This looks like a bug, alright. JTS returns the expected answer of
POLYGON ((1 0, 0 0, 0 1, 1 1, 1 0.5, 1 0),
(1 0.5, 0.5 0.9, 0.5 0.1, 1 0.5))
Martin
Pete Gerritson wrote:
I found what I think to be a bug in the Intersection routine.
I defined a polygon with the following wkt: POLYGON( (0 0, 1 0, 1 1, 0
1, 0 0), (.5 .1, 1 .5, .5 .9, .5 .1)) This polygon has an exterior
ring that is a square, and one interior ring that is a triangle. The
interior ring touches the exterior ring in one point. I believe this
is considered a valid polygon.
When I call the GEOSIntersection method using this polygon for both
parameters, I would expect to get back the same polygon. Instead I
get back a geometry collection of 8 linestrings and a polygon:
GEOMETRYCOLLECTION (LINESTRING (0.0000000000000000 0.0000000000000000,
1.0000000000000000 0.0000000000000000), LINESTRING (1.0000000000000000
0.0000000000000000, 1.0000000000000000 0.5000000000000000), LINESTRING
(1.0000000000000000 0.5000000000000000, 1.0000000000000000
1.0000000000000000), LINESTRING (1.0000000000000000
1.0000000000000000, 0.0000000000000000 1.0000000000000000), LINESTRING
(0.0000000000000000 1.0000000000000000, 0.0000000000000000
0.0000000000000000), LINESTRING (0.5000000000000000
0.1000000000000000, 1.0000000000000000 0.5000000000000000), LINESTRING
(1.0000000000000000 0.5000000000000000, 0.5000000000000000
0.9000000000000000), LINESTRING (0.5000000000000000
0.9000000000000000, 0.5000000000000000 0.1000000000000000), POLYGON
((1.0000000000000000 0.0000000000000000, 0.0000000000000000
0.0000000000000000, 0.0000000000000000 1.0000000000000000,
1.0000000000000000 1.0000000000000000, 1.0000000000000000
0.5000000000000000, 1.0000000000000000 0.0000000000000000),
(1.0000000000000000 0.5000000000000000, 0.5000000000000000
0.9000000000000000, 0.5000000000000000 0.1000000000000000,
1.0000000000000000 0.5000000000000000)))
The last polygon appears to be equivalent to my input, but with one
extra point in the exterior ring. The 8 linestrings appear to be the
pieces of the boundary of the polygon.
I believe this is a bug. If I change the input polygon so that the
interior ring doesn't touch the exterior ring, I get the expected result.
Thanks,
Pete Gerritson
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Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022
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