# `Quiddity.Face`
[🔗](https://github.com/d-led/ex.quiddity/blob/main/lib/quiddity/face.ex#L1)

One face of a part: what kind of surface it is, how big it is, which way it
faces, and what it touches.

"Face" here is the B-rep face — one trimmed piece of one surface — which is the
unit quiddity's records are made of. A feature is a *set* of them
(`Quiddity.Recognition.owner/2` goes the other way), so a caller reading a
face's `surface` is reading what it is made of, not what it is for.

# `t`

```elixir
@type t() :: %Quiddity.Face{
  area: float() | nil,
  bounds: map(),
  centroid: [float()] | nil,
  edges: [non_neg_integer()],
  index: non_neg_integer(),
  neighbours: [non_neg_integer()],
  normal: [float()] | nil,
  reversed: boolean(),
  surface: map()
}
```

# `describe`

```elixir
@spec describe(t()) :: String.t()
```

One line about the face, in the words a person would use.

A `.step` file is a list of surface entities; a reader wants "cylindrical wall,
4 mm radius, 125.7 mm², facing +x". This is that sentence, and it is what the
MCP tools print, so a model reads the same words a person would.

# `diameter`

```elixir
@spec diameter(t()) :: float() | nil
```

The diameter of a round face, in millimetres, or `nil`.

# `kind`

```elixir
@spec kind(t()) :: String.t()
```

What kind of surface the face is: `"plane"`, `"cylinder"`, …

# `parse`

```elixir
@spec parse(map()) :: t()
```

A face, as the engine answered it.

# `radius`

```elixir
@spec radius(t()) :: float() | nil
```

The radius of a cylindrical or spherical face, in millimetres, or `nil`.

A hole's wall is a cylinder, so this is the hole's radius — but a cylinder can
also be a boss or a pin, which is why the *family* comes from the recognition
and not from here.

---

*Consult [api-reference.md](api-reference.md) for complete listing*