[postgis-devel] Looking for a good ST_ChangeEdgeGeom semantic

[Paul Ramsey]

I think your concept works fine in my head. I worry that it might be
brittle in practice since so many of these changes will be very minor
single vertex shifts leading to test polygons that are very thin and
narrow, precisely the thing that leads to topology failures in GEOS.
However I don't let have a similar concept as easy to explain.
Also note you'll have to do more than just join up the edges due to
the ease of constructing an invalid polygon that way (morph a straight
line into an "S" shape, you get the classic invalid figure eight
polygon).

P.

On Fri, Feb 17, 2012 at 12:30 AM, Sandro Santilli <strk at keybit.net> wrote:
> 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------'
>
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