[postgis-users] Self-intersecting polygon gives ST_IsSimple=true

[Jukka Rahkonen]

Jukka Rahkonen kirjoitti 2017-05-08 15:48:

> Ok, it was defined in JTS 1.13
> http://javadox.com/com.vividsolutions/jts/1.13/com/vividsolutions/jts/geom/Geometry.html#isSimple()
> 
> "Valid polygonal geometries are simple, since their rings must not
> self-intersect. isSimple tests for this condition and reports false if
> it is not met. (This is a looser test than checking for validity)."
> 
> And now in JTS 1.15 it is
> 
> "Polygonal geometries are simple by definition, so isSimple trivially
> returns true. (Note: this means that isSimple cannot be used to test
> for (invalid) self-intersections in Polygons. In order to check if a
> Polygonal geometry has self-intersections, use Geometry.isValid())."
> 
> I believe that there is some good reason for this change but doesn't
> it put too much responsibility for the user to know not to test
> polygons with IsSimple? Why does it accept polygon as input at all?


I found alternative documentation about IsSimple from current JTS source 
code from Geometry - IsSimple
https://locationtech.github.io/jts/javadoc/org/locationtech/jts/geom/Geometry.html

Perhaps IsSimpleOp is updated but IsSimple not
https://locationtech.github.io/jts/javadoc/org/locationtech/jts/operation/IsSimpleOp.html

I can see from https://github.com/bjornharrtell/jsts/issues/205 that JTS 
used to behave like it is now again behaving but it was changed by Dr. 
JTS in https://sourceforge.net/p/jts-topo-suite/code/662/.

I can still find the same comments about polygon validity in 
https://github.com/locationtech/jts/blob/master/modules/core/src/main/java/org/locationtech/jts/operation/IsSimpleOp.java 
beginning from line 205

  /**
    * Computes simplicity for polygonal geometries.
    * Polygonal geometries are simple if and only if
    * all of their component rings are simple.
    *
    * @param geom a Polygonal geometry
    * @return true if the geometry is simple
    */


Awaiting what Dr. JTS has to say but perhaps I should ask it through the 
JTS list. On the other hand, this is not any real problem for me but I 
am mostly curious. By a fast comparison with documentation of other 
implementations it is pretty common to interpret that polygons with 
self-intersecting rings are not simple which does make sense for a 
layman. If Dr. JTS has changed his mind at least twice the 
interpretation is obviously not self-evident.

-Jukka Rahkonen-