On 20 June 2013 10:26, TONGARI <tongari95@gmail.com> 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?