[geos-devel] Noding linework

[Martin Davis]

What can I say, except "Welcome to my world"?   It's not easy making 
things robust.  I don't have any more ideas at present a fix or new 
approach that would make more of the test cases work.

Martin


On 7/15/2011 6:55 AM, Sandro Santilli wrote:
> On Thu, Jul 14, 2011 at 08:17:55PM -0700, Martin Davis wrote:
>> Yes, union() is supposed to fully node linework.
>>
>> This case works fine in JTS 1.11 (and JTS 1.12) - i.e. running the
>> equivalent of polygonize( union( boundary(A),  B).  The output from
>> the union checks out as being fully noded, and the polygonize
>> produces 2 valid polygons.
>>
>> So possibly this is one of those numerical issues which shows up in
>> GEOS in native code, but not in JTS in Java?  This maybe one of
>> those cases where GEOS and JTS need to be traced through in parallel
>> (which seems really painful, but I think you've done this in the
>> past, right?)
> Found.
>
> Contrary to JTS, which engages SnapRounding on first robustness failure,
> GEOS first tries with a pure CommonBitsRemover approach:
>
> Trying with original input.
> Original exception: TopologyException: found non-noded intersection between LINESTRING (1.72293e+06 4.78835e+06, 1.72293e+06 4.78835e+06) and LINESTRING (1.72293e+06 4.78835e+06, 1.72293e+06 4.78834e+06) at 1.72293e+06 4.78835e+06
> Trying with Common Bits Remover (CBR)
>
> I've seen in the past already that re-adding previously removed common
> bits may introduce invalidities (we discussed this in another occasion).
> That's because some of the lowermost bits fall off possibly moving the
> vertices to fall on a given grid. Evidentily this was the case in GEOS,
> where the output from Union against the shifted geometries was fully noded
> but became non-noded when re-adding common bits.
>
> Indeed going SnapRounding directly gives a fully noded output !
> But it gives failures (found non-noded intersection) in a
> couple of testcase:
>
>   bug360.xml (http://trac.osgeo.org/geos/ticket/360) [JTS works]
>   bug392.xml (http://trac.osgeo.org/geos/ticket/392) [JTS throws an exception!]
>
> --strk;
>
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