Re: [boost] Formal Review: Boost.Polygon starts today August 24, 2009

Subject: Re: [boost] Formal Review: Boost.Polygon starts today August 24, 2009
From: Arash Partow (arash_at_[hidden])
Date: 2009-09-02 10:18:48

Barend Gehrels wrote:
>> I don't know whether convex hull should be considered as a basic 
>> polygon algorithm.
> It is the base of some other algorithms, e.g. the polygon diameter 
> (furthest point pair, e.g. 
> http://pagesperso-orange.fr/colin.barker/lpa/anti_pod.htm). But indeed 
> it can be discussed if it is really basic. It is prescribed by OGC and 
> implemented by most spatial databases.
> 

I'm not sure what you mean by "basic polygon algorithm". If you could qualify that phrase I'm sure it would help myself and everyone else wondering what that means as I never knew there were "degrees" of polygon algorithms.

A convex hull is by definition, simply, a set of points. The cardinality of such a set is infinite. From a geometers point of view a hull is its own structure or entity and has its own set of operations. Just because a couple of GIS applications you've work with in the past seem to have a representation of CHs that resemble a polygon doesn't mean that has to be the case. Again the concept you mentioned above regarding rotating calipers works on a set of points, these may seem like semantic differences to you if all you do is think in 2d or 3d all day long, but if you're planning on proposing a general purpose library that is expected to extend to multi dimensions and provide more than just GIS functionality then you have to start thinking about these other issues.

> This is a difficult one, the one that I wanted to avoid indeed, so I 
> omitted it from my reply. As a Boost-user, always working with and 
> interested in geometry, this library in its current form is of no use 
> for me. But of course, I'm part of the team of another geometry library, 
> having most of this one and more... so why would I use this one... 
> Therefore I feel I can't vote.
> 

In short this is a discussion about the applicability and fitness of a polygon library, not a "mine-is-better-than-yours" debate.

Arash Partow