Outcropping faults: sheets meet the upper surface (specify-long, bowl cavity, remeshed cap)

[lmoresi]

The science case the fault meshes exist for: plate-boundary faults intersect the free surface. Spike-proven first (outcrop_sheet_spike.py, two dead ends measured and recorded), then production in place_sheet, serial and parallel.

• Specify-long contract (ruling 2026-08-11): sheets may extend past the domain. _clip_sheet_to_box clips each triangle against the box planes — cut points snap exactly onto the plane, computed in canonical edge order so shared edges dedupe bitwise — and _outcrop_chain extracts the trace as one ordered polyline on one wall. Box-edge and multi-wall outcrops refuse with the reason (recorded residuals, with curved/deformed tops — the manifold-prep slot).
• The bowl cavity: wall vertices inside the cap become victims; _closed_shell_3d returns dropped cells' wall faces as CAP faces (other wall contact still refuses); the manifold check runs on shell ∪ cap. The cap is pre-meshed by the 2-D fill in the wall plane's own coordinates (rim verbatim, outcrop embedded) because a geo surface bounded by the discrete rim RESAMPLES it — measured, and MeshOnlyEmpty does not prevent it. The 3-D fill takes the loop [shell + discrete cap] with the sheet embedded, outcrop nodes shared by tag.
• Explicit relabelling: the cap's new wall faces carry exactly the (label, value) pairs the replaced faces carried, with full closures — joins cannot recover new points, and an unlabelled boundary patch silently loses its Dirichlet conditions.

Measured, serial and np=2..4: identical facet counts, P2 quadratic FE-exactness (2e-11–5e-11) with a Dirichlet condition gripping the remeshed Top, and full Top-label coverage asserted face-by-face. Interior sheets byte-for-byte unaffected: suites 0853–0856 (44 serial) and the placement ptests green. New contract test in test_0854. The outcropping thin-volume BAND (cap-with-hole + interior skin as one closed loop) is the remaining instalment; the plan file carries its design.

Underworld development team with AI support from Claude Code

[lmoresi]

Adversarial review (self, posted per project practice):

What we attacked: the cap is the first fill piece whose triangulation is OURS rather than gmsh's-under-gates, so its two failure modes got explicit probes. Resampling: the geo-surface route produced 84 renumbered rim nodes / volume 1.31 / Euler −46 before the pre-meshed discrete cap; the moved-node gate now covers rim and chain, and the cap-triangle-count gate refuses a remeshed cap. Label loss: the coverage assertion in test_0854 walks every boundary face on the top plane — an unlabelled patch cannot pass, and the FE oracle with Dirichlet on the remeshed Top would diverge from the quadratic if the condition lost grip anywhere.

Scope honesty: one flat wall per sheet; box-edge outcrops, multiple traces, closed traces, and curved/deformed tops refuse with their reasons (the last is the manifold-prep slot and matters for free-surface runs — recorded in the plan). The clip has no projection band yet: a sheet stopping short of the wall is honestly buried, not snapped; the band is a stated refinement.

Process note for the record: the working-directory drift bit again mid-landing (the branch was created in the wrong checkout and the main repo briefly sat on a feature branch — restored to development immediately). All git operations in this session now name their repo explicitly.

Underworld development team with AI support from Claude Code

[Copilot]

Pull request overview

Adds “outcropping sheet” support to place_sheet() so a fault surface can extend beyond the domain, be clipped to the box, and (when it intersects a single wall plane) open a cavity onto that wall with a pre-meshed/remeshed cap that preserves boundary labels. This extends the placed-surface tooling to cover the stated plate-boundary science case where faults intersect the free surface.

Changes:

• Clip input sheet triangles to global box bounds and extract a single-wall outcrop chain to drive an “open wall” cavity case.
• Extend the 3-D cavity carve + shell closure + gmsh fill path to support an open-wall cap (pre-meshed in 2-D and embedded into the 3-D fill).
• Add a new regression/contract test asserting the outcropping case preserves Top boundary labelling and retains P2 exactness under Dirichlet BCs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/underworld3/utilities/place_surface.py	Implements sheet clipping + outcrop detection; supports open-wall cavity with a pre-meshed cap; explicitly re-applies boundary labels to remeshed cap faces.
tests/test_0854_place_sheet.py	Adds a new test exercising an outcropping sheet that intersects the Top wall and verifies label preservation and FE exactness.

Suppressed comments (2)

src/underworld3/utilities/place_surface.py:1704

• on_plane can only store one wall code per point, but corner/edge points lie on multiple planes. The current assignment overwrites earlier axes, so _outcrop_chain() may fail to detect multi-wall/box-edge outcrops (which are supposed to be refused) and can misclassify boundary edges.

    on_plane = np.full(len(out_pts), -1, dtype=np.int64)
    for axis in range(3):
        on_plane[out_pts[:, axis] == lo[axis]] = 2 * axis
        on_plane[out_pts[:, axis] == hi[axis]] = 2 * axis + 1
    return out_pts, np.array(out_tris, dtype=np.int64), on_plane

src/underworld3/utilities/place_surface.py:2468

• wall_pairs currently includes label values even when the value is negative (typically meaning “unlabelled”). When those pairs are later applied to the full transitive closure, this can overwrite existing non-negative label values on shared rim points/edges with the negative value, silently stripping labels that were not meant to be changed. Only propagate non-negative label values from the replaced faces.

                    for val in values.getIndices():
                        if all(lab.getValue(p) == int(val)
                               for p in cap_face_pts):
                            wall_pairs.append((name, int(val)))

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