[mkgmap-dev] Squaring off of land outlines

Wed Jan 29 09:50:52 GMT 2020

Hi Mike,

the size filter "looks" at the polygons which are to be added to the map. Those polygons are the result of a long chain of split and merge processes followed by rounding which - sometimes - produce rather small shapes. The sea multipolygon is split into parts of no more than 255 points. So, the more detailed the mapping of the coastline the smaller those parts.
This process is not optimal, see also the todo list (1).
I've done a few tests with the default option (--min-size-polygon=8) and default style with the options given in examples/sample.cfg and I think most of the dropped elements at low resolutions should have been kept. For small map tiles like yours even the background (0x4b) polygon is dropped.
The effect of the filter reg. img size is rather small thus it also has nearly no effect on throughput times.

My suggestion would be to change the default to 0 for the --min-size-polygon option. Suggested reasonable values should be in the range of 0-8 and maybe print a warning if --min-size-polygon is used with a value > 8.

Gerd
(1) http://www.mkgmap.org.uk/dev/todo

________________________________________
Von: Mike Baggaley <mike at tvage.co.uk>
Gesendet: Mittwoch, 29. Januar 2020 10:22
An: 'Gerd Petermann'; 'Felix Hartmann'; 'Development list for mkgmap'
Betreff: RE: [mkgmap-dev] Squaring off of land outlines

In the case where this is causing squaring off of the sea polygon, the
coastline is not usually small even at the low resolution. So why is the
size filter discarding it?  Is it because the object is crossing a tile
boundary and the part within a tile is small? If that is the case, it would
make sense to not discard any object that is touching the tile boundary.

Regards,
Mike

-----Original Message-----
From: Gerd Petermann [mailto:gpetermann_muenchen at hotmail.com]
Sent: 29 January 2020 01:20
To: Felix Hartmann <extremecarver at gmail.com>; Development list for mkgmap
<mkgmap-dev at lists.mkgmap.org.uk>
Subject: Re: [mkgmap-dev] Squaring off of land outlines

Hi all,

the default style adds buildings only with resolution 24. The size filter is
disabled at this resolution. The area_size() style function allows to filter
small polygons at this resolution.
The size filter compares the width or height of the bounding box of the
object AFTER all points where rounded to the given resolution.
The rounding can produce a bbox which is smaller than the original object
and thus produce the gaps in sea. I am pretty sure that you will also see
those gaps with huge natural=wood or natural=water multipolygons if they are
mapped in great detail when the size filter is used with default value
(--min-size-polygon=8)
I think this default is too high and the documentation even suggests to use
a higher value.

Gerd

________________________________________
Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von
Felix Hartmann <extremecarver at gmail.com>
Gesendet: Dienstag, 28. Januar 2020 23:52
An: Development list for mkgmap
Betreff: Re: [mkgmap-dev] Squaring off of land outlines

Performance of map on device is drastically improved you you kick out those
tiny polygons. Especially in cities where people have detail mapped all
buildings. If you don't have building=yes or building=* in your style - then
you can likely live with the filter disabled (not sure how much bigger the
map data gets).

On Tue, 28 Jan 2020 at 21:41, Gerd Petermann
<gpetermann_muenchen at hotmail.com<mailto:gpetermann_muenchen at hotmail.com>>
wrote:
Hi Mike,

"sea" is build from shapes (= polygons).
The size filter is not explained in the style manual, but in the options
--min-size-polygon and --polygon-size-limits.
Unfortunately the filter works a bit unpredictable at low resolutions, so in
my eyes it is a good idea to disable.
I've never fully understood the purpose of this filter. I think it should
improve performance, but it seems to decrease quality even more.

You may play with --min-size-polygon=0 to see what happens when the filter
is disabled.

Gerd

________________________________________
Von: Mike Baggaley <mike at tvage.co.uk<mailto:mike at tvage.co.uk>>
Gesendet: Dienstag, 28. Januar 2020 21:21
An: 'Gerd Petermann'; 'Development list for mkgmap'
Betreff: RE: [mkgmap-dev] Squaring off of land outlines

Brilliant! A big improvement! It still doesn't exactly match the land
polygon, but that doesn't matter for my purposes. I suggest changing the to
do list so it says use the same algorithm for shapes, lines and sea. I also
suggest adding a sentence to the style manual in the explanation of
mkgmap:skipSizeFilter - the description doesn't explain the purpose or what
the effect is of passing the size filter.

