Yes, correct.On 20 June 2013 15:18, TONGARI <tongari95@gmail.com> wrote:

> 2013/6/20 Mateusz Loskot <mateusz@loskot.net>

>> 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?

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)

OK, thanks!