Edit mode with boundary cell selection, ID assignment and save

[pcolt]

Summary

• See all exterior boundary cells (including those not explicitly in the file)
• Click-select individual boundary cells
• Assign BoundaryID values to selected cells
• Save the result as .vtu

Key design points

• Exterior edge extraction: For each volume cell, enumerates sub-cells (edges in 2D, faces in 3D). Sub-cells shared by exactly 1 volume cell are exterior boundary.
• Interior boundary preservation: File boundary cells that are interior (shared by 2 volume cells, like deal.II's inner ring markers) are preserved in the merged dataset.
• Picking: Uses vtkCellPicker with PickFromListOn() restricted to the boundary actor, triggered via interactor observer on LeftButtonPressEvent.
• No changes to vtk_pipeline.py: The existing visualization pipeline is untouched; edit mode uses its own actors.

Data flow

File load → build_visualization() [existing]
         → extract_all_boundary_subcells(full_dataset) [new]
         → build_merged_boundary_dataset() [new]
         → actors added to renderer (hidden)

Edit ON  → show merged_bnd_actor, hide original bnd_actor
         → install pick observer on interactor

Click    → observer fires → vtkCellPicker.Pick() → toggle cell in selection_set
         → update_selection_actor() → render

Assign   → modify BoundaryID array on merged_bnd_dataset
         → rebuild LUT + coloring → clear selection → render

Save     → combine vol cells + edited bnd cells → write .vtu

Edit OFF → hide merged actors, restore original bnd_actor

Screencast.from.2026-03-05.13-06-51.webm

[pcolt]

Few fixes are probably needed like updating the legend color bar after a new boundary Id with new color is assigned

[pcolt]

Legend update was fixed

Screencast.from.2026-03-05.16-52-41.webm

[pcolt]

Group selection by adjacency with angle threshold was added. I would need few simple 3D grids in .vtk/.vtu format to test those new features for 3D grids too.

Group selection by connectivity and angle threshold

1. Adjacency graph (build_adjacency_graph): iterates every cell in the merged boundary dataset and records its edges.
   Cells sharing an edge become neighbors. For 1-D cells (boundary lines in 2-D meshes) the key is a single shared point.

2. Cell normals (compute_cell_normals): for each 2-D cell, computes the cross product of two edge vectors from its first three vertices and normalizes it. For 1-D line cells it produces the 2-D perpendicular in the XY plane.

3. Flood-fill (flood_select): BFS starting from the clicked cell. A neighbor is added to the selection only if the absolute value of the dot product between the two neighbor cells is greater than the cosine of the threshold angle.

Pick / Rotate mode toggle

Previously, left-click camera rotation was fully disabled in edit mode to avoid conflicts with cell picking. Now a Pick / Rotate toggle appears at the top of the edit drawer.

Screencast.from.2026-03-06.17-21-24.webm

Screencast.from.2026-03-06.17-21-50.webm

[luca-heltai]

With 2listic/coral#50 you should be able to generate 3d grids with the dealii backend.

[luca-heltai]

I'm not an expert on this, but it is usable, and I like it. For me, it is a go.