[OSM-dev] Advice sought on polygon-with-hole drawing

Cartinus cartinus at xs4all.nl
Thu Mar 13 23:51:30 GMT 2008


On Friday 14 March 2008 00:05:49 Frederik Ramm wrote:
> for each way with area tagging
>    if way is member of multipoly relation
>       if role is "inner"
>          ignore way
>       else
>          combine way and all "inner" members of relation
>          generate drawing instruction for combined path
>       end
>    else
>       generate drawing instruction for simple path
>    end
> end
>
> The tags that determine how the way is drawn would always be those of
> the "outer" way. Those of the "inner" way(s) would be completely ignored,
> unless they, in turn, were "outer" ways in another relation (or unless
> a non-area rule would apply to them). Tags on the relation would be
> ignored as well.

If I understand this correctly, then for a forested island in a lake you'd 
need to have the island twice in the osm data. Once for the hole in the lake 
and once for the forest on the island.

How about:
for each way with area tagging
   if way is member of multipoly relation
      if role is "inner"
         if tags "inner" not equal tags on "outer"
            generate drawing instruction for simple path
         else
            ignore way
         end
      else
         combine way and all "inner" members of relation
         generate drawing instruction for combined path 
      end
   else
      generate drawing instruction for simple path
   end
end

At least this is what I understood the discussion about multipolygons last 
week tried to achieve.

-- 
m.v.g.,
Cartinus




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