# src/test/fractions.c

# Computation of volume fractions from a levelset function

```
#include "grid/cartesian.h"
#include "fractions.h"
int main()
{
origin (-0.5, -0.5);
init_grid (32);
```

The interface is a circle centered on the origin and of radius 0.25. We chose this radius because it leads to degenerate cases where the interface intersects the grid exactly on vertices.

We initialise a levelset function on the vertices of the grid.

```
vertex scalar φ[];
foreach_vertex()
φ[] = sq(0.25) - sq(x) - sq(y);
```

We then use this function to compute the corresponding volume and surface fractions.

```
scalar c[];
face vector s[];
fractions (φ, c, s);
```

To check that this is correct, we draw the corresponding facets, reconstructed using either `c`

and `s`

(on stdout), or only `c`

(on stderr).

```
output_facets (c, stdout, s);
output_facets (c, stderr);
}
```

This gives this figure where “exact” uses *c* and *s* and “VOF” uses only *c*.