[mkgmap-dev] --generate-sea: Flooded islands and MultiPolygonRelation

Thu Nov 5 16:33:16 GMT 2009

Hello,

I found several posible problems why MuliPolygonRelation produces flooded
islands.

1. Inner ways are added to outer in order they come from osm data. If you
add natural=sea to lines instead of polygons you will get the idea why this
is possible problem. Should allways first add the closest inner way to outer
way and repeat until there no more inner ways. This will prevent lines
running over allready added landmass and self-intersecting outer polygon.

2. Winding of inner ways. If inner way has different winding (i.e.
counter-clockwise) to the outer way, you will get self-intersecting polygon
as InsertPoints in MultiPolygonRelation does not take care of this.
Reversing the coastline way should hopefully have no other impact.

3. Adding the last point of inner way (which should (?) be the same as the
first)

I attached modified MultiPolygonRelation.java (from r1340), however I have
very very limited Java skills so this needs a complete rework as it is
painfully (unusably) slow (i.e. calculating the closest way over and over
again). My tests with this modification reduced number of flooded islands
(i.e. on Croatian coastline) at least when zoomed in.

Some islands still get flooded when zooming out. I am not able to find out
why this happens, but some islands notriously like to flood more than others
(i.e. OSM id 8838012) .. Actually, usually it is a whole chain that gets
'cut off' from the land-mass. If one island gets 'flooded', all islands
linked to it also get flooded, i.e. polygon is broken... This might or might
not have to do something with the polygon splitter. I think the proper way
of solving this might be the polygon splitter as in that case we might be
able to add points without delta shift which will reduce artifacts (linking
lines).

Or maybe it would be much easier to simply add the islands as i.e.
natural=land without subtracting them from the sea polygon and draw over the
sea  (via .typ polygon draw-order) ?

Hope this is useful and somebody more skilled will be able to address above
issues.

Thanks,
Ivan
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