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

[Kevin Weller]

OK, WKT it is!  Here ya go:


WORKING PRIMARY:
POLYGON ((1355.2556682087008539 2641.0482955551756277,  
2675.2549625870647105 2642.4131547503075126, 2695.5193111868375127  
3962.2575986814194948, 1375.5200168084738834 3960.8927394862880647,  
1355.2556682087008539 2641.0482955551756277))

WORKING NEGATING:
POLYGON ((1700.4537532531219313 3631.2728433022930403,  
1705.5198404030647907 3961.2339542850709222, 2035.5196639976556980  
3961.5751690838537797, 2030.4535768477128386 3631.6140581010758979,  
1700.4537532531219313 3631.2728433022930403))

WORKING RESULT:
POLYGON ((2035.5196639976556980 3961.5751690838537797,  
2695.5193111868375127 3962.2575986814194948, 2675.2549625870647105  
2642.4131547503075126, 1355.2556682087008539 2641.0482955551756277,  
1375.5200168084738834 3960.8927394862880647, 1705.5198404030647907  
3961.2339542850709222, 1700.4537532531219313 3631.2728433022930403,  
2030.4535768477128386 3631.6140581010758979, 2035.5196639976556980  
3961.5751690838537797))


BROKEN PRIMARY:
POLYGON ((1355.2556682087008539 2641.0482955551756277,  
2675.2549625870647105 2642.4131547503075126, 2695.5193111868375127  
3962.2575986814194948, 1375.5200168084738834 3960.8927394862880647,  
1355.2556682087008539 2641.0482955551756277))

BROKEN NEGATING:
POLYGON ((2030.4535768477128386 3631.6140581010754431,  
2035.5196639976556980 3961.5751690838537797, 2365.5194875922470601  
3961.9163838826366373, 2360.4534004423039733 3631.9552728998587554,  
2030.4535768477128386 3631.6140581010754431))

BROKEN RESULT:
POLYGON ((2035.5196639976556980 3961.5751690838537797,  
2695.5193111868375127 3962.2575986814194948, 2675.2549625870647105  
2642.4131547503075126, 1355.2556682087008539 2641.0482955551756277,  
1375.5200168084738834 3960.8927394862880647, 2035.5196639976556980  
3961.5751690838537797), (2035.5196639976556980 3961.5751690838537797,  
2030.4535768477128386 3631.6140581010754431, 2360.4534004423039733  
3631.9552728998587554, 2365.5194875922470601 3961.9163838826366373,  
2035.5196639976556980 3961.5751690838537797))


- Kevin


On Nov 28, 2007, at 2:27 PM, Martin Davis wrote:

> 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
>>
>> ------------------------------------------------------------------------
>>
>> _______________________________________________
>> geos-devel mailing list
>> geos-devel at lists.osgeo.org
>> http://lists.osgeo.org/mailman/listinfo/geos-devel
>
> -- 
> Martin Davis
> Senior Technical Architect
> Refractions Research, Inc.
> (250) 383-3022
>
> _______________________________________________
> geos-devel mailing list
> geos-devel at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/geos-devel

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