allow saving vertex normal in save_obj

[dhbloo]

Although we can load per-vertex normals in load_obj, saving per-vertex normals is not supported in save_obj.

This patch fixes this by allowing passing per-vertex normal data in save_obj:

def save_obj(
    f: PathOrStr,
    verts,
    faces,
    decimal_places: Optional[int] = None,
    path_manager: Optional[PathManager] = None,
    *,
    verts_normals: Optional[torch.Tensor] = None,
    faces_normals: Optional[torch.Tensor] = None,
    verts_uvs: Optional[torch.Tensor] = None,
    faces_uvs: Optional[torch.Tensor] = None,
    texture_map: Optional[torch.Tensor] = None,
) -> None:
    """
    Save a mesh to an .obj file.

    Args:
        f: File (str or path) to which the mesh should be written.
        verts: FloatTensor of shape (V, 3) giving vertex coordinates.
        faces: LongTensor of shape (F, 3) giving faces.
        decimal_places: Number of decimal places for saving.
        path_manager: Optional PathManager for interpreting f if
            it is a str.
        verts_normals: FloatTensor of shape (V, 3) giving the normal per vertex.
        faces_normals: LongTensor of shape (F, 3) giving the index into verts_normals
            for each vertex in the face.
        verts_uvs: FloatTensor of shape (V, 2) giving the uv coordinate per vertex.
        faces_uvs: LongTensor of shape (F, 3) giving the index into verts_uvs for
            each vertex in the face.
        texture_map: FloatTensor of shape (H, W, 3) representing the texture map
            for the mesh which will be saved as an image. The values are expected
            to be in the range [0, 1],
    """

[bottler]

Is the idea that, if they want, the user can supply verts_normals or faces_normals or both? I think so, and I think the logic for dealing with verts_normals and the logic for dealing with faces_normals should have nothing to do with each other. There's no need for a save_normals flag which makes them interact.

[dhbloo]

Hi, the logic is that both verts_normal and faces_normal are required to save normals in the OBJ file. faces_normal are used to index the verts_normal, and for the case when each vertex has only one corresponding normal, it should be the same with faces.

[bottler]

Ah right, I misunderstood completely. Could the inputs be called normals and faces_normals_idx instead of verts_normals and faces_normals? I think it would be clearer.

[dhbloo]

Sure, I've changed it. I took the name similar to verts_uvs and faces_uvs, but it is indeed a bit misleading.

[bottler]

perhaps faces_normals = faces * 3 or something to make it more obvious that the format is correct.

[dhbloo]

updated the test with more normals per face

[facebook-github-bot]

@bottler has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@bottler merged this pull request in 092400f.