Negative distance buffer

[joao...@…]

I’ve detected a possible bug or at least a bizarre behavior when performing a buffer query with negative distances (e.g.: SELECT st_buffer(a.the_geom, -5.725)...). In some cases interior rings are not being used to perform the inner buffer generation (this is illustrated by the reddish features in the image attachment) although for most of the cases rings are correctly used (green features).

Tests using polygon/multipolygon themes revealed the same result. I've

compared the areas of buffered and non-buffered polygons with very small distances, and in some cases the negative buffered area is bigger revealing the inclusion of the rings areas.

Performing an intersction positevely identifies the problematic polygons:

SELECT a.gid,

a.the_geom

FROM original a, negative_buffer b WHERE st_intersects(a.the_geom, b.st_buffer) AND a.gid<>b.gid;

Enviroment: -- WinXP SP2 -- PostgreSQL 8.2.0.7 -- PostGIS 1.3.3 (data imported under 1.3.2)