Hi Gerd I'm getting worried by rounding-to-resolution and how often it creates self-intersection. When doing the shapeSplitter.splitShape, we found that, unless operating with high-precision points (ie there were problems at 24bit precision), it could fail because of self-intersection. The polygon points limit filters got around this by tricky backtracking. This option isn't available when the overview map is being generated as the problem points have been frozen at very low res. Also thinking that shapeMerge might need to operate at highPrecision. I'm just trying do work out if the different levels have their own copy of shapes pre-filtering and how this effects what shapeMerge does as makeMapAreas steps through the levels. Ticker On Thu, 2021-05-20 at 09:07 +0000, Gerd Petermann wrote:
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@lists.mkgmap.org.uk> im Auftrag von Gerd Petermann <gpetermann_muenchen@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@lists.mkgmap.org.uk> im Auftrag von Ticker Berkin <rwb-mkgmap@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@lists.mkgmap.org.uk> im Auftrag von Ticker Berkin <rwb-mkgmap@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
_______________________________________________ mkgmap-dev mailing list mkgmap-dev@lists.mkgmap.org.uk https://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev _______________________________________________ mkgmap-dev mailing list mkgmap-dev@lists.mkgmap.org.uk https://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
mkgmap-dev mailing list mkgmap-dev@lists.mkgmap.org.uk https://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev _______________________________________________ mkgmap-dev mailing list mkgmap-dev@lists.mkgmap.org.uk https://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev