Geometry Boost List: [ggl] Problems with the difference between two polygons

Subject: [ggl] Problems with the difference between two polygons
From: Barend Gehrels (barend)
Date: 2011-07-17 09:05:17

Hi Enrico,

On 16-7-2011 19:32, Enrico Leoni wrote:
> Hi, in the example below I am having problems.

Thanks for the report.

> s1, the difference between pa and pb, should have three polygons (each
> with an outer ring) but one is missing (this also happens with
> ttmath).

I get different results. For me, the first result s1 is OK. However, if 
I then take the other difference from p-s1, I get indeed 2 of expected 
3. I will have a closer look.

> Furthermore s1 seems to be self-intersecting but it shouldn't.

Right, it considers duplicate points as self-intersecting. I will solve 
that. In the meantime, if you add "unique" after the first call to 
difference, they are removed and it is OK. I could have done that in the 
results of the intersections, but it is the intention that duplicate 
points are not generated...

Anyway, it will be solved.

> I'm using the last trunk revision and Visual Studio 2010.

It might also be good to call boost::geometry::correct to your input 
polygons, because they are not closed. But that does not solve your 
second problem (maybe it solves the first).

Regards, Barend