[mkgmap-dev] Problems with sea in overview map

Wed May 19 12:43:11 BST 2021

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

________________________________________
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 13:14
An: Development list for mkgmap
Betreff: Re: [mkgmap-dev] Problems with sea in overview map

Hi Gerd

I don't see that rounding can cause self-intersecting polygons - it
might produce sides that touch each other.

I imagine that DP can cause self-intersection and this is more likely
on shapes typical of the result of a hole-cut. I wasn't presuming it
would fix them.

RemoveObsoletePointsFilter should be able to remove the cut to a hole
that has become a point. However this is run after DP. Maybe swapping
the order would improve matters.

Ticker

On Wed, 2021-05-19 at 10:56 +0000, Gerd Petermann wrote:
> Hi Ticker,
>
> I think the self-intersections are a result of the rounding of
> polygons with holes. DP just cannot fix those.
>
> I don't want to "delay hole cutting per resolution". I want to remove
> the small holes. It doesn't matter if precomp-sea is used or not, it
> works for both.
>
> 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 12:46
> An: Development list for mkgmap
> Betreff: Re: [mkgmap-dev] Problems with sea in overview map
>
> Hi Gerd
>
> I often see other polygons becoming self-intersecting at lower
> resolutions (or at least GPSMapEdit does). If the DP filter is doing
> this then maybe this should be tacked first, with an alternate
> version
> for polygons - I have no idea how this would be implemented.
>
> Should shapeMergeFilter somehow consider each polygon as if its
> points
> are rounded to resolution, with adjacent duplicates removed. This
> should allow more merging and will be much less likely to be
> inhibited
> by the 256 point limit. Doing this, small holes/islands are likely to
> disappear.
>
> I don't like the idea of trying to delay hole cutting per resolution,
> and I don't see quite how it would work for pre-comp-sea
>
> Ticker
>
> On Wed, 2021-05-19 at 07:06 +0000, Gerd Petermann wrote:
> > Hi all,
> >
> > I've identified a few problems with the handling of natural=sea in
> > areas with lots of islands, e.g. coast of Norway. Thouands of
> > rather
> > small polygons are created, esp. when also natual=land is rendered.
> > 1. Typical tiles are rather large, they span several precompiled
> > sea
> > tiles if --precomp-sea=sea.zip is used. As of now, the polygons
> > from
> > different sea-tiles are never merged. This might be improved to
> > reduce file size and possibly also improve rendering
> > 2. Even at resolution 12 the curent code distributes the thousands
> > of
> > tiles into multiple sub divisions before ShapeMerger is executed
> > for
> > each subdiv. Shapes (polygons) are only merged when the number of
> > points for one shape stays inside the IMG limit of 256 points, so
> > the
> > merger cannot do much.
> > 3. Douglas-Peucker (DP) filter is used for the merged shapes which
> > contain holes. This gives weird results because the invisible lines
> > which connect the holes are causing heavy self-intersections at low
> > resolutions. I think this is one reason for white triangles in the
> > sea.
> >
> > I am experimenting with these ideas:
> > - At low resolutions the ShapeMerger could merge more so that
> > shapes
> > with more than > 256 points are produced, assuming that DP filter
> > will remove many of them. Not sure if this works with shapes that
> > have holes.
> > - At low resolutions it is likely/wanted that small islands
> > disappear, so we should remove those first, together with the lines
> > that connect them with the outer ring.
> > - The simplified sea polygons without holes should be merged as
> > much
> > as possible before using any filter, maybe DP should be executed
> > first on the merged shapes so that fever subdivs are created.
> >
> > Each method has its own effects and I'm searching for the best
> > combination and order.
> >
> > A lot of this is only possible because we now have Tickers code
> > that
> > allows multipolygon splitting without using
> > java.awt.geom.Area.intersect() . Tickers code in ShapeSplitter
> > seems
> > to be able to handle the holes properly.
> >
> > Gerd
> >
> >
> > _______________________________________________
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> > mkgmap-dev at lists.mkgmap.org.uk
> > https://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
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