[geos-devel] Difference operation not always working as expected

[Martin Davis]

Kevin,

Please post the input and output WKT or WKB for your good and bad 
cases.  That's the only way we can attempt to debug this problem.

Kevin Weller wrote:
> All,
>
> Hopefully it's not merely due to my own idiocy, but I'm experiencing 
> some weird results from difference and snap operations that may or may 
> not imply GEOS3 bugs.  I'm using the latest and greatest from trunk.  
> Here's what I'm trying to do:
>
> I've got a smaller polygon (which I call the negating polygon) which 
> partially covers a larger polygon (which I call the primary polygon).  
> The negating polygon is always flush with one side or corner of the 
> primary polygon.  The goal is to subtract the negating polygon from 
> the primary polygon such that there is a resulting "notch" in the 
> primary.  You can see an example of this actually working by looking 
> at 
> http://www.asapwebsoft.com/files/working_primary_and_negating_plot.png and 
> http://www.asapwebsoft.com/files/working_result_plot.png
>
> Sometimes, with only slightly different input (in this case, I move 
> the negating polygon to the East), it doesn't work.  See 
> http://www.asapwebsoft.com/files/broken_primary_and_negating_plot.png and 
> http://www.asapwebsoft.com/files/broken_result_plot.png (interior ring 
> not shown on the plot, but I've checked, and it's there)
>
> In the latter case, instead of a notch in the resulting polygon, I get 
> an interior ring (hole), even though the negating polygon is as close 
> to overlapping as I can get it.  I've even tried using the 
> GeometrySnapper to snap the negating polygon to the primary polygon, 
> in case they weren't quite lined up, but no luck.  In fact, snapTo 
> seems to have no effect for any of the inputs I provide, so the 
> function must consider the polygon edges in question to be flush 
> already.  But I still get holes instead of notches.
>
> I'm scripting this in Ruby, and an excerpt from my Ruby driver code is 
> at http://www.asapwebsoft.com/files/geos_client_code_excerpt.rb
>
> Also, the current output from geos_tests.rb (showing a handful of 
> failures, probably not related, but I don't know that for sure) is at 
> http://www.asapwebsoft.com/files/test_output.txt
>
> I'm not exactly sure what to try next, but I'm hoping that someone out 
> there has some ideas.
>
> Oh, in the process of working on this, I've added the snapping 
> features to the C API and the SWIG bindings...hopefully correctly.  
> I'll be happy to share those changes if the community wants them.  
> Since I'm not a committer, what would I do?  Send patches to the list?
>
> - Kevin
>
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-- 
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022