Hi Gerd I've now looked at your example (mp-gap2.osm). With my toMapUnit/toHighPrec patch the problem almost disappears. I don't see any significant difference when I also apply your mp-cut-coord- pool.patch. I can't see a way of fixing these gaps/overlaps without disabling a lot of filter point-removal, maybe by setting Coord.preserved() and ensuring added points for polygon cutting are on both the inner and outer polygon, which I thought they were, with the algorithm cutting through the complete structure and then bits being merged later if possible. Ticker On Thu, 2023-10-05 at 16:37 +0100, Ticker Berkin wrote:
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@lists.mkgmap.org.uk> im Auftrag von Ticker Berkin <rwb-mkgmap@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@lists.mkgmap.org.uk> im Auftrag von Gerd Petermann <gpetermann_muenchen@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@lists.mkgmap.org.uk> im Auftrag von Ticker Berkin <rwb-mkgmap@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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