[geos-devel] bad input data or robustness issue?

Russell Strong russell at strong.id.au
Mon Mar 3 04:43:36 EST 2008

Hi all,

I'm trying to dissect up a bunch of nearly round overlapping polygons, 
ie.. trying to find areas of overlapping radar coverage given a set of 
radar locations and ranges.

However, trying this with a few polygons, testing each case I can think 
of works very well.  Add a few more and I get all sorts of errors including:
* side location conflict
* stuck in endless loop ( due to intersect and difference operations 
that produce the same  polygons as we started with )
* non-noded intersection
* no outgoing dirEdge found

I've included some test code that shows all of these,  ( uncomment 
various tests in main ).  If anyone has some time to run these and 
comment I'd appreciate it.  I've spent 5 solid days on it and I'm out of 

it basically works like this:

I keep a link list of "areas" which contain a geometry.  I then compare 
each geometry against each other ( except for self ).  If it intersects, 
I add the intersection and the 2 differences to the list (if they exist) 
and remove the source areas.  I keep going until I've compared every 
area against every other area and found no non-empty intersections.

One other thing that I found was the Overlaps can return true but the 
intersection of the two polygons returns true for isEmpty?!?!?  (I was 
using overlaps instead of intersects in the split_areas comparison)

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