Fix BoundaryChainNoder to split chains at self-touch nodes

[dr-jts]

Fixes the BoundaryChainNoder to split chains at self-touch nodes. This is required by CoverageUnion to produce correct OGC topology for inverted polygons and exverted holes.

For example, the coverage:

GEOMETRYCOLLECTION (POLYGON ((1 9, 9 9, 9 6, 5 7, 2 6, 1 9)), POLYGON ((1 1, 1 9, 2 6, 4 3, 5 7, 7 3, 9 6, 9 1, 1 1)))

produces a polygon with two holes:

POLYGON ((9 9, 9 6, 9 1, 1 1, 1 9, 9 9), (5 7, 7 3, 9 6, 5 7), (2 6, 4 3, 5 7, 2 6))

Before this fix, the output was an inverted polygon with a single self-touching ring.

[pramsey]

Porting to GEOS, this pre-existing test now throws a TopologyException:

auto input = GEOSWKTReader_read(m_reader,
        "GEOMETRYCOLLECTION(POLYGON ((0 0, 0 1, 1 1, 1 0, 0 0)), POLYGON ((1 0, 0.9 1, 2 1, 2 0, 1 0)))");

auto result = GEOSCoverageUnion(input);

What does JTS do? @dr-jts

[pramsey]

Answering my own question, it does the same thing as GEOS:

[dr-jts]

it does the same thing as GEOS:

The TopologyException is expected behaviour, since the input is an invalid coverage (the polygons overlap). Are you seeing something that needs fixing in JTS?

[pramsey]

It’s a change in behaviour. Previously the behavior was that invalid coverages were just returned unchanged when fed through coversge union

…

On Tue, Jul 8, 2025 at 10:55 AM Martin Davis ***@***.***> wrote: *dr-jts* left a comment (locationtech/jts#1134) <#1134 (comment)> it does the same thing as GEOS: The TopologyException is expected behaviour, since the input is an invalid coverage (the polygons overlap). Are you seeing something that needs fixing in JTS? — Reply to this email directly, view it on GitHub <#1134 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAA3ZW5FCTJAIZURQRZ63633HQAXJAVCNFSM6AAAAAB6JA6JKOVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTANBZHAZDGOJSHA> . You are receiving this because you commented.Message ID: ***@***.***>

[dr-jts]

It’s a change in behaviour. Previously the behavior was that invalid coverages were just returned unchanged when fed through coversge union

Actually it's always been possible that TopoloigyExceptions are thrown for invalid coverages. See the Javadoc on OverlayNG.CoverageUnion here. But the doc on the coverage.CoverageUnion class should make this explicit.

[dr-jts]

Added doc about CoverageUnion exceptions in 0e5ac9a