GEOSBuffer emits invalid buffering a collection

[komzpa]

Follow-up on #978. I replaced Unary Union by Buffer(0), given that all my inputs are polygons. I believe on mailing lists Martin mentioned Overlays-NG are going to be similar to it, so may be worth checking it.

PostGIS query for reference:

09:42:59 [gis] >  select w_geom from tmp_osm_population_raw_nowater where p_geom && '(-7885150.676182366,5282400.9061096404)'::point::geometry::box2d and not ST_IsValid(ST_Buffer(w_geom, 0)) ;
NOTICE:  00000: Self-intersection at or near point -7885150.676182366 5282400.9061096404
LOCATION:  pg_notice, lwgeom_pg.c:265
NOTICE:  00000: Self-intersection at or near point -7885150.676182366 5282400.9061096404
LOCATION:  pg_notice, lwgeom_pg.c:265
Time: 1229,412 ms (00:01,229)

Input collections are elementwise valid:

09:40:41 [gis] >  select ST_IsValid((ST_Dump(w_geom)).geom) from tmp_osm_population_raw_nowater where p_geom && '(-7885150.676182366,5282400.9061096404)'::point::geometry::box2d and not ST_IsValid(ST_Buffer(w_geom, 0)) ;
NOTICE:  00000: Self-intersection at or near point -7885150.676182366 5282400.9061096404
LOCATION:  pg_notice, lwgeom_pg.c:265
NOTICE:  00000: Self-intersection at or near point -7885150.676182366 5282400.9061096404
LOCATION:  pg_notice, lwgeom_pg.c:265
┌────────────┐
│ st_isvalid │
├────────────┤
│ t          │
│ t          │
│ t          │
│ t          │
│ t          │
│ t          │
│ t          │
│ t          │
│ t          │
│ t          │
│ t          │
│ t          │
│ t          │
│ t          │
└────────────┘
(14 rows)

Exhibit 1:

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Exhibit 2:

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[dbaston]

Can you please provide all arguments to GEOSBuffer in this ticket?

What do exhibits 1 and 2 represent?

[mdavis]

Confirmed that JTS buffer(0) exhibits the same issue. It produces an invalid output on both geometries above (of which the first is a subset of the second).

This is almost certainly due to a noding robustness failure, in an area with several very close high precision vertices:

A
[]  POLYGON(108)
Ring-CW  Vert[0 11]  POINT ( -7885150.676182368 5282400.906109639 )
Ring-CW  Vert[0 12]  POINT ( -7885150.676182364 5282400.90610964 )
Ring-CW  Vert[0 13]  POINT ( -7885150.676182369 5282400.906109639 )

Some light precision reduction might solve this problem, in this case.

[mdavis]

Snap-rounding is highly applicable to buffer construction, and should solve problems like this. It's probably too much impact to try and add it to the current (complex) buffer code, so that may have to wait for a rewrite using the upcoming OverlayNG framework. Although at that point UnaryUnion should work as well.

[strk]

I'd like to test this with current GEOS but there's in input in this ticket ? Martin where did you find the input ? Both exhibit1 and exhibit2 are self-intersecting multipolygons. In both cases Buffer(0) output a still-invalid geometry, while UnaryUnion fixes the invalidity. I don't think by contract Buffer(0) is to be guaranteed to output a valid geometry from an invalid one ?

[mdavis]

The data is in #979 and #978.

Yes, there is no contract about buffer(0) fixing invalid geometry (although it actually does for many cases). We should focus on ensuring that OverlayNG works with this data (and it is included as part of the OverlayNG robustness unit tests ​here

[strk]

The test is already included in GEOS testsuite, data from 979 passes in current GEOS so I'm closing this