Feature edges

feature_edges(mesh, feature_angle=30) returns the edges of a surface that matter to anyone meshing, loading or inspecting it — the sharp creases, the open boundary, the non-manifold junctions and the pairs of faces that disagree about which side is out — as a mesh of line cells. It is the crease test decimate, decimate_volume and smooth pin nodes with, exposed for inspection and for picking the edges a boundary condition lives on.

import meshioplusplus as mp

mesh = mp.read("part.stl")

edges = mp.feature_edges(mesh)                          # 30 degrees, every category
edges = mp.feature_edges(mesh, feature_angle=60)        # only the sharper creases
edges = mp.feature_edges(mesh, boundary=False)          # creases only, not the open rim
edges, report = mp.feature_edges(mesh, return_report=True)
report["num_feature"], report["num_boundary"], report["num_non_manifold"], report["num_inconsistent"]

mp.write("part_edges.vtp", edges)                       # view it on top of the surface

Which edges

Every edge of the surface's faces is classified from the faces that use it:

Category	feature:kind	Rule
non-manifold	3	used by three or more faces
boundary	2	used by one face
inconsistent	4	used by two faces that walk it the same way (they disagree about "out")
feature	1	used by two faces whose normals are more than feature_angle degrees apart

An edge in several categories is labelled by the first selected one in the order of the table. For an inconsistent pair the angle is measured after reorienting one face, so a flat but inconsistently wound pair is inconsistent and not sharp. Only the two faces that share an edge are compared: a smooth surface that is merely coarsely tessellated has no feature edges however few triangles it has. A face with no area gives no angle, so its edges are never sharp.

The edges of a quad or polygon are its own ring edges; a non-planar quad never reports its diagonal. Higher-order cells contribute their corners.

Volume meshes

When the mesh has volume cells, the surface examined is their skin: the faces used by one selected cell (the global face table orients each outward). region="name" restricts to a named cell region, of surface cells or of volume cells whose skin is taken — so the skin of a sub-volume includes its interface with the rest of the mesh.

Output

The input's points, verbatim and not compacted, so a line's node ids are the input's own; one line block in ascending (low, high) endpoint order; and two cell-data arrays:

Array	Type	Content
feature:kind	int64	1 feature, 2 boundary, 3 non-manifold, 4 inconsistent
feature:angle	float64	the dihedral angle in degrees; NaN where it is undefined

Point data, field data and point regions ride through; cell and side regions name cells the edge mesh does not have and are dropped with a warning. The report counts every category over the whole surface, whether or not it was selected for output.

How decimate and smooth use it

Since v16.23.0 the three operations that preserve features pin the endpoints of every sharp or non-manifold edge this test reports (open edges stay their own preserve_boundary / fix_boundary decision). Before, each carried a per-vertex test that compared every pair of faces around a vertex, adjacent or not, which pinned every vertex of a coarsely tessellated sphere and let a decimation do nothing. smooth on a surface mesh also used to look only at the boundary polyline, in the XY plane; it now also holds the creases of the surface itself, and measures the polyline's corners in 3-D.

CLI

meshioplusplus feature-edges IN OUT [--angle DEG] \
    [--no-feature] [--no-boundary] [--no-non-manifold] [--no-inconsistent] \
    [--region NAME] [--json] [--quiet]

Other languages

mio_feature_edges_opts opts;
mio_feature_edges_opts_init(&opts);
opts.feature_angle = 45.0;
mio_feature_edges_report report;
mio_mesh* edges = mio_feature_edges(mesh, &opts, &report);

edges = m%feature_edges(feature_angle=45.0_real64, num_feature=nfeat)

r = feature_edges(mesh; feature_angle=45)
r.mesh, r.num_feature

r <- mio_feature_edges(mesh, feature_angle = 45)

const r = m.featureEdges(mesh, 45);
r.mesh.cell_data['feature:kind'];

It is also a settings-pipeline step, {"Op": "FeatureEdges", "FeatureAngle": 30, "Feature": true, "Boundary": true, "NonManifold": true, "Inconsistent": true, "Region": ""} — see pipelines — and the MCP tool feature_edges.