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

Subject: Re: [geometry] Why closure?
From: TONGARI (tongari95_at_[hidden])
Date: 2013-06-20 10:18:57

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?