[mkgmap-dev] [mp: PATCH] Multipolygon handling - decomposed polygons

Sat Jan 9 09:13:24 GMT 2010


On 09.01.2010 10:08, WanMil wrote:
> Am 09.01.2010 00:05, schrieb Johann Gail:
>    
>>      
>>> Changes:
>>> * Polygons with inner polygons are now divided into several polygons.
>>> It's much faster and the PolygonSplitter cannot destroy them any more.
>>>
>>>        
>> Hello WanMil,
>>
>> this change could have some other side effects. (As observed and
>> described in another thread)
>> If the polygons gets small enough, then the SizeFilter will remove parts
>> of them. I.e. if a big forrest gets cut into smaller pieces, some of
>> them may be small enough to be catched by the SizeFilter.
>>
>> Is it a must for splitting them up? Or was it just done to serve the
>> PolygonSplitter?
>>
>> Regards,
>> Johann
>>      
> Johann,
>
> the old findCpa method was quite slow. Its  complexity is O(n^3) at its
> worsty worst case and usually O(n^2).
>
> Therefore I started to implement the new algorithm without the findCpa
> method. The current implementation may have problems with small polygons
> but it should be possible to perform some remerging.
>
> Another idea is to add some specific tags by the mulitpolygon algorithm
> that link to the other pieces of the formerly big forrest. This could be
> evaluated by the SizeFilter?
>    
That would be good. Another thing would be to implement a variable 
setting for Multipolygon inner cutouts based on type (put it into the 
style-file). I think on resolution 24 and 23 it is clear that we want to 
have multipolygon inner cutout. In 22 I am not to sure. From 21 down I 
would prefer only lakes and sea to make use of inner, but all other 
objects to be displayed as if there was only the outer. That should be 
added to style-file like "mp 22" meaning 22 is the last resolution were 
inner polygons are cut out.
> WanMil
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