[postgis-tickets] [PostGIS] #3099: ST_curveToLine bad conversion for some arcs

[PostGIS]

#3099: ST_curveToLine bad conversion for some arcs
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  Reporter:  claudeunger  |      Owner:  pramsey
      Type:  defect       |     Status:  new
  Priority:  critical     |  Milestone:  PostGIS 2.1.9
 Component:  postgis      |    Version:  2.1.x
Resolution:               |   Keywords:  ST_curveToLine
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Comment (by tiiipponen):

 Hi

 I include here file “arc_stroking_lw_arc_center.txt”. [[BR]]
 It has fixed function “lw_arc_center” from file lwalgorithm.c. [[BR]]
 http://postgis.net/docs/doxygen/2.1/dc/dc0/lwalgorithm_8c_source.html
 [[BR]]

 Original function returned incorrect centroid and radius for arc geometry.
 [[BR]]
 This attached function is minimum correction to fix the problem. [[BR]]

 I have tested mathematics of attached function only with Python in Safe
 Software FME environment. [[BR]]
 I also include Python function in file arc_stroking_python.txt for
 reference. [[BR]]

 My Python function has also some extra functionality, [[BR]]
 but implementing that in current PostGIS library is more complicated.
 [[BR]]
 I still would like to some day see following functionality in PostGIS
 ST_CurveToLine function. [[BR]]
 Tolerance parameter. In GIS data this is important. [[BR]]
 If user gives “Tolerance parameter” value > 0, then “vertex count”
 parameter means minimum vertices in stroked arc. [[BR]]
 For example given tolerance with “vertex count”=0 means, that stroked arc
 can even be straight line if tolerance allows it. [[BR]]

 I am ready to test new fixed ST_CurveToLine function heavily when it is
 available.

 Timo

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Ticket URL: <https://trac.osgeo.org/postgis/ticket/3099#comment:6>
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