[mkgmap-dev] shapes with holes

Tue Jan 29 19:28:19 GMT 2013

>
> Some notes:
> I don't understand why you calculate the distance between a point and a
> segment and not between two points?
> In the sea generator, the outer polygon is a big bbox. If I connect the
> points
> all inner shapes are connected to the corners of the bbox (if I don't care
> about
> visibility). My idea is that if I create the connection with the shortest
> line
> it is impossible that another shape is crossed.

Yes, that should work - good.

>
>
> WanMil wrote
>> Of course you can create your own distance calculation without the
>> spherical part. It is required in the Quadtree because it checks if the
>> checked point is more far away than a given gap. Another reason is that
>> one Garmin map unit does not have a constant length. Was that your
>> problem with Line2D.ptSegDistSq()?
>
> I really don't know. I find completely different nodes for the connections
> when I use the simple calculation. I'll sleep again about this ;-)
>
>
> WanMil wrote
>> Your cut procedure still requires that all parts of mkgmap that use the
>> resulting ways do not use the java.awt.geom.Area class. So another hard
>> second part of the implementation is to find an adequate replacement for
>> this.
>
> Yes, I have to find a way to split the resulting polygon into parts with
> less than
> 250 nodes. I don't yet have a good idea how to do this.

I think it is not a good idea to focus on the 250 nodes limit. This 
creates an implicit rule about the further processing after the 
multipolygon has been calculated. And that will fail at some time in 
future. (e.g. the java Area object is also used in the AreaClipper)

Without changing the splitting with the java Area object you might also 
change the cutting procedure a little bit. Instead of completely 
surrounding the inner polygons you might create singular polygons 
without holes by partly surrounding the inner polygons. The algorithm 
should not too complex if you have the information about the shortest 
distances. I have implemented a similar algorithm a long time ago when I 
tried a similar approach. But I abandoned because the calculations of 
the shortest distances took much too long (I used a visibility graph 
calculated by an external library).

This change makes it possible to keep the java Area calculations until 
we find a good replacement.

WanMil

>
> Ciao,
> Gerd

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