NGSolve Periodic Boundary Condition Regression Bug Report

NGSolve
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NGSolve Periodic Boundary Condition Regression Bug Report

Forum Post Title

Periodic boundary conditions broken with OCC geometry Identify() in version 6.2.2406 and later

Summary

A regression bug has been discovered in NGSolve version 6.2.2406 and all subsequent versions (up to 6.2.2506 as of November 2025) where the Identify() method for periodic boundary conditions with OCC geometry no longer works correctly. The identification information is lost during mesh generation when using Glue(), resulting in zero identifications and no periodic boundary conditions being applied.

Problem Description

When using Identify() on OCC geometry faces combined with Glue(), the identification information is lost during mesh generation. This affects code that previously worked correctly in versions 6.2.2404 and 6.2.2405.

Symptoms

  • mesh.ngmesh.GetIdentifications() returns an empty list (0 identifications)
  • Periodic boundary conditions are not applied
  • No error messages are generated
  • Mesh generation completes successfully but without periodic identification

Code to Reproduce

from ngsolve import *
from netgen.occ import *
from netgen.meshing import MeshingParameters

iron = Box((0,0,0),(1,1,1))
iron.name = "iron"
iron.maxh = 0.2
internal_domain = Sphere(Pnt(0,0,0.0), r=2)*Box((0,0,0), (5,5,5))
internal_domain.name = "air"
internal_domain.maxh = 1.0
internal_domain.faces.Min(Z).name = "gnd"
internal_domain.faces[0].name = "outer"

external_domain = Sphere(Pnt(3,0,0.0), r=2)*Box((3,0,0), (3+5,5,5))
external_domain.name = "air"
external_domain.maxh = 1.0
external_domain.faces.Min(Z).name = "gnd"
external_domain.faces[0].name = "outer"

# Set periodic identification
external_domain.faces[0].Identify(internal_domain.faces[0], "id")
domain = Glue([iron, internal_domain, external_domain])

geo = OCCGeometry(domain, dim=3)
mp = MeshingParameters(maxh=20.0, segmentsperedge=1, grading=0.7)
mesh = Mesh(geo.GenerateMesh(mp)).Curve(2)

# Check identifications
identifications = mesh.ngmesh.GetIdentifications()
print(f"Number of identifications: {len(identifications)}")  # 6.2.2406+: 0

Version Comparison Results

Tested Versions

Version Identifications Status Notes
6.2.2404 20 pairs ✓ Working Verified working
6.2.2405 20 pairs ✓ Working Recommended version
6.2.2406 0 pairs ✗ Broken Bug introduced
6.2.2501 0 pairs ✗ Broken
6.2.2502 0 pairs ✗ Broken
6.2.2503 0 pairs ✗ Broken
6.2.2504 0 pairs ✗ Broken
6.2.2505 0 pairs ✗ Broken
6.2.2506 0 pairs ✗ Broken Latest as of Nov 2025

Working Versions (6.2.2404, 6.2.2405) Output

Number of identifications: 20
[(3.0, 2.0, 0.0), (0.0, 2.0, 0.0)]
[(3.0, 0.0, 2.0), (1.2246467991473532e-16, -2.999519565323715e-32, 2.0)]
[(5.0, -2.4492935982947064e-16, 8.057930683288193e-14), (2.0, -2.4492935982947064e-16, 8.057930683288193e-14)]
[(3.0, 1.868120247864882, 0.7142315727530197), (0.0, 1.868120247864882, 0.7142315727530197)]
[(3.0, 1.4142121958551752, 1.4142149288896946), (0.0, 1.4142121958551752, 1.4142149288896946)]
[(3.0, 0.714227962521057, 1.868121628147648), (0.0, 0.714227962521057, 1.868121628147648)]
...（total 20 pairs）...

Broken Versions (6.2.2406+) Output

Number of identifications: 0

✗ No identifications found

Visual Comparison

Version 6.2.2404 - Working ✓

ngsolve_6.2.2404_working
ngsolve_6.2.2404_working1200×1080 61.7 KB

Figure 1: NGSolve 6.2.2404 with periodic boundary conditions correctly applied. Red lines show identified point pairs where periodic boundary conditions are enforced.

In the working versions above, the two spherical regions’ boundary surfaces (outer) are correctly identified. The corresponding points on the left and right regions are connected by red lines, indicating where periodic boundary conditions are applied.

Version 6.2.2406 - Broken ✗

ngsolve_6.2.2406_broken
ngsolve_6.2.2406_broken1200×1080 58.7 KB

Figure 3: NGSolve 6.2.2406 - No identification lines visible, periodic boundary conditions not applied.

In version 6.2.2406 and later, no red identification lines are displayed, confirming that the identification information has been lost and periodic boundary conditions are not applied.

Suspected Cause

The regression appears to be related to changes in either the Glue() method or the Identify() method between versions 6.2.2405 and 6.2.2406. The identification information set before Glue() is being discarded during the geometry merging process.

Possible causes:

  1. Glue() may be removing identification information during geometry processing
  2. The faces may have incompatible topology after boolean operations (sphere intersections)
  3. OCC geometry identification may not work correctly with complex boolean operations in newer versions

Workaround

Recommended: Downgrade to 6.2.2405

pip install --force-reinstall ngsolve==6.2.2405

Important: After installation, restart your Jupyter kernel if using Jupyter Notebook.

Alternative Approaches (if downgrade is not possible)

  1. Use CSG Geometry instead of OCC

    • CSG geometry with PeriodicSurfaces() method is more reliable
    • However, this provides less geometric flexibility

  2. Use simpler geometries

    • Avoid complex boolean operations (sphere intersections)
    • Simple box geometries may work

Environment

  • OS: Windows
  • Python: 3.12
  • NGSolve: 6.2.2404-6.2.2506 tested
  • netgen-mesher: Corresponding versions
  • Verification Date: November 24, 2025

Request for Development Team

Could the development team please investigate this regression? This is a breaking change that affects existing code using periodic boundary conditions with OCC geometry.

Key questions:

  1. What changed in version 6.2.2406 regarding Glue() or Identify() methods?
  2. Is this an intended behavior change or a bug?
  3. Is there a new recommended approach for periodic boundary conditions with OCC geometry?

Thank you for your excellent work on NGSolve!

Additional Resources

Visualization Export Methods

For saving visualizations from NGSolve webgui:

Method 1: VTK Export (Recommended)

from ngsolve import VTKOutput

vtk = VTKOutput(ma=mesh, coefs=[gfu], names=["solution"], filename="output", subdivision=2)
vtk.Do()
# Open in ParaView for high-quality image export
# ParaView: https://www.paraview.org/download/

Method 2: OS Screenshot Tool

  • Windows: Win + Shift + S to capture WebGuiWidget display
  • Note: WebGL canvas auto-capture via JavaScript is restricted

References

End of Report