[postgis-devel] Looking for a good ST_ChangeEdgeGeom semantic

[Sandro Santilli]

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;

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