3.14. DWG Compatibility

DWG is AutoCAD’s native binary format, and IQcad reads and writes it in process — there is no temporary DXF, and no external converter to install. Opening a .dwg file is the same operation as opening an .iqz: File › Open… and pick it. Saving to DWG is File › Save As… with the DWG filter, or File › Export › DWG….

Because the format is binary and undocumented in places, this chapter states plainly what is supported, what is approximated, and where the fallbacks are.

3.14.1. Supported Versions

Version	Magic	Read	Write	Notes
R13 / R14	`AC1012` / `AC1014`	—	—	Recognised, then handed to the fallback converter.
R2000	`AC1015`	✓	✓	The most widely readable output. Choose this when the file must open in old or third-party software.
R2004	`AC1018`	✓	✓
R2007	`AC1021`	✓	✓	Compressed three-stream objects.
R2010	`AC1024`	✓	✓
R2013	`AC1027`	✓	✓
R2018	`AC1032`	✓	✓	The default output version.
R2025	`AC1036`	—	—	Not yet supported.

Every written version is validated offline against the checksums a conforming reader verifies, and files at all six versions are accepted by the ODA File Converter and open in AutoCAD.

The output version is set once, in File › Settings › DWG › Save Version — see Application Settings.

3.14.2. What Round-Trips

Read and written natively:

Group	Objects
Geometry	LINE, CIRCLE, ARC, POINT, ELLIPSE, LWPOLYLINE, 2D and 3D POLYLINE with their vertices, SPLINE, XLINE, RAY, 3DFACE, SOLID, TRACE.
Annotation	TEXT, MTEXT, ATTRIB, ATTDEF, all seven DIMENSION sub-types plus arc-length and jogged-radius dimensions, MULTILEADER, legacy LEADER, TOLERANCE, SHAPE (a single glyph from an `.shx` shapefile).
Fills and masks	HATCH with its pattern and boundary, WIPEOUT, IMAGE (raster references).
Structure	BLOCK definitions, INSERT references, MINSERT arrays, LAYER, LTYPE, STYLE, DIMSTYLE, TABLESTYLE, LAYOUT records and VIEWPORT windows.
Meshes	`AcDbSubDMesh`, polyface meshes and M × N polygon meshes.
Tables	`ACAD_TABLE`, with its cell contents and a Standard table style.
Solids	REGION, 3DSOLID and BODY. The ACIS geometry is carried through unchanged — encrypted SAT on R2000–R2010, and the SAB prototype section on R2013 and R2018 — so a solid authored in AutoCAD comes back out of IQcad as the same solid. IQcad displays its wireframe extent; it does not model or edit ACIS geometry.
Civil 3D	COGO points and the point-group tree, TIN surfaces (including the surface’s own stored triangulation, hidden faces and deleted faces), parcels and alignments. See Exporting Survey Objects below.

Objects IQcad has no equivalent for are counted and reported rather than silently dropped, and proxy entities are recorded under the class they stand in for.

3.14.3. Opening and Re-saving Untouched

If a drawing is opened and saved again without being edited, and the target version matches the source, IQcad re-emits the original bytes exactly. Nothing is re-encoded, so nothing can be lost. The moment an object is added, removed or changed the drawing goes through the normal encoder instead.

This makes “open it here, save it, send it on” safe even for drawings using features IQcad does not model.

3.14.4. Backends and the Fallback Chain

IQcad tries backends in order and takes the first that succeeds:

Native (in-process) — the built-in reader/writer. The default, and the fastest by a wide margin.
LibreDWG CLI — the external dwg2dxf / dxf2dwg tools.
ODA File Converter — the Open Design Alliance’s free converter.

Both CLI backends work by converting through a temporary DXF, so they are slower and lose anything DXF cannot carry. They exist to cover file versions the native engine does not read (R13/R14, R2025).

The chain stops immediately on success and on a corrupt file — a damaged file surfaces an error rather than being quietly retried, because corruption is not something a different backend will fix. It moves on to the next backend only for an unsupported version, an unsupported object, or a backend that is not installed.

Set the preference in File › Settings › DWG › Reader Backend. Pin a CLI converter there if you need to reproduce its behaviour or suspect a problem in the native path.

Installing a CLI converter (only needed for the fallbacks):

Linux — sudo apt install libredwg-tools, or build from https://github.com/LibreDWG/libredwg.
Windows — the LibreDWG release binaries, or the free ODA File Converter from https://www.opendesign.com/guestfiles/oda_file_converter.

3.14.5. Exporting Survey Objects

Survey points and TIN surfaces have no counterpart in plain DWG: they are Civil 3D custom objects, which ordinary CAD programs cannot read. IQcad therefore lets you choose how they leave the drawing, in File › Settings › DWG › Survey objects.

Mode	What is written
Civil 3D objects	Genuine Civil 3D objects: an `AeccDbPointsNode` carrying the COGO points with the group and query tree above it, a `Root` entry in the named-object dictionary, `AeccDbSurfaceTin` for surfaces, and `AeccDbFace` / `AeccDbAlignment` for parcels and alignments. Civil 3D lists these in the Prospector and edits them as its own. Plain AutoCAD and other CAD programs show nothing for them.
Points + surface, with labels	The default. Geometry any CAD can read: a `POINT` per COGO point, a `TEXT` label for its number and description, and a `3DFACE` per TIN triangle. Civil 3D rebuilds a surface from the 3DFACEs in a single command.
Points + surface, geometry only	The same, without the text labels.
Do not export	Neither points nor surfaces are written. Use this when the recipient only wants the drafting content.

Tip

Send Civil 3D objects to a Civil 3D user and with labels to anyone else. If you do not know which, the labelled geometry mode is the safe choice — it is readable everywhere and still rebuilds into a surface.

Reading survey objects needs no setting: Civil 3D points, point groups, surfaces, parcels and alignments in an incoming file are decoded into IQcad’s own civil objects automatically, and appear in the Prospector.

3.14.6. Known Limitations

Multiline styles — the writer emits one Standard MLINESTYLE. A multiline’s geometry and its element pair survive; a bespoke named style does not.
ACIS solids — carried through, not modelled. IQcad cannot edit, boolean or section a 3DSOLID.
Point descriptions — Civil 3D COGO points written in Civil 3D objects mode carry number and position, not the description field.
R2000 is still the safest target for third-party software, even though all six versions are written and validated. The Save Version list carries a stale “not yet supported” note beside R2007; the version is in fact written and accepted.

If a file will not open, switch the reader backend to ODA File Converter in the settings and try again; that tells you quickly whether the problem is the file or the native reader.