Overlay ops accept GeometryCollections when headers indicate that they don't

[dbaston]

Example from Geometry.h:

	/**
	 * \brief
	 * Returns a Geometry representing the points making up this
	 * Geometry that do not make up other.
	 *
	 * @throws util::TopologyException if a robustness error occurs
	 * @throws util::IllegalArgumentException if either input is a
	 *         non-empty GeometryCollection
	 *
	 */

Actual behavior (through CAPI):

    GEOSGeometry* g1 = GEOSGeomFromWKT("GEOMETRYCOLLECTION (POINT (1 1), LINESTRING (2 2, 3 3)) ");
    GEOSGeometry* g2 = GEOSGeomFromWKT("GEOMETRYCOLLECTION (POINT (3 3), LINESTRING (1 1, 2 2)) ");

    GEOSGeometry* g3 = GEOSDifference(g2, g1);
    /* No error passed to user, g3 != NULL */

    printf("%s\n", GEOSGeomToWKT(g3));
    /* prints LINESTRING (1.0000000000000000 1.0000000000000000, 2.0000000000000000 2.0000000000000000) */

Related to ​https://trac.osgeo.org/postgis/ticket/3341 and possibly ​https://trac.osgeo.org/geos/ticket/725

[dbaston]

The excerpt from Geometry.h was intended to be:

	/**
	 * \brief
	 * Returns a Geometry representing the points making up this
	 * Geometry that do not make up other.
	 *
	 * @throws util::TopologyException if a robustness error occurs
	 * @throws util::IllegalArgumentException if either input is a
	 *         non-empty GeometryCollection
	 *
	 */
	virtual Geometry* difference(const Geometry *other) const;

[strk]

See also #752

[strk]

Ticket retargeted after milestone closed

[pramsey]

Is the remediation to change the headers?

[dbaston]

Call it a wontfix, if the overlay ops are going to be replaced anyway?

[pramsey]

Well, changed under the covers, maybe not at an interface level. If it's a doco fix, no reason not to do it, I'm just not sure what ops to change :)

[dbaston]

Depends if the new overlay impl is going to handle geometry collections, though?

Note that JTS errors out on collections (​https://github.com/locationtech/jts/blob/master/modules/core/src/main/java/org/locationtech/jts/geom/Geometry.java#L1384). Has GEOS really been extended to properly handle these cases that JTS avoids?

[Mike Taves]

I don't think these operations are working as expected. Here is an odd example with shapely with GEOS 3.9.1:

from shapely.geometry import Point, GeometryCollection

poly_list = [Point(i, 0).buffer(0.7, resolution=10) for i in range(2)]
A = GeometryCollection(poly_list)
res = A.difference(Point(1, 0).buffer(0.2))
print(res)

the result is "POLYGON EMPTY". If the first geometry were replaced with a unary-unioned polygon, the result would be a normal looking polygon with a hole punched thru.

The odd thing is when the number of coordinates used in the GeometryCollection is reduced, an exception is raised:

poly_list = [Point(i, 0).buffer(0.7, resolution=9) for i in range(2)]
A = GeometryCollection(poly_list)
res = A.difference(Point(1, 0).buffer(0.2))
# TopologyException: side location conflict at 0.5 -0.4861824369638621

---

A PostGIS 2.5.2 / GEOS 3.6.2 equivalent of the example is:

-- runs, but returns GEOMETRYCOLLECTION EMPTY
SELECT ST_AsText(ST_Difference(a, b))
FROM (
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geometry as a,
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geometry as b
) f;

-- ERROR:  lwgeom_difference: GEOS Error: TopologyException: side location conflict at 0.5 -0.4861824369638621
SELECT ST_AsText(ST_Difference(a, b))
FROM (
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geometry as a,
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geometry as b
) f;

---

Unless there is a robust solution to get expected behaviour with consistent results, GEOS should do what JTS does and raise an error if one of the inputs is a GeometryCollection.

On a related note, I see the ​PostGIS 2.2 manual for ST_Difference note "Do not call with a GeometryCollection as an argument", but this appears to be removed from the current release. Was this intentional?

[pramsey]

mdavis is going to add geometry collection support into JTS OverylayNG, and then we can sync up and maybe clean out wrappers in downstream projects. The semantics seem not-impossible to suss out.

[robe]

Retargeting in prep for GEOS 3.10.0 release