Re: [Jts-topo-suite-user] Invalid polygon resulting from Vectorization

[Martin D. <mtn...@gm...>]

Michael's suggestion is the easiet way to handle this situation, although
it does have some pitfalls as noted in the FAQ:

http://tsusiatsoftware.net/jts/jts-faq/jts-faq.html#G


On Tue, Apr 29, 2014 at 5:36 PM, Michael Bedward
<mic...@gm...>wrote:

> Hi Rob,
>
> Doing poly.buffer(0) on that one seems to give what you are after.
>
> Michael
>
>
> On 30 April 2014 07:44, Rob Emanuele <rde...@gm...> wrote:
> > Hi JTS folk,
> >
> > This is Rob from the GeoTrellis team. We have an algorithm that performs
> > vectorization against raster data that does a RegionGroup operation over
> the
> > raster and then vectorizes each region along the cell borders.
> >
> > We're wrapping JTS in scala code (you can find it here:
> >
> https://github.com/geotrellis/geotrellis/tree/master/feature/src/main/scala/geotrellis/feature
> ).
> > This is replacing some earlier wrapping work contained in GeoTrellis <=
> > 0.9.X. One difference between the two wrappers is that the current
> Polygon
> > checks to see that the JTS geometry is valid in the constructor (as does
> > each geometry type).
> >
> > This is causing one of our previous Vectorization unit tests to fail. In
> > this case, the Raster looks like this:
> >
> >   X  X  X  X  X
> >   X  P  P  P  X
> >   X  P  X  P  X
> >   X  X  P  P  X
> >   X  X  X  X  X
> >
> > Where X represents NoData, and P represents a value (The letter choice is
> > arbitrary). The algorithm creates a vector who's border LineString goes
> > around the perimeter of the region of P's.
> >
> > The problem is, the algorithm creates a border with two points being the
> > same:
> >    ______
> >   |    __   |
> >   |__| _|   |
> >        | __ |
> >
> > (apologies for the very rough ASCII art, I hope it translates).
> >
> > So because the border intersects itself, it's considered an invalid
> polygon.
> > The algorithm could possibly changed to consider the outside perimeter as
> > the LineRing and the space in the middle as a hole; this would require
> some
> > significant changes to an already complex algorithm, and I'm not sure it
> > could handle all of the edge cases.
> >
> > My question is, is there a way to solve this in JTS? To take a LineString
> > that is invalid for the border of the polygon, and separate it out into a
> > valid border and a hole, and to create a valid polygon from it, in a
> generic
> > way?
> >
> > Thanks,
> > Rob
> >
> >
> >
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