in [stl2lnas](https://github.com/AeroSim-CFD/stl2lnas) repository.
```

These geometries are defined by the `domain.bodies` field in the configuration file

```yaml
simulation:
  - name: BuildingSimulation

    domain:
      bodies:
        solid-body1:
          lnas_path: "lnas/solid-body1.lnas"
        solid-body2:
          lnas_path: "lnas/solid-body2.lnas"
```


The Lagrangian mesh points are taken as the vertexes of triangular meshs.
The averaged area of triangles adjacent to the vertex is considered to each point.
For that reason, **it's important to use a `.stl` of sufficiently regular elements**.
The figure below illustrates the difference between an inadequate and an ideal mesh for IBM.

```{figure} /_static/img/user_guide/02_config/mesh_regularity.png
:alt: Mesh regularity
:align: center

Meshes regularity (left bad, right good)
```

The mesh confection can be performed by any external tool such as [FreeCAD](https://www.freecad.org/) or [MeshLab](https://www.meshlab.net/) that can generate a `.stl` mesh. 
After thath, the `.stl` can be used to generate a `.lnas` file through [stl2lnas](https://github.com/AeroSim-CFD/stl2lnas).

## Data extraction

It's very common to extract data from a body, such as $\rho$ for pressure calculation afterwards.
This can be done by specifying bodies for extraction in `data.series.series_name.bodies` field.

It's important to notice that the extraction is done using the **resolution from the original mesh**.
The original vertices and triangles from the original `.lnas` are the ones used to extract data from the body (considering all bodies transformations and such).

```{important}
Make sure that your mesh has the desired spatial resolution for data extraction.
If the mesh is more refined than required, or less refined, it should be updated for use.
```

Another capability is to extract data from a surface.
It's possible to specify a probe using a body as `body_name.surface_name` to consider only the triangles from a `.lnas` surface.