Fix #7942 : ExtendWallsToUnderside produces wrong geometry on second call

Closes: (link issue here)

Summary

bpy.ops.bim.extend_walls_to_underside glitched when applied twice to the same wall — once per slope of a ridge roof. The first call produced correct geometry, but the second call introduced spurious extra vertices (e.g. 14 instead of 10), resulting in a malformed gable profile.

Root Cause

clip_wall_to_slab builds an IfcPolygonalFaceSet clip solid from the downward-facing slope face by extruding its exact vertex footprint upward. When the operator is applied twice for a symmetric ridge roof, the two clip solids share an exact ridge edge — their boundaries are coincident in 3D.

OCCT's boolean engine treats this as a kissing-solid condition: two volumes that touch at a shared face or edge rather than overlapping volumetrically. This is numerically degenerate and causes OCCT to introduce extra vertices at the coincident boundary, resulting in a corrupted gable profile.

Fix

Instead of extruding the exact face footprint, clip_wall_to_slab now builds the clip solid's bottom face as a rectangle on the slope plane, sized to the tight bounding box of the original face vertices in slope-plane coordinates, extended by a small margin (1 project unit) in each tangent direction.

This margin pushes each clip solid slightly past the ridge into the adjacent slope's territory, turning the shared ridge edge into a volumetric overlap between the two solids. OCCT handles overlapping DIFFERENCE operands correctly — material already removed by the first boolean simply remains absent.

The construction uses an orthonormal basis spanning the slope plane (tangent1, tangent2 derived from the face normal), projects the face vertices onto those axes to find the tight extents, then adds the margin before building the clip bmesh. The extrusion height (max_z - min_z) and all downstream logic (add_boolean, mark_manual_booleans) are unchanged.

Changes

File

Change

tool/model.py — clip_wall_to_slab

Build per-face clip solid from a margined slope-plane rectangle instead of exact face footprint; one operand per face instead of one combined mesh

tool/model.py — get_slab_clipping_bmesh

Add faces.ensure_lookup_table() after recalc_face_normals

core/model.py — extend_wall_to_slab

Minor cleanup

bim/module/model/wall.py — ExtendWallsToUnderside

Minor cleanup

Testing

Tested against a ridge roof with two slopes applied sequentially to both a wall (IfcWall) and a covering (IfcCovering). Both now produce correct gable profiles after two operator calls.

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