Posted by LordOfBikes on Jun 14, 2022; 6:14pm
URL: https://forum.librecad.org/Double-lines-and-unwanted-connections-tp5721810p5721813.html

Marcin,

your DXF contains special polylines. It needed some investigation to figure this out.

What LibreCAD shows are the vertices contained in these polylines. So basically I think libdxfrw reads the DXF without issues. But LibreCAD seems not to interpret the special flags set on the vertices.
From the polyline flags I suppose the vertices should be used to create a spline and they represent fit or control points.

The DXF is really old. AC1009 is DXF R11/R12 format and thus pre 1995!
It lacks many information, like variables and tables.

Because R12 has no support for SPLINE entities, I come to the conclusion that this was the early way to save splines in DXF before 1995. The R13 format, released 1995, brought SPLINE support, thus I assume polyline splines are deprecated, even when they are still contained in modern DXF reference.

In fact I've never seen this before, it must be a rarely used feature. And it is not implemented in LibreCAD.
Possibly it was never implemented because it was already deprecated when LibreCAD begins.