Simplify processing algorithm never removes polygon's first/last vertex

[uclaros]

Describe the bug

When using the simplify processing algorithm, if a polygon's first/last vertex is within the specified simplification tolerance it is not removed, resulting a non-simplified polygon. The expected behavior would be to remove the first/last vertex pair and add a new last vertex to the first available vertex.

How to Reproduce

• Add a polygon layer
• Digiitize a polygon so that the first vertex is in the middle of a straight segment
• Run simplify with a tolerance that would cause the first vertex to be removed
• First vertex is still there

QGIS and OS versions
69faea8 on Debian testing

[uclaros]

It seems like an upstream issue in GEOS, since QgsGeometry::simplify() calls GEOSTopologyPreserveSimplify_r. Or maybe this is by design in geos?

edit: Opened a geos ticket: https://trac.osgeo.org/geos/ticket/1064#ticket

[alexbruy]

@uclaros GEOS ticket is closed, does it work for you now? I still can replicate using the polygon from the GEOS ticket.

[uclaros]

@uclaros GEOS ticket is closed, does it work for you now? I still can replicate using the polygon from the GEOS ticket.

It seems to be partially fixed.
With GEOS 3.12.0-CAPI-1.18.0 only the following two cases still fail:

QgsGeometry.fromWkt('Polygon ((0 42, 42 100, 100 42, 42 42, 0 42))').simplify(1)
QgsGeometry.fromWkt('Polygon ((100 42, 42 42, 0 42, 42 100, 100 42))').simplify(1)