Outie

Turns every face of a polygon mesh to point out, like an outie, even when the mesh is not closed.

A port of libigl's reorient_facets_raycast, written in Rust and used from Python, the reference code for Takayama, Jacobson, Kavan and Sorkine-Hornung, A Simple Method for Correcting Facet Orientations in Polygon Meshes Based on Ray Casting, 2014.

How

Faces that share edges are gathered into patches first, so a patch is decided as one. Rays are traced in Rust by parry, in parallel on every core, and numpy arrays pass in without being copied.

Use

import outie

flip, patch = outie.reorient_facets_raycast(vertices, faces)   # vertices n by 3; faces rings of corner numbers
faces = [f[::-1] if turn else f for f, turn in zip(faces, flip)]

libigl's function, with its name, arguments and outputs: per face, whether to turn it, and the patch it belongs to. The paper's faces are polygons, and so are outie's: a face is a ring of corners, any number of them, or its outer ring and then its holes, each a ring; faces are given as a list of them or an m by k array; given triangles, it is libigl's function exactly. Each face is cut into triangles only to cast rays from, and is decided whole, with its patch. The settings are keyword arguments with libigl's defaults: rays_total, rays_minimum, facet_wise, use_parity, and seed so a run repeats.

One addition: occluders, triangles as corners, k by 3 by 3, that rays can hit but that are never turned: the ground a model stands on, or the things around it. A model with no floor is otherwise as open below as above. The ground belongs exactly at the model's lowest point, not below it.

Mending a face that crosses itself is the caller's.

Build

The core is Rust, in src/lib.rs, bound to Python by PyO3 and built by maturin; uv sync builds it, and uv run pytest tests it. Releases are a wheel for each system, so installing needs no Rust.

Licence

MPL-2.0, as libigl is: this is a port of its code, taken at libigl commit 7100764. The method is its authors'.