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Tommaso Bianchi
695932ad87
A filleted solid arrived on the plate as a broken model: the slicer reported
"8 non-manifold edges" on an 80x50x12 box with r=3 on all edges, and advised
repairing it in another CAD application -- the exact round trip the Design tab
exists to remove. The same box without the fillet committed cleanly.
Measured rather than guessed. Each of the 8 bad edges is degenerate, both
endpoints the same vertex:
open tri=145 edge=1 face=5 v59(3.000000 3.000000 0.000000) v59(3.000000 3.000000 0.000000)
open tri=538 edge=1 face=6 v87(3.000000 3.000000 12.000000) v87(...)
... one per corner, 8 corners
OCCT triangulates a degenerate surface parameterization with a triangle at the
pole; a corner sphere patch has exactly one. Its two pole nodes are distinct in
the per-face triangulation and collapse to a single vertex when the faces are
welded, leaving a zero-area triangle whose v->v edge can never pair with a
neighbour. its_face_neighbors counts it as open, and the field the object panel
prints as "non-manifold edges" is in fact stats.open_edges.
So the geometry was never wrong -- the B-rep volume matches the Steiner formula
for a box dilated by a ball to 0.016%. Only the bookkeeping was.
Dropping those triangles after the weld removes 8 of 3492 and takes open_edges
to 0. Zero area, so nothing about the shape changes. tri_face is compacted in
the same pass, since it must stay index-aligned with the triangle list that the
face picking and per-body colouring both index into.
Guarded by a new [CadDocument] case that asserts open_edges == 0, no degenerate
triangle survives, and both per-triangle maps still match the triangle count.
The existing suite only ever checked B-rep volumes and areas, which is why a
mesh defect this visible went unnoticed: 150 cases / 2049 assertions green on
both forks.
snaporca-agw