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Zero-width artefacts in rotated plans (design note)

Status: Done (D-114) · Date: 2026-10-03 · Decisions: D-088, D-090, D-111, D-113, D-114; ADR-002, ADR-004, ADR-010

A revision after S6, run as D-111 sets out: built once, test-first, on a feature branch in a worktree; reviewed by a second agent; scripts/verify.ps1 before the merge; the headless Rhino check once; recorded in one decision-log entry. No version change or tag unless the fix changes output; pushing waits for the owner.

Problem

Rotated and moved plans can carry zero-width artefacts in zone parts and surfaces: spikes (a vertex going out and straight back), excursions through a repeated vertex, and slivers about 1e-6 m wide that have no partner on the adjacent side. S6 found them when the stepped band (SYN-TYP-012) was rotated and moved by Transform Plan (131.4° + (15.08, −153.9), 30° + (5.3, −2.1)) and passed through every simplifier and aggregator. Validation accepts them; Convert2IDF drops them with SliverOmitted before pairing surfaces (ADR-010). They come from clipping on the integer grid (the D-090 limit) somewhere in Lod.Core.

Tasks

  1. Reproduce. Failing tests in tests/Lod.Integration.Tests that detect the artefacts directly on zone parts and surfaces (a spike, a repeated vertex, a ring or piece narrower than ToleranceSettings.Distance relative to its perimeter), for the rotated stepped band and at least one other family, with every simplifier × aggregator at the two placements.
  2. Locate. For each artefact, the step that creates it (PolygonOps, VertexGrid/UnionTiles, LayoutSurfaceBuilder, Storeys, SingleZoneMerge, PerimeterCore, or the transform).
  3. Fix at the source, test-first: remove spikes and zero-area excursions where they arise, identically on both sides of a shared edge, without moving any real vertex; documented in ADR-002 (and ADR-004 if a rule about the grid changes).
  4. Downstream. If the pipeline no longer produces them, Convert2IDF's sliver handling stays as a guard; its warning should then not fire on any generated plan (assert it in the export tests).

Acceptance criteria

  • The reproduction tests pass; no generated, rotated plan carries a spike, repeated vertex, or sliver in any zone part or surface.
  • No tolerance is widened and no validation check relaxed (AGENTS.md rules 12 and 14); existing snapshots stay byte-identical unless a documented change forces it.
  • scripts/verify.ps1 passes; the rhino-smoke specs and the IDD check of the export tests still report no problems.