[gdal-dev] Possible error in orientation of converted arcs from dxf

[Jan Heckman]

Hi all,
Sorry for a somewhat long post, but here we go:
I am converting a .dxf having a sequence of simple lines and arcs which
form a continuous (poly)line, (correctly) ordered by EntityHandle. The arcs
are converted (to shp) as well as displayed (Qgis) in the opposite
direction as the simple lines. Checking the codes and docs, this appears
wrong.
Below the minimal relevant information, I can add the complete .dxf, but I
think attached files are frowned upon?

Docs cited:
https://help.autodesk.com/view/OARX/2024/ENU/?guid=GUID-0B14D8F1-0EBA-44BF-9108-57D8CE614BC8
ARC (DXF)
The following group codes apply to arc entities.
Arc group codes
Group code Description
100 Subclass marker (AcDbCircle)
39 Thickness (optional; default = 0)
10 Center point (in OCS) DXF: X value; APP: 3D point
20, 30 DXF: Y and Z values of center point (in OCS)
40 Radius
100 Subclass marker (AcDbArc)
50 Start angle
51 End angle
210 Extrusion direction (optional; default = 0, 0, 1) DXF: X value; APP: 3D
vector
220, 230 DXF: Y and Z values of extrusion direction (optional)

https://ezdxf.readthedocs.io/en/stable/tutorials/dxf_primitives.html
The arc goes always in counter-clockwise orientation around the z-axis more
precisely the extrusion vector of OCS, but this is beyond the scope of this
tutorial.

The case (extracted, whole .dxf file available):
AcDbEntity
  8
0000-001_09_AND1N
  6
Continuous
 62
    18
100
AcDbCircle
 10
130974.661837
 20
497502.478468
 30
0.0
 40
7576.094430000001
100
AcDbArc
 50
305.5444330000001
 51
344.795475
  0
LINE
  5
2E
330
1A0
100

The arc is converted (.shp) and drawn (qgis) from end angle to start angle,
without a 210 code to alter default behavior, which obviously is the other
way around, noting counter clock around (positive) Z-axis.
The applicability of this convention also follows from the (correct)
position of the arc in the conversion and drawing.
Only begin- and endpoints are interchanged.

Please advice,
Thanks in advance,
Jan
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