[postgis-devel] Looking for a good ST_ChangeEdgeGeom semantic

[Sandro Santilli]

Great, thanks a lot Paul.
I've slept over it and internalized that as well.

And "Homeorpheus" came to the rescue !

I woke up and while explaining the problem of "figuring out when
the restriction is violated" to my daughter I got enlightened by
this simple algorithm:

 If the polygon formed by the _old_ and the _new_ edge
 contains any node, then the rule is violated.

It seems to work pretty well. Only corner case is the closed edge
being reverted, but a single corner case I can live with :D

So, for your delectation, and hoping your mail client uses monospace
fonts, here are the cases I've shown with the algorithm applied:

Source:

 o========o
       o

Targets:

 o========o
 |     o  |      NOT VALID (a node inside)
 '--------'
 
 
 
 o==,-----o
 |  |  o         VALID
 '--'
 
 o==,-----o
 |  |  o .-.     VALID
 |  `----' |
 '---------'
 
    ,---------.
    `-------. |
 o==,-----o | |
 |  |  o    | |  VALID
 |  `-------' |
 '------------'

Exciting !

--strk;


On Thu, Feb 16, 2012 at 10:07:10AM -0800, Paul Ramsey wrote:
> 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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  ,------o-. 
  |   __/  |    Delivering high quality PostGIS 2.0 !
  |  / 2.0 |    http://strk.keybit.net
  `-o------'