[geos-devel] symDifference, malfunction or misunderstanding?

[Paul Ramsey]

I think you're misinterpreting your output, it includes either a nor
b, it looks like this. And reading back your multilinestring, it's the
same as the one JTS is outputting.

P

On Fri, Nov 27, 2009 at 5:18 PM, Frederik Ramm <frederik at remote.org> wrote:
> Dear all,
>
>   apologies for using up about 95% of bandwidth on this list recently. I
> hope that my numerous queries, and your answers, will at least help others
> in my situation in the future through Google et al.
>
> I have a problem with symDifference (in geos-3.0.0 as shipped with Ubuntu
> jaunty, but using the 3.2.0rc2 doesn't change things). Perhaps it is a
> misunderstanding on my part.
>
> I have two symmetric linear rings formed like adjoining angular brackets,
> like this:
>
> +------+------+
> |      |(a)   |
> |   +--+--+   |
> |   |     |   |
> |   +--+--+   |
> |      |(b)   |
> +------+------+
>
> They touch in the middle. Here's the WKT:
>
> LINEARRING (1 4, 3 4, 3 3, 2 3, 2 2, 3 2, 3 1, 1 1, 1 4)
> LINEARRING (3 4, 5 4, 5 1, 3 1, 3 2, 4 2, 4 3, 3 3, 3 4)
>
> If I compute the intersection between both, using x->intersection(y), then I
> get what one would expect, namely the two segments labelled "a" and "b" in
> the above diagram:
>
> MULTILINESTRING ((3.00 4.00, 3.00 3.00), (3.00 2.00, 3.00 1.00))
>
> When I do a "symDifference" between the two, I would expect to get the
> original shape minus the "a" and "b" segments, something like
>
> +------+------+
> |             |
> |   +--+--+   |
> |   |     |   |
> |   +--+--+   |
> |             |
> +------+------+
>
> Instead, I get a four-part Multilinestring that contains the original shape
> minus the segment "b", but including the segment "a":
>
> MULTILINESTRING ((1.00 4.00, 3.00 4.00), (3.00 3.00, 2.00 3.00, 2.00 2.00,
> 3.00 2.00), (3.00 1.00, 1.00 1.00, 1.00 4.00), (3.00 4.00, 5.00 4.00, 5.00
> 1.00, 3.00 1.00), (3.00 2.00, 4.00 2.00, 4.00 3.00, 3.00 3.00))
>
> Just to be on the safe side, I then combined both linear rings into a
> multilinestring (using Union), and then computed the difference between this
> union and the intersection - again expecting to get the shape minus "a" and
> "b", and again getting the shape minus "a" but including "b".
>
> Is this a bug in GEOS, or a bug in my approach?
>
> I'm attaching the full C++ test file.
>
> Bye
> Frederik
>
> #include <stdio.h>
> #include <geos/geom/PrecisionModel.h>
> #include <geos/geom/GeometryFactory.h>
> #include <geos/geom/Geometry.h>
> #include <geos/io/WKTReader.h>
>
> using namespace geos::geom;
> using namespace geos;
>
> GeometryFactory *global_factory;
>
> int main(int argc, char **argv)
> {
>    PrecisionModel *pm = new PrecisionModel(2.0, 0, 0);
>    global_factory = new GeometryFactory(pm, -1);
>    delete pm;
>
>    io::WKTReader *reader = new io::WKTReader(global_factory);
>    Geometry *a = reader->read(
>        "LINEARRING (1 4, 3 4, 3 3, 2 3, 2 2, 3 2, 3 1, 1 1, 1 4)"
>        );
>    Geometry *b = reader->read(
>        "LINEARRING (3 4, 5 4, 5 1, 3 1, 3 2, 4 2, 4 3, 3 3, 3 4)"
>        );
>
>    Geometry *inter = a->intersection(b);
>    Geometry *diff = a->symDifference(b);
>
>    printf("intersection: %s\n", inter->toString().c_str());
>    printf("sym difference: %s\n", diff->toString().c_str());
> }
>
> --
> Frederik Ramm  ##  eMail frederik at remote.org  ##  N49°00'09" E008°23'33"
> _______________________________________________
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>
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