[mkgmap-dev] low-res-opt branch: error message from ShapeSplitter for self-intersecting multipolygon

Tue Jun 8 08:43:49 BST 2021

Hi Ticker,

my understanding is that Garmin software has no problems with this kind of self-intersection.
In this particular case the problem could be solved in method MultipolygonRelation.joinWays(), as 4 ways are connected at the same point, but self-intersecting shapes are also possible as simple ways.  While they are wrong for OSM they should be allowed for MapShape since Garmin allows them, too.

A possible way to fix those shapes is something like this:
                        Path2D.Double path = Java2DConverter.createPath2D(polygonPoints);
                        path.setWindingRule(Path2D.WIND_EVEN_ODD);
                        List<List<Coord>> shapes = Java2DConverter.areaToShapes(new java.awt.geom.Area(path));
So, maybe ShapeSplitter could do this with the input shape in case of error and try again with shapes?

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, 8. Juni 2021 08:54
An: mkgmap-dev at lists.mkgmap.org.uk
Betreff: Re: [mkgmap-dev] low-res-opt branch: error message from ShapeSplitter for self-intersecting multipolygon

Hi Gerd

The shape is a figure-of-8 so shapesSplitter needs to give some error.

If this isn't an original OSM shape, it points to some problem with
earlier processing, probably shape-merging. Although it might render
(could be device dependent), further processing (eg overview merging)
could come to grief.

Ticker

On 07/06/2021 10:24, Gerd Petermann wrote:
> Hi Ticker,
>
> okay, no problem, I'll work on the faster-mp branch in the mean time
> Maybe I'll merge it first into the low-res-opt branch.
>
> Gerd
>
> ________________________________________
> Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Ticker Berkin <rwb-mkgmap at jagit.co.uk>
> Gesendet: Montag, 7. Juni 2021 11:23
> An: Development list for mkgmap
> Betreff: Re: [mkgmap-dev] low-res-opt branch: error message from ShapeSplitter for self-intersecting multipolygon
>
> Hi Gerd
>
> The shape I'm getting in my build has a major problem with a twist
> between the 2 parts. I'm having trouble reconciling it with the OSM
> map. I've attached the original shape trace.
>
> I agree that some of the messages do need tweaking, but, in this cases
> there is a significant problem.
>
> I'll consider the types of messages etc in the new version I'm working
> on.
>
> I'm away for a few days and won't be able to do much remotely.
>
> Ticker
>
> On Mon, 2021-06-07 at 07:22 +0000, Gerd Petermann wrote:
>> Hi Ticker,
>>
>> please check https://files.mkgmap.org.uk/download/512/63240024.o5m
>> with (unpatched) r4756 and these options --generate
>> -sea=multipolygon,floodblocker --preserve-element-order --order-by
>> -decreasing-area
>> --allow-reverse-merge
>>
>> Produces an error message
>> SCHWERWIEGEND (ShapeSplitter):
>> e:\osm_out_work\norway\20210511_095323\63240024.o5m: Vertical split
>> 36691968 failed on shape at
>> http://www.openstreetmap.org/?mlat=65.979056&mlon=12.308790&zoom=17 P
>> ossibly a self-intersecting polygon
>> for this invalid multipolygon :
>> https://www.openstreetmap.org/relation/5294624
>>
>> The result doesn't look wrong, so I think there should either be no
>> error message or we need code to remove those self-intersections or
>> ignore invalid MP like that?
>>
>> Gerd
>> _______________________________________________
>> mkgmap-dev mailing list
>> mkgmap-dev at lists.mkgmap.org.uk
>> https://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
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