[postgis-devel] Possible bug in intersection of polygons

[Juan Pedro Pérez Alcántara]

Hi, I may have found a bug in polygon intersection. I've carefully checked
everything I could think of and updated all libraries to the latest
version, to rule out the possibility of a GEOS error or something, but the
error still persists.

postgis_full_version() is:

POSTGIS="3.3.1 3786b21" [EXTENSION] PGSQL="150" GEOS="3.11.0-CAPI-1.17.0"
PROJ="9.1.0" LIBXML="2.9.13" LIBJSON="0.15" LIBPROTOBUF="1.3.3"
WAGYU="0.5.0 (Internal)"

The system is Linux.

Here is a brief summary of the problem:

[image: Screenshot from 2022-11-02 12-19-38.png]

As you can see in the screenshot, I have several polygons with a clean
topology. A st_intersection of them, isolating only LINESTRING and
MULTILINESTRING solutions, returns the green lines. As you can see, the
clean topology produces clean results: all frontiers are correctly
identified but the segment in black. I've thoroughly analyzed this segment,
studying the as_ewkt() representation of both polygons and identifying the
common primitive, they are identical coordinate by coordinate, but I'm at a
loss at why st_intersection is unable to compute this exactly common
primitive.

I've all assets available to reproduce this issue if developers think it is
of interest, but I'm not going to dump them all here for brevity. Please
ask me for further details if you think this is of any interest.

Thanks for your attention and your time,

---

Juan Pedro Pérez Alcántara

jp.perez.alcantara at gmail.com
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