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

Subject: Re: [geometry] Why closure?
From: Barend Gehrels (barend_at_[hidden])
Date: 2013-06-20 17:53:40

Hi,

On 20-6-2013 18:30, TONGARI wrote:
> 2013/6/21 Barend Gehrels <barend_at_[hidden] <mailto:barend_at_[hidden]>>
>
>     Hi,
>
>
>     On 20-6-2013 11:26, TONGARI wrote:
>
>
>         I'm not quite understand why we need traits::closure for Ring
>         & Polygon, aren't these always closed?
>         What should I expect if the closure is open? Should I see Ring
>         as Linestring then?
>
>
>     No (about the last question).
>
>     Apparently the doc about closure should be updated. However, this
>     page explains it clearly:
>
>     http://www.boost.org/doc/libs/1_53_0/libs/geometry/doc/html/geometry/reference/enumerations/closure_selector.html
>
>
> Yah, I somewhat missed that page, seems it does contain the info I needed.
> Since I'm writing my 2d graphics lib, I always think the lib should 
> close it anyway for drawing, but I don't know how it matters in the 
> context of geometry operations...
>

Sure, basically every algorithm  needs polygons to be closed. Therefore 
we have the closeable_view which closes them automatically. Likewise we 
have the reversible_view which walks through the points in opposite 
direction, if the rings are counter clockwise. So basically (in a system 
where x is going up) we consider, internally, with help of these two 
views, all polygons as closed and clockwise.

Regards, Barend