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Subject: Re: [geometry] #8376: difference of non-closed polygons returns closed polygon
From: Menelaos Karavelas (menelaos.karavelas_at_[hidden])
Date: 2014-10-10 05:17:12

Hi Volker.

On 10/10/2014 11:39 πμ, Volker Schöch wrote:
>> I tried this example as well (see attached program and output).
>> What I see is that the output has 18 points (I can see that from both the dsv printout of the geometry, as well as from bg::num_points). So the resulting multi-polygon is not closed, but when you output it through bg::wkt, it is shown as closed.
> Thank you very much for double-checking! In the mean time I, too, found that things may not be what they appear to be, but I could not yet nail it. Apparently, not only wkt but also read_wkt mix up the closedness. I got caught by this because I have a number of unit tests in place based on read_wkt, which are now triggering like crazy.
> I'm glad that it's just wkt/read_wkt, and not the actual algorithm, that is at fault.
> Should I file a ticket for the wkt/read_wkt issue? It interferes with debugging, unit-testing, and with reporting issues, too!

I am quite new to Boost.Geometry, so please take my writings with a
grain of salt.
The behavior of bg::wkt was changed in boost 1.56 on purpose. The
rationale for the change is that the WKT format does not account for
open polygons; open areal geometries is a BG "thingy". So in order to
have valid WKT descriptions of geometries, open areal geometries are
outputted as if they were closed (again, to have a valid WKT description).

I do not want to encourage you or discourage you to file a ticket. You
are free to do as you want, and in fact filing a ticket will give us the
motivation to look over the issue again. I just wanted to let you know
of the current status.

All the best,

- m.

> Regards
> Volker
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