[postgis-tickets] [PostGIS] #3929: Unexpected rounding from ST_SnapToGrid

[PostGIS]

#3929: Unexpected rounding from ST_SnapToGrid
-----------------------+---------------------------
  Reporter:  michaudm  |      Owner:  pramsey
      Type:  defect    |     Status:  closed
  Priority:  medium    |  Milestone:  PostGIS 2.4.2
 Component:  postgis   |    Version:  2.4.x
Resolution:  wontfix   |   Keywords:
-----------------------+---------------------------

Comment (by michaudm):

 Hi Paul,

 I'm not a C programmer so I don't know if the problem can be solved in
 Geos/PostGIS, but in java,


 {{{
 rint(311.4/0.1)*0.1 -> 311.40000000000003
 }}}

 while

 {{{
 round(311.4/0.1)*0.1 -> 311.4
 }}}

 If I use rint, I obtain exactly the same result as in your example while
 with round, I get correct result.

 Moreover, I noticed the following comment in the makePrecise method of JTS
 code :


 {{{
 if (modelType == FIXED) {
     return Math.round(val * scale) / scale;
     // return Math.rint(val * scale) / scale;
 }
 }}}

 Not sure how to interpret these results as I'm not a IEEE double precision
 expert, but maybe there is something to do, especially if it makes JTS and
 Geos behaviour closer.

--
Ticket URL: <https://trac.osgeo.org/postgis/ticket/3929#comment:2>
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