Fixture attachment paths on trusses

Separate attachment domains

GDTF Magnet geometries describe discrete structural connection locations. They remain the source for truss-to-truss joining and are neither copied nor expanded for fixture mounting. A fixture can be mounted continuously along a main chord, so Perastage derives runtime attachment paths from the truss geometry instead. The derived paths are an application inference, not data defined by MVR or GDTF.

Resolution and coordinate spaces

core/truss_attachment_paths owns the GUI-independent resolver. Its curve-ready path value contains a local polyline, an instance-transformed world polyline, a stable runtime identifier, optional estimated thickness and detection diagnostics. Resolution first reads truss.gdtfSpec through the safe read-only archive reader. It resolves each Structure's named Model, composes its full Geometry/Structure Position hierarchy, and tries usable Structure geometry before other model-bearing GDTF geometries. Only after all usable GDTF sources are exhausted does it try MVR Geometry3D/Symbol geometry; oriented bounds are the final fallback. Priority is based on the resource actually being analyzed, not the mutable sourceRepresentation import hint.

The current GDTF standard identifies Structure geometry and its model but does not explicitly enumerate the individual fixture-mounting chord paths. Therefore ExplicitStandardStructure remains reserved: geometry-derived Structure paths use GdtfStructureGeometry, generic GDTF models use GdtfModelGeometry, and the individual chords remain Perastage runtime inference.

Paths are never written to a GDTF archive, MVR document, ChildPosition, project extension, or any other persistent field. Committing a fixture snap stores only the existing fixture transform and standard scene group relationship.

Straight-chord analysis

The first implementation measures a clear dominant bounds axis, then intersects the indexed mesh with 17 evenly spaced transverse planes. Nearby intersection points form occupied cross-section regions. Regions are tracked between planes and accepted only when they persist through at least 70 percent of the truss and remain transversely stable. This persistence rule rejects short members and diagonal braces without assuming that a truss has two, three, or four chords.

The analysis operates on the triangles after loader node/object transforms have been applied. It therefore works for separately authored members and for one welded or monolithic mesh. Centralized constants in truss_attachment_paths.cpp define axis dominance, sampling, clustering, persistence, and stability tolerances. Ambiguous dimensions, malformed indices, non-finite vertices, and unstable tracks fail conservatively.

Each accepted track currently becomes a straight two-point polyline whose span comes from the track's occupied sample range rather than unrelated global mesh extents. Radius remains unset unless local geometry can support a reliable estimate. Snapping uses the generic ClosestPointOnPath operation, so sampled curves, closed paths, and multi-segment corner paths can be added without changing the snap contract.

Cache, fallback, and viewers

Local analysis is cached by canonical geometry path, file size, and modification time. Instance transforms are applied after lookup. Moving or rotating a truss therefore does not parse or analyze its geometry again, while replacing the resource invalidates its entry. The cache is process-local and is not serialized. Each viewer owns its resolver, and analysis requests load vertex/index data without decoding textures or initializing wxWidgets image handlers. This keeps attachment resolution safe when an overlay is first requested from a movement event. If a preferred geometry resource is readable but produces no reliable paths, the resolver retains its diagnostics and continues to the next eligible source before selecting the bounds fallback.

Fixture snapping uses reliable paths first and records the selected runtime path, parameter, provenance, and confidence in transient SnapResult state. If no analyzable geometry or stable chord is available, the previous oriented-bounds surface calculation remains as an explicitly low-confidence ApproximateBoundsFallback. In the 3D viewer, continuous fixture paths are acquired within a 16-logical-pixel screen aperture. Camera-depth separation does not reject a visible path; depth and full world distance are deterministic tie-breakers after screen distance. Perspective-correct reconstruction resolves the selected screen position back onto the actual runtime polyline, so the committed result remains a complete 3D translation and retains its continuous path parameter. Calls without a fixture projection retain the existing world-space behavior, including the 2D viewers and headless callers.

Fixture overlays project every point and draw continuous segments from the same resolved polylines; they do not expose truss connector Magnets. Truss and truss-group overlays continue to show the independent discrete connector candidates, whose acquisition and ranking are unchanged.

The detector currently supports straight trusses with a clearly dominant axis. Curved, circular, corner, unusually sparse, and heavily occluded geometry will need structure-aware or curve-fitting resolvers before they can produce reliable paths and currently use the conservative fallback.