Thanks for your assistance,
Mike

-----Original Message-----
From: Gerd Petermann
[mailto:gpetermann_muenchen at hotmail.com<mailto:gpetermann_muenchen at hotmail.c
om>]
Sent: 28 January 2020 19:03
To: Mike Baggaley <mike at tvage.co.uk<mailto:mike at tvage.co.uk>>
Subject: AW: [mkgmap-dev] Squaring off of land outlines

Hi Mike,

some small rather small sea polygons are filter by the size filter.
Change the statememnt for sea polygons to this:
natural=sea {add mkgmap:skipSizeFilter=true; name '${name}'} [0x32
resolution 14]

See also the corresponding rule in the default style:
natural=sea {add mkgmap:skipSizeFilter=true; set mkgmap:drawLevel=2} [0x32
resolution 10]

Gerd

________________________________________
Von: Mike Baggaley <mike at tvage.co.uk<mailto:mike at tvage.co.uk>>
Gesendet: Dienstag, 28. Januar 2020 19:39
An: 'Gerd Petermann'
Betreff: RE: [mkgmap-dev] Squaring off of land outlines

Thanks Gerd. I has been considering trying to find the problem myself, but
I'm not familiar with the writing sections of code, and they are complicated
to get to grips with.

Cheers,
Mike

-----Original Message-----
From: Gerd Petermann
[mailto:gpetermann_muenchen at hotmail.com<mailto:gpetermann_muenchen at hotmail.c
om>]
Sent: 28 January 2020 16:16
To: Mike Baggaley <mike at tvage.co.uk<mailto:mike at tvage.co.uk>>; 'Development
list for mkgmap'
<mkgmap-dev at lists.mkgmap.org.uk<mailto:mkgmap-dev at lists.mkgmap.org.uk>>
Subject: AW: [mkgmap-dev] Squaring off of land outlines

Hi Mike,

okay, I'll have a closer look now.

Gerd

________________________________________
Von: Mike Baggaley <mike at tvage.co.uk<mailto:mike at tvage.co.uk>>
Gesendet: Dienstag, 28. Januar 2020 16:51
An: 'Gerd Petermann'; 'Development list for mkgmap'
Betreff: RE: [mkgmap-dev] Squaring off of land outlines

Hi Gerd, the problem I have is not the one in the to do list. If I add the
polygon fill for natural=coastline, this does produce a different outline
than the one for the line, but both are reasonable approximations and
neither contains unexpected artefacts. The sea generation approximation is
wildly different and contains unwanted artefacts.  Please see the attached
image that contains land in brown, sea in blue and an area of white that is
not filled by either. I believe this is a bug in the sea generation code. I
would expect sea generation to use the same algorithm as for filled areas so
that holes are not left. I'm not too worried about the line being slightly
different.

Regards,
Mike

-----Original Message-----
From: Gerd Petermann
[mailto:gpetermann_muenchen at hotmail.com<mailto:gpetermann_muenchen at hotmail.c
om>]
Sent: 28 January 2020 14:59
To: Mike Baggaley <mike at tvage.co.uk<mailto:mike at tvage.co.uk>>; 'Development
list for mkgmap'
<mkgmap-dev at lists.mkgmap.org.uk<mailto:mkgmap-dev at lists.mkgmap.org.uk>>
Subject: AW: [mkgmap-dev] Squaring off of land outlines

Hi Mike,

See last point in the todo list: http://www.mkgmap.org.uk/dev/todo
I think what you see is exactly the result of the two different algos.
I can only suggest to omit the rendering of polygon outlines at resolutions
below 24.

