Geometry Boost List: Re: [geometry] Equivalence between (multi-)polygons with spikes (3)

Subject: Re: [geometry] Equivalence between (multi-)polygons with spikes (3)
From: Barend Gehrels (barend_at_[hidden])
Date: 2014-11-24 13:53:16

Hi Volker,

I had to digest your replies a little ;-) Hopefully I can give some more 
answers now and later.

Volker Schöch wrote On 20-11-2014 13:06:
>
> bg> E.g. your sample (in the ticket) with two points, where a spike is 
> removed, results in a polygon with only one point. That is IMO a 
> perfect result from remove_spikes. Because the spike is removed. 
> remove_spikes should do nothing more than that... Why should it remove 
> a remaining point - that point is not a spike. And remove_spikes 
> should not change anything but spikes...
>
> bg>
>
> bg> That the result is still not as desired, OK, I understand that 
> but... that should be implemented in another function (e.g. correct).
>
> Thank you for the explanation. I'm not yet convinced. What exactly is 
> the definition of a "spike"? How is a single point not a spike? It's a 
> boundary that doesn't contain/represent any points of the polygon.
>

No, a single point is not a spike. It is not a valid polygon, but a 
spike goes forwards and backwards over the same path, so a segment but 
it can also make a corner (or more corners).
If you consider a diamond-shape with a spike at one of its corners, the 
spike is removed but exactly the point where the spike leaves the 
rectangle is kept, of course. So the point at the base of the spike is 
kept. That is why that single point is kept...

> It might be debatable whether a single point is a spike and should be 
> removed by remove_spikes(). However, it seems crystal clear to me that 
> when removing spikes, the result cannot be a single point.
>

For me it is not that clear. I had to think about it but I think the 
current implementation is right. What if you have a polygon with two 
holes. One of two holes is spike-only, so a single point remains. Of 
course it is not valid, but should remove_spike remove that single 
point? That results in a hole with zero points (empty). Should it then 
remove that empty hole from the interior-ring container ? I really think 
that is outside its responsibility.
Same story for multi-polygons, suppose 5 of the 10 polygons of a 
multi-polygon are spike-only. Resulting in 5 single-point geometries. 
Should they be removed from the multi-polygon resulting in 5 ?

> Again: What exactly is a spike? Which points constitute a spike? In 
> the example there was a polygon representation that consists of only a 
> spike, i.e., the polygon does not have any points. /It consists of a 
> spike!/ Removing the spike is removing the polygon representation, 
> seems obvious to me. The representation does not enclose any points, 
> it is a spike, it should be removed. What's so special about one point 
> or the other, that some points are removed from the representation but 
> one remains?
>

Well, yes, these are good questions and we might consider removing the 
only point from the spike, leaving an empty polygon or interior rings 
with partly empty rings or multipolygons with partly empty polygons 
after that.

Taking into account the whole remove_spikes operation exists and is 
released is partly your responsibility ;-) you also have some influence 
and I'm curious what you find now.

So after calling remove_spikes the user should call correct, which 
should remove rings which are empty from interior rings and polygons 
which are empty from multi-polygons (it currently does not do that, but 
from "correct" you might expect that).

Regards, Barend