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

Mon Jun 14 16:39:22 BST 2021

Hi Gerd

Made some progress! I needed to disable the EPS_HP=2 tolerance along
the cut line otherwise there were correct cases that could be
interpreted incorrectly. I also needed not to chuck away very small
areas otherwise your test complained. The major problem was valid
shapes that had parts with zero area when cut and passed through the
same point on the cut-line as another line.

The main tiles in xxx.zip now build without error with order-by and no
-order-by, even with ShapeMergeFilter maxPoints = 65535.

However, the overview builder, when --no-order-by and --improve
-overview, fails splitting. The shape presented to ShapeSplitter has
obvious self-intersections (with --order-by, shape merge is disabled in
the overview builder)

Ticker

On Mon, 2021-06-14 at 08:28 +0000, Gerd Petermann wrote:
> Hi Ticker,
> 
> I am still testing with the files in 
> https://files.mkgmap.org.uk/download/507/xxx.zip
> 
> I applied the attached patch to enable massive merging and simulating
> possible splits and your splitShapeFix_10_lowRes.patch and I see
> several error messages.
> The previously attached example comes from xxx\63240004.osm.pbf
> 
> My options:
> --output-dir=e:\ld  --gmapi --generate-sea=multipolygon,floodblocker 
> --preserve-element-order --order-by-decreasing-area
> --allow-reverse-merge   --improve-overview --min-size-polygon=2
> -c e:\xxx\template.args
> 
> 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, 14. Juni 2021 10:19
> An: Development list for mkgmap
> Betreff: Re: [mkgmap-dev] low-res-opt branch: error message from
> ShapeSplitter for self-intersecting multipolygon
> 
> Hi Gerd
> 
> Any form of self-intersection might cause problems to ShapeSplitter.
> Not sure what you mean by "connected hole visited.." - I presume
> something like the outer edge of the shape is clockwise, with a cut
> leading to a hole that is also clockwise; for this to happen the
> in/out
> lines from the edge to the hole conceptually cross.
> 
> I'm just running low-res-opt on BritishIsles with ShapeMergeFilter
> maxPoints=65535 to see if I can get a problem to work on. What is
> your
> test area?
> 
> Also, the latest development versions of the suspect code areas are
> in
> different branches. Should I attempt to have a composite with the
> faster-mp bits of MP handling and low-res-opt version of
> ShapeMergeFilter?
> 
> Your attached example looks OK and I wouldn't have expected it to
> cause
> any difficulties for the later versions of ShapeSplitter. A possible
> difficulty might be an almost horizontal line close to the in/out
> lines
> to the lower shape. I hope my attempt to allow a bit of tolerance
> where
> calculated points on the cut-line end up 1/2 HP units the wrong way
> round isn't causing other problems. Do you have this as an example I
> can test?
> 
> Ticker
> 
> 
> On Sun, 2021-06-13 at 14:17 +0000, Gerd Petermann wrote:
> > Hi Ticker,
> > 
> > OK, my current understanding is that ShapeSplitter may fail to
> > split
> > 8-like shapes or shapes where the connected hole is visited in the
> > "wrong" direction. Those problem cases are possibly created by
> > 1) MultipolygonRelation.joinways()
> > 2) MultipolygonCutter
> > 3) ShapeMergeFilter (maybe)
> > 
> > Do you see a way to avoid them in those routines? Attached is
> > another
> > example that doesn't work.
> > 
> > Gerd
> 
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