[mkgmap-dev] Gaps in surfaces

Wed Oct 4 14:57:33 BST 2023

Hi Gerd

I've been thinking about how to stop the small ranges of decimal degrees from generating
MapUnit/delta=+32 and MapUnit+1/delta=-32.

Without changing the MapUnit value, but truncating the highPrec calc, eg, in Coord.java

	private static int toHighPrec(double degrees) {
		final double CVT_HP = ((double)(1 << HIGH_PREC_BITS)) / 360;
		return (int)Math.floor(degrees * CVT_HP);
	}

this problem almost goes away, with deltas between -31 .. +32 except for 2 instances of delta of
-32 I get while building the Britain-and-ireland.osm.pbf. I need to work out why these are
happening.

Although it is unnatural to have this range rather than -32..+31, it doesn't matter. It probably
could be fixed by using Math.ceil instead or reversing where the delta goes.
getAlternativePositions() will generated delta values approx -48..+48. 

I don't get failures from LineClipperTest with this change.
I do get failures from GmapsuppTest, SimpleRouteTest & SortTest, but I think these are not
significant to this issue.

As far as the gap between areas is concerned, I haven't looked at this yet but I'll see if my
change has similar effects to your patch to Coord.

Ticker

On Wed, 2023-10-04 at 08:16 +0000, Gerd Petermann wrote:
> Hi Ticker,
> 
> I've uploaded the test case: https://files.mkgmap.org.uk/detail/564
> I had to modify the data a bit since the default style doesn't render natural=heath
> 
> Gerd
> 
> ________________________________________
> Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Gerd Petermann
> <gpetermann_muenchen at hotmail.com>
> Gesendet: Montag, 2. Oktober 2023 16:58
> An: Development list for mkgmap
> Betreff: Re: [mkgmap-dev] Gaps in surfaces
> 
> Hi Ticker,
> 
> the word overflow might be confusing. The problem is that we want to use only 6 bits for the
> delta, but we store values from -32 .. 32.
> The special case with Michael example is that one coord with such an extreme delta is used
> (after converting to double) in Area.subtract() and the returned coord is converted back but
> get's the other extreme. In the end, only the 24 bit value is written to the map, and that
> causes the gap.
> 
> 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, 2. Oktober 2023 15:55
> An: Development list for mkgmap
> Betreff: Re: [mkgmap-dev] Gaps in surfaces
> 
> Hi Gerd
> 
> Considering no_hp-overflow_alpha.patch:
> 
> It seems wrong to change a 24bit coordinate. Utils.toMapUnit() is well defined to do the
> expected rounding with the conversion.
> 
> The actual deltas are local to Coord.java and, apart from their use by
> getAlternativePositions,
> are just used to get back to the highPrec coord value.
> 
> The deltas are stored as byte, so the value of +32 causes no arithmetic problems generally.
> 
> getAlternativePositions(), however, should handle any complications with the +32, but it looks
> like it mostly does, bumping up or down the 24bit coord if the abs(deltas) are > 16. It it
> really possible that modLxxDelta can overflow a byte here?
> 
> Haven't looked at LineClipperTest yet.
> 
> 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.
> > 
> 
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