[geos-devel] [GEOS] #1074: Setting precision on some but not all geometries causes intersection results to use the smallest nonzero gridSize

[GEOS]

#1074: Setting precision on some but not all geometries causes intersection
results to use the smallest nonzero gridSize
--------------------------+--------------------------
 Reporter:  Brendan Ward  |      Owner:  geos-devel@…
     Type:  defect        |     Status:  new
 Priority:  major         |  Milestone:
Component:  Default       |    Version:  3.8.0
 Severity:  Unassigned    |   Keywords:
--------------------------+--------------------------
 According to the comments in "geos_c.h.in" for GEOSGeom_setPrecision_r:
 "
  * Note that operations will always be performed in the precision
  * of the geometry with higher precision (smaller "gridSize").
  * That same precision will be attached to the operation outputs.
 "

 Given inputs:
 * "POLYGON ((0 0, 0 0.9, 0.9 0.9, 0.9 0, 0 0))"
 * "POLYGON ((0.75 0, 0.75 0.75, 1.75 0.75, 1.75 0, 0.75 0))"

 I then used GEOSGeom_setPrecision_r with a gridSize of 0.5 for the first
 geometry, which returns
 "POLYGON ((0 0, 0 1, 1 1, 1 0, 0 0))", and reports a gridSize of 0.5 via
 GEOSGeom_getPrecision_r.

 Performing an intersection via GEOSIntersection_r, I get back
 "POLYGON ((1 0.75, 1 0, 0.75 0, 0.75 0.75, 1 0.75))"
 which reports a gridSize of 0.5 (second geometry reports a gridSize of 0).

 If I then set a precision of 1 on the second geometry, which produces:
 "POLYGON ((1 0, 1 1, 2 1, 2 0, 1 0))" (second geometry then reports a
 gridSize of 1)

 I get back "LINESTRING (1 1, 1 0)" from intersection, which reports a
 gridSize of 0.5


 This may be expected behavior, but the comment in the code implies that
 the higher precision, in this case full precision as denoted by a gridSize
 of 0, would be used.  If so, perhaps the comment could be clarified to
 note something like:
 * if inputs both have nonzero gridSize, operations will be performed using
 the higher precision (smaller "gridSize").
 * if one but not all of the inputs have a nonzero gridSize, and others
 have a gridSize of 0, the smallest gridSize will be used.


 (only tested on GEOS 3.8 via C API)

-- 
Ticket URL: <https://trac.osgeo.org/geos/ticket/1074>
GEOS <http://trac.osgeo.org/geos>
GEOS (Geometry Engine - Open Source) is a C++ port of the Java Topology Suite (JTS).