________________________________________
Von: Mike Baggaley <mike at tvage.co.uk<mailto:mike at tvage.co.uk>>
Gesendet: Dienstag, 28. Januar 2020 15:23
An: 'Gerd Petermann'; 'Development list for mkgmap'
Betreff: RE: [mkgmap-dev] Squaring off of land outlines

Hi Gerd, I have produced a small sample which demonstrates the problem. It
contains one very small island. I have cut down the style to a bare minimum
with just two resolutions: 24 and 14 containing just sea and a coastline
outline. At resolution 24 the sea and coastline are coincident. At 14, they
are wildly different. The coastline at low resolution includes a spur which
is a complication. The ideal sea boundary would be the same as the
coastline, but with the spur removed (or perhaps the spur should not be on
the coastline either). I have included the style, typ file, command line in
a batch file and osm data in the attached zip. Please let me know if you
need anything else.

Cheers,
Mike

-----Original Message-----
From: Gerd Petermann
[mailto:gpetermann_muenchen at hotmail.com<mailto:gpetermann_muenchen at hotmail.c
om>]
Sent: 28 January 2020 08:26
To: Mike Baggaley <mike at tvage.co.uk<mailto:mike at tvage.co.uk>>; 'Development
list for mkgmap'
<mkgmap-dev at lists.mkgmap.org.uk<mailto:mkgmap-dev at lists.mkgmap.org.uk>>
Subject: AW: [mkgmap-dev] Squaring off of land outlines

Hi Mike,

I can try to find out what's wrong but I would need a lot more details about
your map creation process. Could be an error in mkgmap or in the OSM data or
in the --levels option or in the --min-size-polygon option or
--polygon-size-limits or ...

Gerd

________________________________________
Von: mkgmap-dev
<mkgmap-dev-bounces at lists.mkgmap.org.uk<mailto:mkgmap-dev-bounces at lists.mkgm
ap.org.uk>> im Auftrag von Mike
Baggaley <mike at tvage.co.uk<mailto:mike at tvage.co.uk>>
Gesendet: Montag, 27. Januar 2020 19:07
An: 'Development list for mkgmap'
Betreff: Re: [mkgmap-dev] Squaring off of land outlines

HI Gerd, I don't have an old map, but this has been happening for some time
- I am not sure whether it has always done this, but thought it would be
worth trying a few old versions to see whether it is bug that got
introduced, or is one that has always been there. It looks similar to the
problem reported on Feb 18 2017 with "Terraced/pixellated coastline with
--precomp-sea" as the title. I am not using --precomp-sea, but the problem I
am getting is also in the sea outline. NopMap indicated that his problem was
fixed in r3819, however, I have downloaded the source to that version and
compiled it, and it exhibits the same problem for me.

Regards,
Mike

-----Original Message-----
From: Gerd Petermann
[mailto:gpetermann_muenchen at hotmail.com<mailto:gpetermann_muenchen at hotmail.c
om>]
Sent: 27 January 2020 08:36
To: 'Development list for mkgmap'
<mkgmap-dev at lists.mkgmap.org.uk<mailto:mkgmap-dev at lists.mkgmap.org.uk>>
Subject: Re: [mkgmap-dev] Squaring off of land outlines

Hi Mike,

I am not aware of a folder that keeps all older releases or an error that
would be related to this.
Are you sure that you didn't change mgkmap options like --levels or the
style or the typ file?
Do you still have a map which looks better? In that case you should find the
string mkgmap in the img file, e.g. "mkgmap-r4431"

Gerd

________________________________________
Von: mkgmap-dev
<mkgmap-dev-bounces at lists.mkgmap.org.uk<mailto:mkgmap-dev-bounces at lists.mkgm
ap.org.uk>> im Auftrag von Mike
Baggaley <mike at tvage.co.uk<mailto:mike at tvage.co.uk>>
Gesendet: Montag, 27. Januar 2020 01:43
An: 'Development list for mkgmap'
Betreff: [mkgmap-dev] Squaring off of land outlines

Hello,

I am getting squaring off of the outline between land and sea at low zoom
levels (less than 16) - see attached BaseCamp images of Gran Canaria at
resolution 16 and at resolution 14 displayed at the same size. Obviously
some reduction in accuracy is expected at lower zoom level, but not these
artefacts. I'm not sure when this started happening but believe it was some
time ago. I can't seem to find the old releases to try previous versions,
http://svn.mkgmap.org.uk/mkgmap/releases/ just shows me an empty folder. Can
you point me to the correct download folder that contains the previous jar
files, and/or let me know if this is a known problem, and if so whether
there is a workaround?

Thanks,
Mike

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