Re: [Boost-users] Boost.Geometry: Correct use of multi-part polygons

Subject: Re: [Boost-users] Boost.Geometry: Correct use of multi-part polygons
From: Barend Gehrels (barend_at_[hidden])
Date: 2013-02-20 15:36:59

Hi John,

On 20-2-2013 16:29, John Lilley wrote:
>
> Greetings, I am looking for some guidance around the correct creation 
> and use of multi-part polygons.  I am a bit confused about when to use 
> each of the following:
>
> ·Polygon with multiple rings
>
> ·Vector of polygons
>
> ·Polygon set
>
> ·Set/vector of polygons, each potentially containing multiple rings
>
> I gather from this example picture:
>
> http://geometrylibrary.geodan.nl/03__polygon__example_8cpp-example.html
>

Note that this documentation is VERY old. The correct docs are here:
http://www.boost.org/doc/libs/1_53_0/libs/geometry/doc/html/index.html

Examples like above are not included in the doc's, but they still exist 
in the SVN tree.

> That if one wants holes to be opened as a result of intersection, they 
> need to be rings as opposed to polygon sets or vectors of polygons?
>

Hmm, no... If two polygons, or multipolygons, do have the normal winding 
order, any holes are generated automatically. With normal I mean, w.r.t. 
to the specified clockwise order. If you use boost::geometry polygons, 
they are by default ordered clockwise.

In "normal" (multi)polygons, exterior rings are ordered clockwise, 
interior rings are ordered counter clockwise. This is an often-used 
convention.

If your polygons are not like that, just use boost::geometry::correct

> An example would be polygon(s) representing an island chain.  Some of 
> the islands are atolls and as such have a hole in the middle.  What 
> would a representation of this set of islands look like?
>

This is a multi polygon.

> And, what If I don't know if I have "real" rings (interior) vs just a 
> collection of polygons (like I got this from an external source that 
> doesn't have the same model).  If I just throw a pile of points at a 
> polygon and call clean(), will that magic everything up for me?
>

Not clean().  Where did you get that from?

Currently there is not a function like that, but dissolve in the 
extensions would target this functionality. Correct works only for the 
direction of rings, not to remove interior overlaps.

Regards, Barend