[gdal-dev] Correct reading of ellipse entities (ticket #5705) when extrusion vector is (0, 0, -1)

[Pero Brbora]

Hi all

Thanks for feedback, unfortunate :)

I already started working on ticket #5705 "[AutoCAD DXF] Incorrect  
processing of Ellipses with negative extrusion" before I read Dmitry's  
post, and here are my results.

First I tested ticket sample files to check the status with the  
current code base and defect is still there.

After some internet searching I found that:
1. ellipse entity doesn't use OCS (based on that support for ellipse  
entity came latter to AutoCAD/DXF), but this is not confirmed so  
calling ApplyOCSTransformer in TranslateELLIPSE (file ogrdxflayer.cpp)  
may not be needed for ellipses.
(source: theswamp.org - CAD related forum)
2. when extrusion (normal) is [0,0,-1] ellipse is reversed
(source: stackoverflow and my inspection of sample files in text  
editor and QCAD)

Note: When extrusion (normal) is [0,0,1] there is also no need to call  
ApplyOCSTransform, but this applies to all entities. Currently  
transform is applied whenever extrusion data is present (small  
optimisation).

Based on (2) at least defect reported in ticket #5705 can be resolved.

I made a quick fix against a trunk branch and since I have Github  
account (unused until now) I decided to send pull request, it should  
be up now.

I tested it with files from the ticket and my fix produces correct results.

And for proposal I should say that I can write code for DWG read  
support for R2000 (AutoCAD 2000) version files using ogr dxf support  
files as a template. Other options are possible but please let me know  
if you are interested in my work for GSOC 2016.

Anyway I will keep an eye on GDAL mailing list and help you out.

Side note:
On Building on Unix wiki page  
(https://trac.osgeo.org/gdal/wiki/BuildingOnUnix) there is a typo:
----
% export LD_LIBRARY_PATH=/path/to/install/prefix/lib:$PATH
----
should read:
----
% export LD_LIBRARY_PATH=/path/to/install/prefix/lib:$LD_LIBRARY_PATH
----

IMHO as everyone says.
Thanks bye