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DXF2BIM — a deterministic 2D ⇄ BIM round trip (technical note)

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Redhuan D. Oon ¡ 2026-10-17 ¡ MIT licensed ¡ status: preliminary (basic shapes)

Abstract

We describe a deterministic, rule-based method that (1) reduces a BIM model to a standard 2D drawing set (plans, elevations, roof plan, section) whose every grid line, dimension, level and door swing is derived from IFC relationships and measured geometry, and (2) rebuilds a BIM model from such a 2D set — walls, floor slabs, roof, openings and door/window fills — exporting IFC4 with a storey structure. The meaning the geometry cannot carry is stated as metadata embedded in the DXF itself, each marking recording its source (file, model or user). A qualify verdict gates the build: either the set is complete, or a machine-readable list of blocking items is returned. Nothing is guessed: an unknown value is a named gap. No AI or machine-learning inference runs at any step of the pipeline. On a two-storey residential model (the Duplex) the round trip reproduces every converted element within 1 mm of the reference run and exports 96 wall pieces, 2 slabs, 1 roof, 14 doors and 22 windows on 2 storeys.

1 ¡ Problem

Turning 2D drawings into BIM is usually manual (link the drawing, trace it) or probabilistic (recognise symbols). Our own earlier attempts at the first leg — producing the 2D set from a model — failed in instructive ways (measured on the earlier writer, BIMCompiler 2D_Layout/python/drawing_writer*.py):

Earlier rule Consequence
exterior vs inner wall by element name ('Ext' in name) inner load-bearing walls not gridded
grid snapped to a 100 mm module, merged within 0.20 m dimensions no longer equal the model
dimensions written as text figures typed, not measured
door hinge always at the opening's min-x; swing side by which side has more furniture door swings matched the model 6 of 14 times (≈ chance)
plans drew only the elements of that storey walls spanning two storeys missing from the upper plan
elevations: everything within 1.0 m of the face, or every triangle edge inner walls and clutter in elevations

The common cause: meaning was inferred from names, furniture and snapping, and no test compared the drawing back to the model, so errors were invisible and fixed by hand.

2 ¡ Method

  1. Semantics from IFC relationships, not names. Load-bearing = a foundation wall runs under the wall's axis; door/window host from IfcRelFillsElement; door hinge from the IFC door operation type (left/right hand) and placement axes; swing side from the asymmetric leaf; radius = leaf width. Missing data → a gap note, never an arc.
  2. Drafting conventions, stated as choices. Plans show everything the cut plane (storey + 1.2 m) passes through, whatever its storey; grid on load-bearing and exterior axes, labels 1…n / A…n skipping I and O, never snapped; three dimension tiers (overall · grid-to-grid · detail) as real DIMENSION entities; slab thickness from a section; interior door heights from the door schedule via the door tag.
  3. Metadata in the DXF. A layer DXF2BIM-MARK holds @KEY=value texts: view/role, level, face, cut, units, layer meanings, tags, schedule rows, wall tops, regions/exclusions. A suffix @BY=model|user records who stated it; none = stated in the file. The file remains an ordinary DXF.
  4. Qualify verdict. Before building, the set is checked: units stated (never taken from the header alone), every storey has a plan with a level, every building face an elevation, every layer mapped, every dimension text equal to its measurement, grid lines on walls, levels consistent, and every pipeline gap resolved. Output: qualifies, or blocking/advisory items with code, sheet, entity handles and position.
  5. Rebuild with provenance. Walls (footprint or line pairs), slabs (from the plan outline and the section), roof (roof plan + level), openings cut from drawn wall gaps, fills from a component library (real meshes; a missing product is a hard failure, never a box). Every element records the DXF handle and layer it came from.
  6. Witnesses against the model, with falsification. Every rule is checked against the source model (the answer key of the round trip), and every check is run once with the rule deliberately broken to prove it fails.

3 ¡ Evidence (Duplex, 2026-10-16)

Each number is read from a witness log; the full number → log-line mapping is in prompts/film_dxf2bim_facts.md (BIMCompiler).

Claim Result Witness
Standard set is deterministic 5 separate processes → one digest test_std2d W1-proc
Grid on the load-bearing / exterior axes, none extra x = 3 axes, y = 4 axes, labels 1–3 / A–D test_std2d W2, W4
Dimensions equal to the model, no overrides 34 DIMENSION entities on the ground-floor plan (3 tiers), each within 0.5 mm test_std2d W3
Door hinge and swing from the IFC 14 / 14 arcs; earlier hand-made set 6 / 14 swing side test_std2d W5 (+ INFO line)
A 50 mm wall shift in the model is detected stale drawing fails grid and dimension checks; regenerated passes test_std2d W7
Standard set qualifies 0 blocking, 75 advisory rosetta_qualify (Node) = Modeller page
Rebuild parity, page vs Node every element within 1 mm, same run fingerprint Modeller witness_dxf_convert C1
IFC export 96 wall pieces, 2 slabs, 1 roof, 14 doors, 22 windows; 2 storeys; all contained witness_dxf_convert C2
Raw drawings refused until complete stripped set blocked; filled through the editor, then convert witness_dxf_meta_editor E2/E5
Units trap caught (independent sample) header mm → 0.80 × 0.56 m; door radii 24–36 in → 20.2 × 14.3 m; units carry their source jscad_units_probe, t8_sandbox
Independent drawing (not authored by us) 17 walls, outline 384 × 192 in = printed 32′ × 16′; height 120 in from its level lines farm_walls, farm_heights

Determinism and genericity. The reader modules contain no building names, layer names or building-specific numbers; the standard writer contains no Duplex constants. All tolerances are named constants recorded in the spec.

4 ¡ Limits (what is not claimed)

  • Basic shapes only — no stairs, MEP, furniture; columns are cut but not yet drawn as gridded column symbols.
  • Rectilinear geometry — skewed/curved walls are reported as gaps; roofs are drawn as flat outlines.
  • Openings above or below the plan cut are not yet shown as gaps from their host relationship (in progress).
  • Blocks (INSERT) in user DXF files are not expanded yet; door/window blocks are therefore not read.
  • The metadata of the standard set is derived from the model (@BY=model). A raw drawing needs a person to state it through the editor; door schedule entry is being added.
  • One round-trip building (Duplex) and one partial independent drawing (Farmhouse). "Any DXF" is not claimed.
  • No novelty claim beyond our search: prior art reviewed (2026-10-11): manual tracing in commercial authoring tools; an open student project reading structural members from user-typed layer names; an open-source script converting lines to IFC curves; a manufacturing tool extruding parts. We found no general, deterministic, open-source DXF-to-BIM tool; a combination of the features above was not found elsewhere in that search. A wider search (commercial tools, research, patents) has not been done.

5 ¡ Reproduce

Everything is MIT licensed on GitHub: BIMCompiler (reader modules deploy/dev/dxf2*.js, writer 2D_Layout/python/std2d.py, tests tests/dxf2bim/, specs docs/DXF2BIM_*.md) and bim-ootb (Modeller modeller/dxf2bim*, modeller/dxf_*, witnesses modeller/tests/witness_dxf_*, lane spec prompts/DXF2BIM_MODELLER_LANE.md). Work landed in BIMCompiler PRs #176–#193 and bim-ootb PRs #2049–#2069, dated 2026-10-16/17. The developer chain is in the user guide.

Development note: the code, tests and this note were written with AI assistance (Claude Code); the pipeline itself makes no AI call. Test drawing credit: American Farmhouse drawings by Jay Osborne (freefarmhouse.com), CC BY-SA, not included in the repository.