Extract Edges

Extract edges from a surface.

from __future__ import annotations

import pyvista as pv
from pyvista import examples

From vtk documentation, the edges of a mesh are one of the following:

1. boundary (used by one polygon) or a line cell

2. non-manifold (used by three or more polygons)

3. feature edges (edges used by two triangles and whose dihedral angle > feature_angle)

4. manifold edges (edges used by exactly two polygons).

The extract_feature_edges() filter will extract those edges given a feature angle and return a dataset with lines that represent the edges of the original mesh.

To demonstrate, we will first extract the edges around a sample CAD model:

# Download the example CAD model and extract all feature edges above 45 degrees
mesh = examples.download_cad_model()
edges = mesh.extract_feature_edges(45)

# Render the edge lines on top of the original mesh.  Zoom in to provide a better figure.
p = pv.Plotter()
p.add_mesh(mesh, color=True)
p.add_mesh(edges, color="red", line_width=5)
p.camera.zoom(1.5)
p.show()

Static Scene

Interactive Scene

We can do this analysis for any pyvista.PolyData object. Let’s try the cow mesh example:

mesh = examples.download_cow()
edges = mesh.extract_feature_edges(20)

p = pv.Plotter()
p.add_mesh(mesh, color=True)
p.add_mesh(edges, color="red", line_width=5)
p.camera_position = [(9.5, 3.0, 5.5), (2.5, 1, 0), (0, 1, 0)]
p.show()

Static Scene

Interactive Scene

We can leverage the pyvista.PolyData.n_open_edges property and pyvista.DataSetFilters.extract_feature_edges() filter to count and extract the open edges on a pyvista.PolyData mesh.

# Download a sample surface mesh with visible open edges
mesh = examples.download_bunny()
mesh

PolyData	Information
N Cells	69451
N Points	35947
N Strips	0
X Bounds	-9.469e-02, 6.101e-02
Y Bounds	3.299e-02, 1.873e-01
Z Bounds	-6.187e-02, 5.880e-02
N Arrays	0

We can get a count of the open edges with:

mesh.n_open_edges

223

And we can extract those edges with the boundary_edges option of pyvista.DataSetFilters.extract_feature_edges():

edges = mesh.extract_feature_edges(boundary_edges=True, feature_edges=False, manifold_edges=False)

p = pv.Plotter()
p.add_mesh(mesh, color=True)
p.add_mesh(edges, color="red", line_width=5)
p.camera_position = [(-0.2, -0.13, 0.12), (-0.015, 0.10, -0.0), (0.28, 0.26, 0.9)]
p.show()

Static Scene

Interactive Scene