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Subject: Re: [geometry] single-point polygons
From: Barend Gehrels (barend_at_[hidden])
Date: 2014-10-13 17:05:55

Hi Volker,

Volker Schöch wrote On 13-10-2014 18:28:
> The more I think about it, the more confused I get. What are the exact
> pre-conditions and post-conditions of your algorithms?
> On
> it says: "Check the Polygon Concept for the rules that polygon input
> for this algorithm should fulfill".
> On
> 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

> 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 non-empty, 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 multi-polygon, and not a
polygon. The multi-polygon 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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