[geos-devel] Bug in buffer function?

[Silke Reimer]

Hallo!

I observed a really strange behavior of the buffer function when using an 
inner buffer. The error occurs with the following geometry:
POLYGON ((
513.4 343.1, 
707.3 344.2, 
707.0 394.9,
797.9 395.4, 
799.2 341.8,
516.9 340.9,
516.7 262.3, 
513.2 262.3,
513.4 343.1
))
Its WKB representation is
0103000000010000000900000033333333330B80409A9999999971754066666666661A8640333333333383754000000000001886406666666666AE78403333333333EF88406666666666B678409A99999999F98840CDCCCCCCCC5C7540333333333327804066666666664E75409A99999999258040CDCCCCCCCC6470409A99999999098040CDCCCCCCCC64704033333333330B80409A99999999717540

Using a buffer of -2 should lead to an inner buffer but leads 
to "GEOMETRYCOLLECTION EMPTY" resp. "POLYGON( EMPTY)" depending on the geos 
version.

Using the same buffer of -2 on a polygon with the same shape but shifted to 
the left by 100 and shifted down by 100, i.e.

POLYGON ((
413.4 243.1, 
607.3 244.2, 
607.0 294.9,
697.9 295.4, 
699.2 241.8,
416.9 240.9,
416.7 162.3, 
413.2 162.3,
413.4 243.1
))

resp.
010300000001000000090000006666666666D679403333333333636E406666666666FA82406666666666866E400000000000F8824066666666666E72403333333333CF854066666666667672409A99999999D985409A99999999396E4066666666660E7A40CDCCCCCCCC1C6E4033333333330B7A409A999999994964403333333333D379409A999999994964406666666666D679403333333333636E40

leads to the expected buffer:
POLYGON((609.172261037685 243.512992930268,609.26383338939 
243.821372982058,609.29996498814 244.211834112356,609.011804031568 
292.911035772954,695.948180429531 293.389233662932,697.151062564334 
243.793477951075,609.172261037685 243.512992930268))
resp. 
01030000000100000007000000DD1965CA600983408D5226706A706E403118AE541C0A83402AF8FDAF487A6E400A300B54660A8340DB175558C7866E405E4CB62C1808834035253F9A934E72405BFC9EDF95BF8540BECC134D3A567240B72B4A6035C98540EB3EDF2B64796E40DD1965CA600983408D5226706A706E40

I tested this on different geos versions among them
3.0.0rc4 and 3.0.0

I would say that this is a bug but it would be nice if somebody could confirm 
this - and perhaps you even have an idea how to solve this? I did not find 
any entry in the bug tracker but maybe I just missed it. In this case I would 
be glad for a hint.

Many greetings,

	Silke

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