Using the SurfaceTool
=====================

The `SurfaceTool` provides a useful interface for constructing geometry.
The interface is similar to the `ImmediateGeometry` node. You
set each per-vertex attribute (e.g. normal, uv, color) and then when you add a vertex it
captures the attributes.

The SurfaceTool also provides some useful helper functions like `index()` and `generate_normals()`.

Attributes are added before each vertex is added:

gdscript GDScript

```
    st.add_normal() # Overwritten by normal below.
    st.add_normal() # Added to next vertex.
    st.add_color() # Added to next vertex.
    st.add_vertex() # Captures normal and color above.
    st.add_normal() # Normal never added to a vertex.
```

When finished generating your geometry with the `SurfaceTool`
call `commit()` to finish generating the mesh. If an `ArrayMesh` is passed
to `commit()` then it appends a new surface to the end of the ArrayMesh. While if nothing is passed
in, `commit()` returns an ArrayMesh.

gdscript GDScript

```
    st.commit(mesh)
    # Or:
    var mesh = st.commit()
```

Code creates a triangle with indices

gdscript GDScript

```
    var st = SurfaceTool.new()

    st.begin(Mesh.PRIMITIVE_TRIANGLES)

    # Prepare attributes for add_vertex.
    st.add_normal(Vector3(0, 0, 1))
    st.add_uv(Vector2(0, 0))
    # Call last for each vertex, adds the above attributes.
    st.add_vertex(Vector3(-1, -1, 0))

    st.add_normal(Vector3(0, 0, 1))
    st.add_uv(Vector2(0, 1))
    st.add_vertex(Vector3(-1, 1, 0))

    st.add_normal(Vector3(0, 0, 1))
    st.add_uv(Vector2(1, 1))
    st.add_vertex(Vector3(1, 1, 0))

    # Commit to a mesh.
    var mesh = st.commit()
```

You can optionally add an index array, either by calling `add_index()` and adding
vertices to the index array or by calling `index()` which shrinks the vertex array
to remove duplicate vertices.

gdscript GDScript

```
    # Creates a quad from four corner vertices.
    # Add_index does not need to be called before add_vertex.
    st.add_index(0)
    st.add_index(1)
    st.add_index(2)

    st.add_index(1)
    st.add_index(3)
    st.add_index(2)

    # Alternatively:
    st.index()
```

Similarly, if you have an index array, but you want each vertex to be unique (e.g. because
you want to use unique normals or colors per face instead of per-vertex), you can call `deindex()`.

gdscript GDScript

```
    st.deindex()
```

If you don't add custom normals yourself, you can add them using `generate_normals()`, which should
be called after generating geometry and before committing the mesh using `commit()` or
`commit_to_arrays()`. Calling `generate_normals(true)` will flip the resulting normals. As a side
note, `generate_normals()` only works if the primitive type is set to `Mesh.PRIMITIVE_TRIANGLES`.

If you don't add custom tangents, they can be added with `generate_tangents()`, but it requires
that each vertex have UVs and normals set already.

gdscript GDScript

```
    st.generate_normals()
    st.generate_tangents()
```

By default, when generating normals, they will be calculated on a per-face basis. If you want
smooth vertex normals, when adding vertices, call `add_smooth_group()`. `add_smooth_group()`
needs to be called while building the geometry, e.g. before the call to `add_vertex()`
(if non-indexed) or `add_index()` (if indexed).