The reason for the "extra" vertex #12 is that the bottom-right vertex of
the L-shaped polygon does not quite lie exactly on the boundary of the
larger polygon. It lies slightly inside, and thus the result of the
difference contains a spike running down to vertex #12.
I was able to resolve this by snapping the larger to the smaller polygon,
using GeometrySnapper with distance 1. The snapping introduces a vertex
matching the bottom right vertex of the L-shaped polygon, and so the
difference does not contain the spike.
This isn't really a bug - the code is working as designed. This is just
one of those tricky situations where close tolerances and the limitations
of finite precision causes some geometric anomalies. The ultimate solution
to this is for JTS overlay operations to incorporate snap-rounding to a
tolerance - but that has not yet been developed.
I'm not sure why there is a difference between the behaviour of
single-sided buffer VS normal buffer. You'd have to post the exact inputs
and calls to allow seeing if there's anything odd going on. But I suspect
that it might be due to "cutting" a line segment to create vertex 13. A
vertex computed along a line segment is highly unlikely to lie exactly on
the line segment, due to the limitations of finite precision.
On Mon, Jun 22, 2015 at 3:37 AM, Paul Meems <[email protected]> wrote:
> I'm reposting my question below which I initial posted to the NTS group.
> Since QGis is experiencing the same issue Diego (NTS) suggested posting it
> to the JTS group.
>
> The main question is if this is a bug in JTS or not.
> The L-shaped polygon was created using a single-sided buffer of a
> linestring made of coordinates from the larger polygon.
> In this case I can also use a 'normal' buffer. When I do so the difference
> is correct.
>
> Thanks,
>
> Paul
>
> ---------- Forwarded message ----------
> I have two polygons which I want to subtract from each other.
> The field polygon it the larger polygon and overlaps the smaller polygon.
>
> When I perform the Difference action the remaining part has one additional
> point on the smaller point which is not expected.
>
> This is the WKT:
> POLYGON ((194908.68715217288 586962.86751464731, 194881.30215215127
> 586952.0195146437, 194879.05315214754 586952.15151464322,
> 194877.20115214764 586954.13551464747, 194831.95715210476
> 587019.88551468146, 194760.91615204382 587122.9405147346,
> 194857.09315212426 587178.23851475632, 194858.9451521278
> 587178.63551475492, 194860.26815212792 587177.44451475318,
> 194873.10015214139 587158.65951475059, 194925.75215218749 587081.797514704,
> 194953.13715220965 587042.10951468861, 194962.79415221984
> 587027.42551468522, 194985.68115224215 586994.0885146677,
> 194986.21015224193 586991.70651466073, 194985.54815224075
> 586989.32551465672, 194908.68715217288 586962.86751464731))
>
> and
>
> POLYGON ((194886.66904433916 586975.65752607386, 194901.74579137619
> 586981.629866985, 194928.53316474415 586990.85093296878, 194935.04290785809
> 586971.94000373222, 194908.68715217288 586962.86751464731,
> 194881.30215215127 586952.0195146437, 194879.05315214754
> 586952.15151464322, 194877.20115214764 586954.13551464747,
> 194831.95715210476 587019.88551468146, 194821.8332618672
> 587034.57164674706, 194838.29986314307 587045.92290405277,
> 194848.42375338063 587031.23677198717, 194886.66904433916
> 586975.65752607386))
>
> Both geometries are in EPSG:28992 (RD_new/Amersfoort, The Netherlands).
>
> I hope you can see the screen shot:
>
>
> The blue L-shaped polygon is the small polygon. The grey-brown polygon is
> the result of the Difference action. As you can see the 12th point is
> unexpected. The border should go from 11 to 13 and so on. Now it passed
> point 13 and created point 12 and then goes back to point 13.
>
>
>
>
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