What is DXF?¶
The common assumption is also the cite of Wikipedia:
AutoCAD DXF (Drawing eXchange Format) is a CAD data file format developed by Autodesk for enabling data interoperability between AutoCAD and other applications.
DXF was originally introduced in December 1982 as part of AutoCAD 1.0, and was intended to provide an exact representation of the data in the AutoCAD native file format, DWG (Drawing). For many years Autodesk did not publish specifications making correct imports of DXF files difficult. Autodesk now publishes the DXF specifications online.
The more precise cite from the DXF reference itself:
The DXF™ format is a tagged data representation of all the information contained in an AutoCAD® drawing file. Tagged data means that each data element in the file is preceded by an integer number that is called a group code. A group code’s value indicates what type of data element follows. This value also indicates the meaning of a data element for a given object (or record) type. Virtually all user-specified information in a drawing file can be represented in DXF format.
No mention of interoperability between AutoCAD and other applications.
In reality the DXF format was designed to ensure AutoCAD cross-platform compatibility in the early days when different hardware platforms with different binary data formats were used. The name DXF (Drawing eXchange Format) may suggest an universal exchange format, but it is not. It is based on the infrastructure installed by Autodesk products (fonts) and the implementation details of AutoCAD (MTEXT) or on licensed third party technologies (embedded ACIS entities).
For more information about the AutoCAD history see the document: The Autodesk File - Bits of History, Words of Experience by John Walker, founder of Autodesk, Inc. and co-author of AutoCAD.
DXF Reference Quality¶
The DXF reference is by far no specification nor a standard like the W3C standard for SVG or the ISO standard for PDF.
The reference describes many but not all DXF entities and some basic concepts like the tag structure or the arbitrary axis algorithm. But the existing documentation (reference) is incomplete and partly misleading or wrong. Also missing from the reference are some important parts like the complex relationship between the entities to create higher order structures like block definitions, layouts (model space & paper space) or dynamic blocks to name a few.
Reliable CAD Applications¶
Because of the suboptimal quality of the DXF reference not all DXF viewers, creators or processors are of equal quality. I consider a CAD application as a reliable CAD application when the application creates valid DXF documents in the meaning and interpretation of Autodesk and a reliable DXF viewer when the result matches in most parts the result of the free Trueview viewer provided by Autodesk.
These are some applications which do fit the criteria of a reliable CAD application:
CAD applications based on the OpenDesignAlliance (ODA) SDK, see also ODA on wikipedia, even Autodesk is a corporate member, see their blog post from 22 Sep 2020 at adsknews but only to use the ODA IFC tools and not to improve the DWG/DXF compatibility
BricsCAD (ODA based)
GstarCAD (ODA based)
ZWCAD (ODA based)
Unfortunately, I cannot recommend any open source applications because everyone I know has serious shortcomings, at least as a DXF viewer, and I don’t trust them as a DXF creator either. To be clear, not even ezdxf (which is not a CAD application) is a reliable library in this sense - it just keeps getting better, but is far from reliable.