
Geometry : 
Subject: Re: [geometry] singlepoint polygons
From: Barend Gehrels (barend_at_[hidden])
Date: 20141013 17:05:55
Hi Volker,
Volker Schöch wrote On 13102014 18:28:
>
> The more I think about it, the more confused I get. What are the exact
> preconditions and postconditions of your algorithms?
>
> On
> http://www.boost.org/doc/libs/1_56_0/libs/geometry/doc/html/geometry/reference/algorithms/difference.html
> it says: "Check the Polygon Concept for the rules that polygon input
> for this algorithm should fulfill".
>
> On
> http://www.boost.org/doc/libs/1_56_0/libs/geometry/doc/html/geometry/reference/concepts/concept_polygon.html
> it says "There should be no cut lines, spikes or punctures". It also
> says "If the input is invalid, the output might be invalid too". Fair
> enough.
>
I can imagine it is not that simple, and we probably should describe it
better.
> What it doesn't say is that a polygon without area is invalid. Let's
> take that at face value, though: It is trivial to imagine, e.g., the
> difference of two nonempty, valid polygons that results in empty
> polygon. Does this mean that, e.g., the difference algorithm can take
> valid input and return invalid output?
>
No: the difference algorithms outputs a multipolygon, and not a
polygon. The multipolygon does not contain any polygons, and is a valid
geometry. So the output is valid.
Having said that, an empty polygon is also considered as valid by
several database packages. Our implementation (is_valid is just
released) reports false to empty polygons, we probably should fix that.
> If I a have a computation that consists of multiple steps, I would
> like to make sure that the input is valid, and then run my computation
> as a sequence of calls to geometry algorithms. Given the above  do I
> have to verify every intermediate result, and make sure that if I find
> an invalid polygon, the algorithm returns half way? What is your
> recommended best practice to deal with this? I am sure that calling
> correct, unique, remove_spikes (in which order?) and is_valid all over
> the place cannot be the answer...
>
No, if the input is valid, the output should be valid too. If it is not
valid, these cases may be reported.
Thanks for your input.
Regards, Barend
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