[postgis-devel] Looking for a good ST_ChangeEdgeGeom semantic

Paul Ramsey pramsey at opengeo.org
Thu Feb 16 10:07:10 PST 2012


The two odd cases you show are impossible without "lifting" the edge
up off the paper and putting it down on the other side of the other
edges. The deformation has to be done without leaving the manifold.
The authors perhaps assumed the reader would internalize that
restriction.

P.

On Thu, Feb 16, 2012 at 10:02 AM, Sandro Santilli <strk at keybit.net> wrote:
> Someone might have been following the ST_ChangeEdgeGeom saga:
> http://trac.osgeo.org/postgis/ticket/1571
>
> The problem is that ISO/SQL-MM specification don't tell
> much about what's allowed and what not when it comes to
> changing the geometry of an edge, except that its endpoints
> must remain the same. So for example you can go from this:
>
>    o--------------o
>
> To this:
>
>    o---,      .---o
>        |     /
>        `----'
>
> Now my problem is, how far can we push this ?
> Should we also allow going from this:
>
>    o--------------o
>    |              |
>    o------E1------o
>    |              |
>    o--------------o
>
> To this:
>
>  .--------E1--------,
>  |                  |
>  | o--------------o |
>  | |              | |
>  '-o              o-'
>    |              |
>    o--------------o
>
> And what about from this:
>
>    .--------------,
>    |              |
>    o------E1------o
>
> To this:
>
>  .--------E1--------,
>  |                  |
>  | .--------------, |
>  | |              | |
>  '-o              o-'
>
> I tend to think "NO", but I don't have a clear definition about it.
>
> It may have to do with considering the edge as made with
> an elastic matter, taking it with your hand and morphing it
> into the new position w/out hitting obstacles. And I think I've
> read about this kind of concept somewhere, related to topology.
>
> Does anyone have better nomenclature for the concept ?
>
> --strk;
>
>  ,------o-.
>  |   __/  |    Delivering high quality PostGIS 2.0 !
>  |  / 2.0 |    http://strk.keybit.net
>  `-o------'
>
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