DWG/DXF: 3D solids (ACIS/SAT data) cannot be imported or exported

[linghengqian]

Problem description

FreeCAD cannot import or export 3D solids stored in DWG and DXF files. In these formats a 3DSOLID/REGION/BODY entity keeps its geometry in a binary ACIS/SAT blob, which FreeCAD cannot read. Opening such a file loses the solids; only plain 2D/wireframe entities arrive. Export has the mirror problem: solids are written only as edges (C++ exporter, R14) or as flattened 2D views (legacy exporter, R12).

DWG adds a second limitation: it can only be opened through an external converter ( https://github.com/LibreDWG/libredwg or the ODA File Converter) that turns it into DXF first, and the documentation states that only 2D geometry is supported. https://github.com/LibreDWG/libredwg 's conversion leaves the ACIS blobs untouched in the DXF, so converting does not help.

Impact: DWG files with 3D solids received from AutoCAD or other DWG-based CAD users cannot be used in FreeCAD. The documented workarounds are a proprietary application that exports STEP, or the paid CAD Exchanger add-on.

Since then, https://github.com/LibreDWG/libredwg 's NEWS lists fixes to 3DSOLID/REGION SAT block decoding and the library has Python bindings, and https://github.com/HakanSeven12/OpenCADStudio (GPL-3.0, so a behavioural reference only) renders 3DSOLID/REGION/BODY entities from DWG by tessellating them. A reduced-scope first step, such as importing the solids as meshes, may be worth investigating.

Workbench affected?

• Core (App, Gui,...)
  • Mod: Core and File format: DXF

Steps to reproduce

1. In AutoCAD or any DWG-based CAD, draw a box with the BOX command so the drawing contains a 3DSOLID entity, and save it as DXF and as DWG. (A minimal example file will be attached as ZIP.)
2. In FreeCAD, File > Open the DXF file.
3. For the DWG file, install https://github.com/LibreDWG/libredwg (or the ODA File Converter), set its path under Edit > Preferences > Import-Export > DWG > Path to file converter, then File > Open the DWG file.

Expected behavior

The solid is imported as a FreeCAD shape (ideally an editable solid; a mesh if that is the only feasible first step), and 3D solids can be written back to DXF/DWG.

Actual behavior

The solid does not appear. According to the documentation ( https://wiki.freecad.org/Draft_DXF ), FreeCAD cannot read the ACIS/SAT data, so only the 2D/wireframe entities of the file are imported. Export writes only edges or flattened 2D views.

Development version About Info (in Safe Mode)

N/A. Documentation consulted: https://wiki.freecad.org/Draft_DXF and https://wiki.freecad.org/FreeCAD_and_DWG_Import

Last known good version (optional)

N/A