[mkgmap-dev] Problems with sea in overview map

Thu May 20 10:07:48 BST 2021

Hi Ticker,

I've attached a test case that shows how the order is important. Depending on the order of shapes to be merged the resulting shape sometimes is self-intersecting, sometimes it isn't.
I've not yet understood what the trigger is.

Gerd

________________________________________
Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Gerd Petermann <gpetermann_muenchen at hotmail.com>
Gesendet: Mittwoch, 19. Mai 2021 18:14
An: Development list for mkgmap
Betreff: Re: [mkgmap-dev] Problems with sea in overview map

Hi Ticker,

yes, I think ShapeMerger can always merge. There seems to be no need to look at the number of points. Maybe 24 is an exception.

Sorting has an effect, but not always to the better.

The merger is already used with rounded coord when the shapes for the overview map are merged.

Attached is a patch that shows what I am playing with. Will continue tomorrow...

Gerd

________________________________________
Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Ticker Berkin <rwb-mkgmap at jagit.co.uk>
Gesendet: Mittwoch, 19. Mai 2021 17:41
An: Development list for mkgmap
Betreff: Re: [mkgmap-dev] Problems with sea in overview map

Hi Gerd

I still haven't deciphered the complicated bits of ShapeMergeFilter.

Could it do the testing of mergeable candidates with rounded-to
-resolution points, then, if merging, convert all the points.

Regardless, the limit checks against MAX_POINT_IN_ELEMENT could use
PredictFilterPoints(). It isn't a major problem if this limit is
exceeded because filtering might take it below again and, if it
doesn't, PolygonSplitter will deal with it.

Regarding ordering, maybe if merge succeeds for some potential shapes,
they should all be done.

Intersecting polygons from rounding of concave shapes with a long
diagonal is slightly insoluble. Maybe something like WrongAngleFixer is
going to be needed. Multipolygon cuts that are vertical or horizontal
might be less of a problem.

I'm considering the effect of different orders of filters, such that if
a small hole becomes a point and cuts to it, after merging, become a
series of spikes going into the shape, removeObsoletePoints removes it.

Ticker

On Wed, 2021-05-19 at 14:12 +0000, Gerd Petermann wrote:
> Hi Ticker,
>
> I think ShapeMergeFilter could be part of the problem. It sometimes
> finds strange ways to merge shapes, esp. when many small parts can be
> merged. Maybe this can be improved by sorting so that it merges from
> top to bottom.
>
> Gerd
>
> ________________________________________
> Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag
> von Ticker Berkin <rwb-mkgmap at jagit.co.uk>
> Gesendet: Mittwoch, 19. Mai 2021 14:14
> An: Development list for mkgmap
> Betreff: Re: [mkgmap-dev] Problems with sea in overview map
>
> Hi Gerd
>
> I've just been experimenting and looking at shapes and rounding-to
> -low
> -resolution can easily cause self-intersections.
>
> It isn't just a problem with sea, or even with multipolygon hole
> cutting. GPSMapEdit reports over 1000 in my test area. This number
> reduces only slightly when I disable DP for polygons. Generally these
> are not noticeable - a little bit of wood missing when zoomed out
> isn't
> obvious, but same is not true of sea.
>
> Ticker
>
> On Wed, 2021-05-19 at 11:43 +0000, Gerd Petermann wrote:
> > Hi Ticker,
> >
> > no need to guess, I use GpxCreator in various places to visualize
> > input and output of filters. I load the gpx files into JOSM and
> > maybe
> > convert to Data layer to understand what happens, e.g. where the
> > start /end point is.
> > Reg. self-intersection:
> > Think of a merged sea shape with several islands in it. The
> > rounding
> > may result in only three or four different nodes, visitied in a
> > more
> > or less random order while the correct shape would be a triangle or
> > rectangle.
> > Not sure if this is only a problem with sea, probably not.
> >
> > Feel free to try other orders of the filters, or maybe different
> > algos to set the preserved flag. There is definitely room for
> > improvements. I've played with this very often and sometimes one
> > area
> > is improved and another is worsened.
> >
> > Gerd
>
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