[mkgmap-dev] Gaps in surfaces

Thu Oct 5 16:37:39 BST 2023

Hi Gerd

I've changed the way mapUnit and highPrec Coords are calculated slightly so that the exact 1/2
unit boundary values are consistent between negative and positive. This gets rid of my couple of
abnormalities with a -32 delta. The old method added or subtracted 1/2 to make ((int)double) do
rounding, which assigns the exact 1/2 to the larger abs value.

toHighPrec(), as in yesterdays change, keeps its bounds consistent with the mapUnit it is
splitting up but is now coded in a similar way to toMapUnit.

I think rounding to get MapUnits is a mistake but changing it now could have drastic
consequences, it should have been floorToNegInfinity, giving the nice cyclic scale -128..+127 <<
24. As it is, we get -128..+128 << 24. 

Equator & Grenwhich Meridian behave well, but I've always had doubts about behaviour around 128

Hope this makes sense - patch attached

Sorry, haven't thought about the actual problem with the gaps yet.

Ticker

On Thu, 2023-10-05 at 07:30 +0000, Gerd Petermann wrote:
> Hi Ticker,
> 
> reg. the change in toHighPrec() :  I must confess that I never doubted why positive values
> should be rounded differently to negative values.
> This is also done in Utils.toMapUnit() and I just adapted that code. If you still want to
> change that I think Math.rint() would get you closer to the original implementation.
> Question is if or why this difference in rounding is/was needed. There should be visible
> effects near equator and at Greenwhich or at 180°.
> 
> Anyhow, I don't think this is the solution for the gap problem which is caused by further
> conversions from highprec int to double and back.
> An alternative solution for that problem would be the attached patch which makes use of the
> coord pool.
> No idea which one has more effects on performance.
> 
> 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, 4. Oktober 2023 15:57
> An: Development list for mkgmap
> Betreff: Re: [mkgmap-dev] Gaps in surfaces
> 
> 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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