# The mesh file
The APM Preprocessor supports mesh files in **.stl** format. The **.stl** file is a commonly used mesh file format which represents bodies with triangular elements. If a body is represented only with triangular elements its mesh is said to be unstructured. Conversely, if a body is represented only with quadrilateral elements its mesh is said to be structured. The images below show an unstructured mesh of a fixed-wing UAV, a structured mesh of a NACA0015 wing, and a hybrid (mixed) mesh of a fixed-wing UAV.
## File formats
### .stl
The **.stl** mesh file format can only store unstructured meshes. However, the APM Preprocessor supports a non-standard **.stl** mesh file format which can store structured meshes. A typical structure of a **.stl** mesh file is shown below:
```
solid "name"
facet normal nx ny nz
outer loop
vertex x y z
vertex x y z
vertex x y z
endloop
endfacet
facet normal nx ny nz
outer loop
vertex x y z
vertex x y z
vertex x y z
endloop
endfacet
...
...
...
endsolid
```
Each mesh element is defined by the `facet` keyword. The `normal` keyword and the `nx`, `ny`, `nz` variables specify the unit normal vector of the element. The `vertex` keyword and the `x`, `y`, `z` variables specify a vertex of the element. Since each element has 3 vertices the `vertex` keyword and the `x`, `y`, `z` variables are repeated three times.
For structured meshes, a fourth vertex is added to each element.
{% hint style="info" %}
A **.stl** file with four vertices per element is non-standard and is specific to the APM Preprocessor.
{% endhint %}
{% file src="" %}
### .ucd
The **.ucd** mesh file format can only store unstructured and structured meshes. A typical structure of a **.ucd** mesh file is shown below:
```
#
.
.
.
# ... ... ...
```
{% file src="" %}
### .wrl
The **.wrl** mesh file format can only store unstructured and structured meshes. A typical structure of a **.wrl** mesh file is shown below:
```
Shape {
geometry IndexedFaceSet {
coord Coordinate {
point [
...
]
}
coordIndex [
... ... ...
...
]
}
}
```
{% file src="" %}
