[mkgmap-dev] White stripes in the sea

Wed Jan 2 20:35:01 GMT 2013

> Hi WanMil,
>
> WanMil wrote
>> So as a result:
>> When you want to remove the sea artefacts you must improve the
>> multipolygon cutting.
>
> just to make sure:
> I already see the small stripes in the precompiled sea tile when I open it
> in JOSM:
> (after converting to xml format)
>
> sea.png <http://gis.19327.n5.nabble.com/file/n5742357/sea.png>

What you see is the result of the multipolygon algorithm that cuts the 
sea areas into distinct polygons to cut out the land area. The white 
stripes are the polygon lines. But there is no space between the 
polygons. So obviously that should not the problem. The problem is that 
Garmins draw algorithm seems to create the white lines.

>
> I think the multipolygons algorithm just adds these ways, it doesn't combine
> them to one
> area which is then "destroyed" by the polygon split algorithm.

It's the mp algorithm task to cut the big sea polygon with holes into 
several polygons without holes. Holes in polygons are not supported by 
the Garmin format (as far as we know?).

> So, the question for me is :
> Could we change the PrecompSeaGenerator or would it help to recombine the
> areas after reading
> the preocompiled sea files?

It would be possible to recombine some polygons that are cut too much 
due to the non optimal mp algorithm. But I guess it is better to improve 
the mp algorithm itself (as I described in my post).

Once again: It does not make sense to improve the PrecompSeaGenerator 
because the main task it does is to calculate the multipolygons. And so 
the only thing that could be improved is the mp algorithm itself.

WanMil