Geometry Boost List: Re: [geometry] Why closure?

Subject: Re: [geometry] Why closure?
From: TONGARI (tongari95_at_[hidden])
Date: 2013-06-20 12:22:36

2013/6/20 Mateusz Loskot <mateusz_at_[hidden]>

> On 20 June 2013 15:18, TONGARI <tongari95_at_[hidden]> wrote:
> > 2013/6/20 Mateusz Loskot <mateusz_at_[hidden]>
> >> On 20 June 2013 10:26, TONGARI <tongari95_at_[hidden]> wrote:
> >> >
> >> > I'm not quite understand why we need traits::closure for Ring &
> Polygon,
> >> > aren't these always closed?
> >>
> >> You can define open polygon or ring, means
> >>
> >> typedef bg::model::polygon<point, false, false> polygon; // open polygon
> >> typedef bg::model::ring<point, false, false> ring; // open ring
> >>
> >> where the following condition does not hold
> >>
> >> last point == first point
> >>
> >> It is specification of the fact if first point is assumed as repeated or
> >> not
> >> at last position.
> >>
> >> > What should I expect if the closure is open?
> >>
> >> Algorithms won't expect physical presence of the repeated first point.
> >>
> >> > Should I see Ring as Linestring
> >>
> >> Ring is also known as linear ring, in case you know OGC geometry model.
> >> IOW, (linear) ring is a specialisation of the linestring kind.
> >
> >
> > So "closure is open" means auto-close for Ring & Polygon, and the closure
> > doesn't change the meaning of Ring & Polygon as we should always see
> them as
> > closed.
> >
> > Am I right?
>
> Yes, correct.
>
> It is also visible from the minimum_ring_size metafunction in closure.hpp,
> which translates to the following requirement, for non-empty ring:
>
> - an open ring shall have at least three points (triangle)
> - a closed ring shall have at least four points  (triangle with first
> point repeated)
> <http://lists.boost.org/mailman/listinfo.cgi/geometry>
>

OK, thanks!