[postgis-tickets] [PostGIS] #4183: Invalid output of ST_ClipByBox2D with an invalid polygon

[PostGIS]

#4183: Invalid output of ST_ClipByBox2D with an invalid polygon
------------------------+--------------------------
 Reporter:  Algunenano  |      Owner:  Algunenano
     Type:  defect      |     Status:  assigned
 Priority:  medium      |  Milestone:  PostGIS GEOS
Component:  postgis     |    Version:  trunk
 Keywords:              |
------------------------+--------------------------
 I start with a valid polygon:
 {{{
 Select St_IsValid('SRID=3857;MULTIPOLYGON(((-8231365.02893734
 4980355.83678553,-8231366.94866817 4980350.2976052,-8231368.82883367
 4980344.52250402,-8231370.96255986 4980333.47364302,-8231372.09200747
 4980327.69798564,-8231372.84505222 4980323.68014243,-8231374.2255656
 4980316.77455645,-8231375.10554947 4980309.86865218,-8231380.6311111
 4980275.9676398,-8231394.82332406 4980186.31880185,-8231394.8569833
 4980133.57928934,-8231367.43081065 4979982.17443372,-8231372.85765532
 4979985.17751813,-8231395.25669799 4980132.93828551,-8231396.69199339
 4980227.59327083,-8231377.30092207 4980340.77515211,-8231368.35061694
 4980357.62467295,-8231365.02893734 4980355.83678553)))'::geometry);
  st_isvalid
 ------------
  t
 }}}


 I pass this polygon through `ST_Simplify and ST_RemoveRepeatedPoints` and
 it comes up as invalid:
 {{{
 Select
 St_IsValid(ST_Simplify(ST_RemoveRepeatedPoints('SRID=3857;MULTIPOL...'),
 19.109257221221924), 19.109257221221924, true));
  st_isvalid
 ------------
  f
 (1 row)
 }}}

 Then I pass it through ST_ClipByBox2D:
 {{{
 Select St_AsEWKT(ST_ClipByBox2D(
     'MULTIPOLYGON(((-8231365.02893734 4980355.83678553,-8231394.82332406
 4980186.31880185,-8231367.43081065 4979982.17443372,-8231396.69199339
 4980227.59327083,-8231365.02893734 4980355.83678553)))'::geometry,
     'POLYGON((-8228215.28157305 4980103.20610541,-8228215.28157305
 4970163.3879457,-8238155.09973276 4970163.3879457,-8238155.09973276
 4980103.20610541,-8228215.28157305 4980103.20610541))'::geometry
 ));

 POLYGON((-8238155.09973276 4970163.3879457,-8238155.09973276
 4980103.20610541,-8231383.67109061 4980103.20610541,-8231367.43081065
 4979982
 .17443372,-8231381.86136498 4980103.20610541,-8228215.28157305
 4980103.20610541,-8228215.28157305 4970163.3879457,-8238155.09973276
 4970163
 .3879457))
 }}}

 The result seen is not the intersection (as expected) but the result of
 subtracting the geometry area to the box.

 In truth this series of chained calls come from St_AsMVTGeom where this
 functions are called inside the same C function, but seeing the individual
 calls makes it easier to spot the issue.

 I've traced this behaviour back to GEOS
 (https://github.com/Algunenano/geos/commit/1737fa1a34e6cc30fb26b2898f607d5db3b176a6).

 Does anyone know if GEOSClipByRect is supposed to handle invalid
 geometries correctly or to just give an invalid output as the one seen
 above?

-- 
Ticket URL: <https://trac.osgeo.org/postgis/ticket/4183>
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