[mkgmap-dev] Multipolygon-Relation checks in mkgmap

Tue Jan 31 17:35:11 GMT 2017

Hi Gerd

ShapeSplitter only works on a single-path shape. Considering the act of
splitting a shape with a single line, you are left with a list of lines
that start and end on the divider. These lines don't cross each other
unless the polygon was self-intersecting which is not allowed. 

Then, just considering one side of the divider, the complexities of how
the other side contorted to define multiple holes in a shape, and
shapes in a hole can all be ignored. Lines going one way relating to
the dividing line define the outer edge of a shape, and if the shape
has direct holes, they must go the other way.

I think the above is true for any non-self-intersecting polygon.

ShapeSplitter has to resort to using the sign of the area of a shape
defined by one of these lines and the divider only when multiple lines
start and end at the same point.

It can handle a shape and hole going through a single dividing point,
so can easily cope with dividing a polygon where there is a single cut
to an inner hole. Without self-intersection, if the dominant direction
of the polygon is, say, anti-clockwise, then the hole must be
clockwise.

Does this make sense?

Ticker

On Tue, 2017-01-31 at 15:41 +0000, Gerd Petermann wrote:
> Hi Ticker,
> 
> thanks, will look again later.
> I am working on a patch for the MultiPolygonCutter. It works without
> java.awt.geom.Area, the basic idea is to connect shapes
> where they are close to each other. I've tried that in 2012, maybe I
> get it working now.
> One problem that I've noticed:
> The code in ShapeSplitter seems to consider some shapes as self
> -intersecting when the connected inner ways are clockwise (or not
> clockwise,
> not sure which one causes trouble). I assume you coded this because
> the shapes produced by java.awt.geom.Area are like that, outer shape
> is
> one direction and inner shapes are in the other direction. I have
> some problems with this behaviour, is it needed ?
> For now I found a work around by reversing the ways so that they are
> ordered. Attached is a patch with what I coded so far, open problems
> are mp-rels at tile boundaries and performance. Be careful, the code
> contains lots of GPXCreator statements and other debug code.
> 
> Gerd
> 
> ________________________________________
> Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag
> von Ticker Berkin <rwb-mkgmap at jagit.co.uk>
> Gesendet: Dienstag, 31. Januar 2017 16:26:41
> An: mkgmap-dev at lists.mkgmap.org.uk
> Betreff: Re: [mkgmap-dev] Multipolygon-Relation checks in mkgmap
> 
> Hi Gerd
> 
> I've think I've finished making all the changes I want to do at the
> moment - It seems to work nicely. Sometime can you merge the branch
> back if you are happy with it.
> 
> Thanks
> Ticker
> 
> 
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