Compound / assembly modeling in EzyCad

[trailcode]

Status: Open — deferred for design discussion; revisit later.

Context

• STEP assemblies (e.g. multi-part crankshaft .stp files) often define many PRODUCT entities in the file. Example: B5D-E1400-00 CRANKSHAFT ASSY YAMAHA FREEGO LTK125.stp contains 9 PRODUCT lines in the STEP text.
• Open CASCADE may still deliver one top-level TopoDS_Shape (often a TopoDS_Compound) after TransferRoots(), rather than nine separate roots—behavior depends on the exporter and reader settings.
• Performance and UX (move, tessellation, selection) are heavy when the whole assembly is one large compound mesh.

Current behavior (EzyCad)

• Each document shape is a Shp (AIS_Shape) holding a TopoDS_Shape. Compounds are already valid B-rep; BRepTools::Write / Read in project JSON preserves topology.
• m_shps is a flat list—no scene hierarchy, assembly tree, or named instances in the document model.
• Export already builds a TopoDS_Compound when merging multiple selected shapes (build_export_shape_).

Gaps (what “compound support” could mean)

1. Workflow: Explode or split a compound into multiple Shp entries for editing; optionally group selected shapes into one compound.
2. UI: Scene tree showing children of compounds; clearer selection of sub-solids (depends on OCCT selection modes).
3. STEP: Optional import strategies (e.g. split roots, or STEPCAF / XCAF) for assembly metadata, names, and structure.
4. Long-term: Full XCAF-based assembly model and richer STEP round-trip.

Suggested directions (when revisiting)

Tier	Scope	Notes
1	Explode / group compounds	TopExp_Explorer or TopoDS_Iterator; replace one Shp with N, or merge N into one compound; undo integration.
2	Tree panel + import tweaks	List m_shps and compound children; optional STEP post-split for multiple roots.
3	XCAF / STEPCAF	Product tree, colors, instance transforms; large effort.

Related work already done (same era)

• PLY import/export, binary file open for import, import_step returns Status, .stp in import filter, batched redisplay for transforms, PLY export via mesh writer (not DEPLY_Provider).

References

• OCCT: TopoDS_Compound, TopExp_Explorer, STEPControl_Reader, optional STEPCAFControl_Reader / XCAF.
• Code: Occt_view::import_step, build_export_shape_, m_shps / to_json shape array in occt_view.cpp.