[mkgmap-dev] Multipolygons and basic question on polygons with holes

Sat Jan 2 14:18:15 GMT 2010

Am 02.01.2010 00:19, schrieb Steve Ratcliffe:
> [...]
> Some observations
>
> 1. The sea polygon is probably cut up into small enough pieces first,
> before the "cuts" to the hole are made.
> 2. Because we do things the other way round, we make the cut a slight
> wedge shape, as you cannot send polygons that have touching segments
> to the polygon splitting code.  This is what makes things look bad.
> 3. If you split a sea polygon with a line that goes "through" (around)
> a hole then you don't need to add a cut.
> 4. Its probably easier to do it by hand, but that might just reflect
> my lack of knowledge of GIS algorithms in this area...
>    
I think it should not be too hard to create an algorithm that decomposes 
a polygon with holes into polygons without holes. I googled for "polygon 
decomposition" and found a paper describing an algorithm to decompose a 
polygon with holes into convex polygons, but this seems to be more than 
we need (and too complex computationally). Right now I have some time so 
I will have a look at it.

Best wishes
Christian