Offset curve issue with closed linestring and miter join

[nyalldawson]

A closed linestring (eg LineString (0 0, 10 0, 10 10, 0 10, 0 0)) gives the correct result when offset by -1 using a round join style:

However if a Miter join style is used, then parts of the result are missing:

Shifting either line endpoint by a tiny amount gives the expected result (eg using LineString (0 0.00000001, 10 0, 10 10, 0 10, 0 0))

[dr-jts]

Interesting. It works fine in JTS:

Possibly related to #1036? Investigation required.

[nyalldawson]

Sorry, I posted my ticket a little early! I should have added that I'm testing using 3.12.1

[nyalldawson]

This is impacting some QGIS users significantly -- could this be prioritised if funding is supplied?

[dr-jts]

To support investigating this, I added geosop operations bufferJoin and offsetCurveJoin. This case can be executed with:

bin/geosop -a "LINESTRING (0 0, 10 0, 10 10, 0 10, 0 0)" offsetCurveJoin N-1 N-5 

LINESTRING (11 -1, 11 11, -1 11)

[nyalldawson]

Thanks @dr-jts !