Outie

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

A Python port of libigl's reorient_facets_raycast, 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 by Embree, through trimesh.

Use

import outie

polygons = outie.orient(polygons)              # a list of rings, each n by 3
faces = outie.orient_mesh(vertices, faces)     # triangles: vertices n by 3, faces m by 3

Both return what they were given, with the faces that pointed in turned round. Polygons are cut into triangles first, each as one consistent piece, and a ring that touches or crosses itself, as a ring with a hole cut into it does, is mended before it is cut. Cleaning the mesh, dropping collapsed or doubled faces, splitting faces that are not flat, is the caller's job before calling. Underneath are libigl's own two functions, with its names, arguments and outputs, for callers who need to know which faces moved or which patch each belongs to:

flip, patch = outie.reorient_facets_raycast(vertices, faces)   # per triangle: turn it? and its patch
faces, patch = outie.bfs_orient(faces)                         # faces wound to agree, and their patches

The paper's settings are keyword arguments with libigl's defaults: rays_total, rays_minimum, facet_wise, use_parity, and seed so a run repeats. Two additions, off unless asked for: closed_by_volume settles a patch that is a closed surface by its signed volume, which is exact, and shoots rays only for the rest; occluders are faces that block rays but are never turned, for the ground a model stands on or the things around it, since a model with no floor is otherwise as open below as above.

Licence

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