[geos-devel] GEOS returning invalid geometries

[strk at refractions.net]

I've checked.
If you wrap your difference() call into a isvalid() func
you get true.
If you feed the isvalid() func what you cut-paste from
difference() output you get false (and an exception thrown).
Carl did probably get the problem.
--strk;

On Sat, Sep 18, 2004 at 09:22:02AM +0200, strk at refractions.net wrote:
> On Fri, Sep 17, 2004 at 11:13:56PM +0000, Carl Anderson wrote:
> > On 09/17/2004 03:47:35 PM, Martin Davis wrote:
> > > I wonder if this is due to floating-point implementation issues?
> > > Being
> > > build on Java, JTS has the luxury of being able to assume that all
> > > floating point is carried out in IEEE754.  GEOS on the other hand is
> > > dependent on the underlying compiler (and presumably processor FPU).
> > >
> > 
> > There is more than one issue
> > First
> >   WKT in postgis uses 15 digits of precision
> >           %.15g in postgis_inout.c
> >   WKT in GEOS uses 16 digits,
> >           MaxSignificantDigits in PrecisionModel.cpp defaults to 16
> >    int decimalPlaces = precisionModel->getMaximumSignificantDigits();
> >        string fmt="%.";
> >        char buffer[255];
> >        sprintf(buffer,"%i",decimalPlaces);
> >        fmt.append(buffer);
> >        fmt.append("f");
> > 
> > The conversion can turn a valid topology into an invalid one
> 
> Conversion between postgis and GEOS do not rely on string output.
> This can be true when 'reading' output on the console.
> Still the isvalid() function would not be confused by this issue.
> 
> --strk;
> 
> > 
> > There is another out there as well, so I will start where you suggest.
> > 
> > > I can't think of any explicit dependencies in JTS/GEOS which would
> > > cause
> > > this, but if another floating point model is used it's possible that
> > > some computations could come out slightly differently.
> > >
> > 
> > C.
> > 
> > Carl Anderson
> > 
> > 
> > 
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