Write a faceted STEP file

Send a tessellated mesh back to a STEP file via OCCT’s STEPControl_Writer. The result is faceted (triangulated), not a B-rep reconstruction, ideal for downstream tools that accept STEP for visualization or CAM but do not require analytical surfaces.

This example takes a real 3D scan, the T-LESS object-5 reconstruction from Primesense RGB-D scans (CC-BY 4.0, Hodaň et al., WACV 2017), decimates it to a CAM-friendly facet count, and writes it to a faceted STEP: the canonical “scanned mesh into a CAD/CAM pipeline” hand-off.

from pathlib import Path
import tempfile

import numpy as np
import pyvista as pv

import pyvista_cad
from pyvista_cad.examples import downloads

Load the real scan. A faceted STEP carries one face per triangle, so decimation is the routine prep before this hand-off.

_, scan_path = downloads.scan_pair_paths()
src = pv.read(scan_path).extract_surface().triangulate().decimate(0.8)
src.field_data['cad.units'] = np.array(['mm'])
src

/home/runner/work/pyvista-cad/pyvista-cad/examples/02_writers/write_step_faceted.py:33: PyVistaFutureWarning: The default value of `algorithm` for the filter
`PolyData.extract_surface` will change in the future. It currently defaults to
`'dataset_surface'`, but will change to `None`. Explicitly set the `algorithm` keyword to
silence this warning.
  src = pv.read(scan_path).extract_surface().triangulate().decimate(0.8)

PolyData (0x7f6fa91aada0)
  N Cells:    8579
  N Points:   4542
  N Strips:   0
  X Bounds:   -4.840e+01, 4.815e+01
  Y Bounds:   -2.843e+01, 2.755e+01
  Z Bounds:   -3.066e+01, 2.916e+01
  N Arrays:   1

PolyData 4,542 points 8,579 cells 1.5 MiB

Bounds⧉X [-4.840e+01, 4.815e+01]Y [-2.843e+01, 2.755e+01]Z [-3.066e+01, 2.916e+01]

Cellsfaces 8,579

• Field Data: (1)
  • cad.units⧉
    (1,)
    <U2

Write a faceted STEP.

path = Path(tempfile.mkdtemp()) / 'tless_obj5_faceted.step'
pyvista_cad.write_step(src, path)

Read back. The output is a faceted STEP; check the cell count tracks the source tessellation.

out = pyvista_cad.read_step(path)
out

MultiBlock (0x7f6fa8110e80)
  N Blocks:   1
  X Bounds:   -4.840e+01, 4.815e+01
  Y Bounds:   -2.843e+01, 2.755e+01
  Z Bounds:   -3.066e+01, 2.916e+01

MultiBlock 1 blocks 1.0 MiB

Bounds⧉X [-4.840e+01, 4.815e+01]Y [-2.843e+01, 2.755e+01]Z [-3.066e+01, 2.916e+01]

• Blocks: (1)
  • Open_CASCADE_STEP_translator_7_9_1_1MultiBlock9 blocks · 1.0 MiB

Render colored by Z.

surface = out.combine().extract_surface()
surface['Z'] = surface.points[:, 2]

pl = pv.Plotter()
pl.add_mesh(surface, scalars='Z', cmap='plasma', show_edges=True, edge_color='black')
pl.show()

Static Scene

Interactive Scene

/home/runner/work/pyvista-cad/pyvista-cad/examples/02_writers/write_step_faceted.py:53: PyVistaFutureWarning: The default value of `algorithm` for the filter
`UnstructuredGrid.extract_surface` will change in the future. It currently defaults to
`'dataset_surface'`, but will change to `None`. Explicitly set the `algorithm` keyword to
silence this warning.
  surface = out.combine().extract_surface()