Buffering a specific closed linestring erroneously produces polygon without hole.

[uclaros]

Applying a buffer of 1m on the following linestring

LineString (278601.0234000000054948 4295292.71929999999701977, 278598.71919999999227002 4295290.49340000003576279, 278589.06283691781572998 4295303.48101469129323959, 278605.49300000001676381 4295297.03689999971538782, 278601.0234000000054948 4295292.71929999999701977)

produces a polygon without a hole, only outer ring. If any of the three almost-aligned nodes is moved, the expected result is returned: a polygon with a hole (one outer ring, one inner ring).

Found on QGIS and confirmed on shapely with geos 3.8.0

[jgrocha]

Buffer wit hthe missing hole in the polygon

[jgrocha]

Confirmed using Postgis with GEOS 3.8.0-CAPI-1.13.1

SQL to reproduce the bug:

CREATE TABLE public.tpoly (
	id serial NOT NULL,
	geom geometry(POLYGON, 3857) not null,
	CONSTRAINT tpoly_pkey PRIMARY KEY (id)
);

CREATE TABLE public.tline (
	id serial NOT NULL,
	geom geometry(LINESTRING, 3857) not null,
	CONSTRAINT tline_pkey PRIMARY KEY (id)
);

insert into public.tline (geom)
values (ST_GeomFromText('LineString (278601.0234000000054948 4295292.71929999999701977, 278598.71919999999227002 4295290.49340000003576279, 278589.06283691781572998 4295303.48101469129323959, 278605.49300000001676381 4295297.03689999971538782, 278601.0234000000054948 4295292.71929999999701977)',3857));

insert into public.tpoly (geom)
select st_buffer(geom,1, 'quad_segs=0') from public.tline where id = 1;

The resulting buffer is attached.

[Mike Taves]

Confirmed, with same behavior with JTS 1.14. And for the record, the input geometry is valid and simple.

The test can be simplified using a default buffer (without parameters).

What makes this bug interesting is that it is depends on the buffer distance, since most other values produce the expected result. And it's not just 1.0.

Here is a simple shapely example that checks 50 buffer distances between 0.1 and 2.0

import numpy as np
from shapely import wkt

g = wkt.loads('LineString (278601.0234000000054948 4295292.71929999999701977, 278598.71919999999227002 4295290.49340000003576279, 278589.06283691781572998 4295303.48101469129323959, 278605.49300000001676381 4295297.03689999971538782, 278601.0234000000054948 4295292.71929999999701977)')
for b in np.linspace(0.1, 2.0, num=50):
    if len(g.buffer(b).interiors) == 0:
        print(b)

shows these buffer distances

0.6428571428571428
0.9918367346938776
1.263265306122449
1.379591836734694

so 8% failure rate.

[mdavis]

Logged as ​JTS 523

[mdavis]

Fixed in JTS by ​PR 525

[Paul Ramsey <pramsey@…>]

In 061656e/git:

Buffering a specific closed linestring erroneously produces polygon without hole. References #1022

[Paul Ramsey <pramsey@…>]

In 56b19e0/git:

Buffering a specific closed linestring erroneously produces polygon without hole. Closes #1022