Closing the reverse-engineering loop

fit_cad turns a skin into a solid in one pass and reports its Hausdorff deviation. Two operations close the loop around it (roadmap 7.7.1, 7.7.2): face_support says how much of each face of the solid the scan actually supports, and fit_cad_iterate refits towards a target, re-segmenting the faces that fall short.

import meshioplusplus

# A scan missing its bottom, the hole closed by repair (which marks the fill).
closed = meshioplusplus.repair(scan, max_hole_edges=0, record_provenance=True)
skin = meshioplusplus.fit_primitives(meshioplusplus.segment(closed, method="primitive"))
solid = meshioplusplus.fit_cad(skin)
marked, report = meshioplusplus.face_support(solid, closed, return_report=True)
report["num_measured"], report["num_inferred"]  # 5, 1: the bottom was guessed

best, rounds = meshioplusplus.fit_cad_iterate(closed, "part.step", part_class="cast", return_report=True)
[r["share_within"] for r in rounds["rounds"]], rounds["stop_reason"]

Support per face

face_support(solid, scan) samples each face of a fit_cad re-mesh (its fit_cad:face cell data; each triangle's four sub-triangle centroids, weighted by a quarter of its area) and measures every sample's distance to the scan. A face is measured when at least measured_share (0.9) of its area lies within tolerance of the scan, inferred below inferred_share (0.1): no scan near it (a hidden interior, the far side of a through-hole, a face the solid closed by construction), and partial between. Scan cells whose hole_data cell data (repair:hole, repair's provenance with record_provenance=True) is at least 0 are the fills repair added, and count as unmeasured, so a hole repair closed before fit_cad reads as inferred. tolerance defaults to 1e-3 of the scan's diagonal.

The solid gains the field data fit_cad:face_support (Int64 per face: 0 measured, 1 partial, 2 inferred) and fit_cad:face_measured (Float64, the share), and the cell data fit_cad:support (each triangle's face's class). The report gives face_share, face_class, the three counts, share_within (the share of the scan's points, on measured cells, within the tolerance of the solid) and tolerance.

A box scanned without its bottom, closed by repair and fitted, has five measured faces and an inferred bottom (tests/cpp/test_fit_cad_loop.cpp).

Targets and the iterating driver

fit_cad_iterate(scan) segments the scan with segment's primitive method, fits with fit_primitives and builds with fit_cad (face_deviation on), in rounds. Each round measures share_within (as face_support does, at fit_cad's tolerance) and explained (the share of the solid's area on analytic or feature faces, fit_cad:type other than 0), then:

1. stops at the target: share_within >= target_within and explained >= target_explained (0: off);
2. stops at no improvement: a round no better than the best so far (by share_within, then explained);
3. stops after max_rounds (4);
4. otherwise re-segments the patches of the worst_faces (3) worst faces (those whose face deviation exceeds the tolerance, largest first, then the largest faceted faces) with the segment tolerance times refine_factor (0.25), splicing their new labels in after the others; with nothing to refine it stops.

The best round's solid is kept (and written to step_path as STEP); each round reports share_within, explained, hausdorff_max, segment_tolerance, num_patches, num_faces and the patches it re-segmented. part_class sets the defaults a part's process suggests:

part_class	target_within	tolerance (of the diagonal)
any (default), analytic	0.90	1e-3
cast	0.85	2e-3
freeform	0.75	4e-3

segment_tolerance and fit_tolerance override the class's tolerance for the first segmentation and for fit_primitives. Deterministic: the same input and options give the same rounds. A box whose top bends in its middle, segmented coarsely, merges the two halves into one patch no plane explains; the second round separates them and explained rises (tests/cpp/test_fit_cad_loop.cpp).

Surfaces

C++ face_support, fit_cad_iterate and fit_cad_part_class_from_name (operations/fit_cad_loop.hpp); Python meshioplusplus.face_support and fit_cad_iterate (C++ core only); both CLIs (face-support SOLID SCAN [OUT] [--tolerance T] [--measured-share S] [--inferred-share S] [--hole-data NAME] [--json], fit-cad-iterate SCAN OUT [--step PATH] [--part-class C] [--target-within S] [--target-explained S] [--segment-tolerance T] [--fit-tolerance T] [--max-rounds N] [--worst-faces N] [--refine-factor F] [--backend B] [--json]); the FaceSupport (Scan) and FitCadIterate pipeline steps; C mio_face_support (mio_face_support_opts, mio_face_support_report, 64 bytes each) and mio_fit_cad_iterate (mio_fit_cad_iterate_opts, 112 bytes; mio_fit_cad_iterate_report, 96; mio_fit_cad_iterate_round); Fortran, Julia, R, Octave/MATLAB, WASM faceSupport and fitCadIterate; the MCP tools face_support and fit_cad_iterate.