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

[Even Rouault]

Hi,

I'm wondering if there woulnd't be a connection with 
https://github.com/OSGeo/gdal/issues/1839#issuecomment-537185826 ?

Even

Le 25/02/2024 à 13:32, Jan Heckman via gdal-dev a écrit :
> 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
>
> _______________________________________________
> gdal-dev mailing list
> gdal-dev at lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/gdal-dev

-- 
http://www.spatialys.com
My software is free, but my time generally not.