[geos-devel] OverlayNG for Testing

[Martin Davis]

On Wed, Jul 29, 2020 at 12:53 PM Paul Ramsey <pramsey at cleverelephant.ca>
wrote:

>
> - 3D! GEOS uniquely has supported adding in Z coordinates to introduced
> coordinates, and that code hasn't been added to JTS (and thus overlayng).
> How this will affect existing expectations for 3D output is unknown, but
> it's sure to be broken in some way.
> - Union on linestrings! OverlayNG extracts maximal length linestrings from
> the unioned graph. Old overlay extracted minimal length linestrings. This
> change is probably "for the best" but it's a change which might impact
> existing logic.
>

Another behavioural change is that now overlay operations do not return
mixed-dimension results.  Instead, a homogeneous geometry containing only
the highest dimension resultants is returned.

So for example, if two polygons intersect in an area and along an edge, the
new overlay will only return the area resultant.  The old overlay returned
a GEOMETRYCOLLECTION containing both a polygon and a line.

Or, if two polygons intersect along an edge and at a point, the new result
is a LineString for the shared edge.  The old overlay returns a GC
ccntaining a line and a point.

The old behaviour seemed like a pain, since most (all?) uses were only
interested in the highest dimension resultants.  And since
GeometryCollections cannot be processed by overlay ops or spatial
predicates, it made chaining together operations painful.

What are people's thoughts on this?  It is probably possible to build in
the old behaviour (with a bit of coding effort), and make it the default
path.  But I think the new behaviour is preferable and more sensible.
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