[mkgmap-dev] Gaps in surfaces

Fri Sep 29 10:03:19 BST 2023

Hi

I was thinking that gaps like this could often happen when adjacent polygons share the points
along the common boundary, but, during the various point-removal optimisations, esp. Douglas
Peucker, different points along this boundary are chosen to represent the 2 polygons.

Regarding LineClipperTest, I don't have the facilities to look at this until next week.

Ticker

On Fri, 2023-09-29 at 06:57 +0000, Gerd Petermann wrote:
> Hi all,
> 
> I've found out that this problem is caused by a flaw in the "high precision" code. Under
> special conditions, two points with almost identical coordinates are internally represented
> with very different values. There is a possible overflow in the delta values, the value +32
> should not occur as it cannot be represented with 6 bits, but the calculations produce this
> value.
> I think I have an ugly fix for this but the resulting map still shows (smaller) gaps and a
> unit test needs corrections, so there is more to do.
> Attached is a patch that checks for this overflow. Work in progress and maybe causes trouble
> in other areas, e.g. in South America where we have negative lat/lon values.
> 
> @Ticker: The unit test für LineClipperTest fails. I am not sure if only the test has to be
> changed (how) or if the code in LineClipper can be improved. I seem to remember that you
> suggested to use
> the code in ShapeSplitter instead.
> 
> ________________________________________
> Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Gerd Petermann
> <gpetermann_muenchen at hotmail.com>
> Gesendet: Dienstag, 26. September 2023 19:58
> An: Development list for mkgmap
> Betreff: Re: [mkgmap-dev] Gaps in surfaces
> 
> Hi Michael,
> 
> okay, I can reproduce the problem. I assume it is caused by the code which subtracts the inner
> area from the multipolygon area.
> No idea if this can be fixed easily.
> 
> Gerd
> 
> ________________________________________
> Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Forstner Michael
> <forstner-m at a1.net>
> Gesendet: Dienstag, 26. September 2023 19:44
> An: 'Development list for mkgmap'
> Betreff: Re: [mkgmap-dev] Gaps in surfaces
> 
> Hello Gerd,
> 
> here is the area: https://www.openstreetmap.org/#map=19/48.57117/13.96920 I
> understand that it is necessary to make rounding. If a point of two surfaces
> is equal in OSM, the point should also be equal in Garmin.
> 
> Thank you!
> 
> Best regards,
> Michael
> 
> 
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