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Subject: Re: [geometry] #8376: difference of non-closed polygons returns closed polygon
From: Adam Wulkiewicz (adam.wulkiewicz_at_[hidden])
Date: 2014-10-10 05:22:14

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!

FYI, some time ago WKT was fixed to always output closed Polygons. AFAIU
this change was made because an output must be consistent and
representation-independent. Not to mention that in WKT language Polygons
are just defined as closed, so it's also a matter of conformance.
Consider e.g. creating WKTs for open polygons in BG and then loading
them using other software.

For convenience:

Is this related to your problem?

Regarding read_wkt AFAIR it checks if the last Point is the same as the
first one and do whatever needs to be done with it. I don't remember if
it also appends the missing closing Point, probably it is.
Assuming that I'm right, maybe it should throw an exception if the input
was invalid (not closed) if that's a problem? We probably shouldn't
assume that we know why the input is incorrect, maybe the last Point is
missing on purpose and should be added, or maybe the last Point is
different by mistake so should be fixed/replaced. But I'm not sure if
the behavior should be changed at this point, plus nobody complains about.

So you should file a ticket if you think that something's wrong or
should be done differently.
Or we could first consider all aspects in detail on the list.


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