[geos-devel] Invalid geometries from GEOS operations

strk strk at keybit.net
Fri Aug 27 10:21:43 EDT 2010

I've spent some days enhancing the XMLTester to check
for validity of returned geometries and found a dozen
or more such cases (6 couple of inputs, multi-ops for
some of them).

All the invalidities come out from GEOS unchecked use of
CommonBitsOp, whereas such operation is known to possibly
yeld collapsed poligons due to loss of precision when 
translating back (re-adding the previously stripped bits).

In JTS, an EnhancedBitsOp class is used to basically add
a validity check around the CommonBitsOp to ensure the
result would be checked before returning.
In GEOS, doing a similar thing would be nice as we could
then try something else (SnapOp). Well, I did and that
fixed 5 over 6 tests. Unfortunately the last test broke
completely, that is it would not return an invalid geometry 
(as it did before) but rather an exception.


Ok, simple question for you now is:
Should GEOS always throw an exception in preference over
returning an invalid geometry ?


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