[mkgmap-dev] Request - set --reduce-point-density= for each resolution

Sat May 1 09:46:05 BST 2021

Hi Felix,

reg. low resolutions:
I think there is also an error in mkgmap which filters e.g. large parts (almost all) of motorways at resolution 14.

Gerd

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Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Felix Hartmann <extremecarver at gmail.com>
Gesendet: Freitag, 30. April 2021 16:55
An: Development list for mkgmap
Betreff: Re: [mkgmap-dev] Request - set --reduce-point-density= for each        resolution

sorry - patch has some typos as i was still playing around with values. here is a working version. These values I feel are good with 1.3 at commandline.

On Fri, 30 Apr 2021 at 22:44, Felix Hartmann <extremecarver at gmail.com<mailto:extremecarver at gmail.com>> wrote:
I hardcoded a patch - as I was not sure someone is interested in it. This is doing the job - but I do feel others could profit as well. And yes I now understood how I could change this for a map starting at resolution 23 as well. I'm still not sure why do we not run the DP filter on resolution 24? I would image a very small value would give some improvement without visually changing anything.

I haven't really looked at polygons yet - but my patch looks pretty good to me if you use a value of 1.3. So just as recommendation for default values. I feel the current default of 2.6 was exactly that low - because at resolution 23 a high value is really bad, while at lower resolutions there is a need for simplification. it's a bad compromise.

On Fri, 30 Apr 2021 at 20:38, Gerd Petermann <gpetermann_muenchen at hotmail.com<mailto:gpetermann_muenchen at hotmail.com>> wrote:
Hi Felix,

you are probably right, the DP filter was not adapted to work with a map with resolution 23 as highest resolution. A quick change would be to disable the filter at level 0, similar to a map with max resolution 24.
The --reduce-point-density sets the error distance for the Douglas-Peucker (DP) algorithm.
If I got you right that's not enough for a map with levels = 0:24, 1:23, ... because you would want to use a rather small error distance for the first two layers and a higher value for the rest.  Sounds reasonable to me. Probably also for --reduce-point-density-polygon.

I'll try to implement both.

Gerd

________________________________________
Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk<mailto:mkgmap-dev-bounces at lists.mkgmap.org.uk>> im Auftrag von Felix Hartmann <extremecarver at gmail.com<mailto:extremecarver at gmail.com>>
Gesendet: Mittwoch, 28. April 2021 05:03
An: Development list for mkgmap
Betreff: [mkgmap-dev] Request - set --reduce-point-density= for each    resolution

As I'm not sure everyone is following the other topic, I would like to ask to introduce the possibility to set different values for the DP filter based on resolution.

Why is this important?

a) maps which are produced with resolution 23 as highest resolution need to have a much lower level than subsequent resolutions.
b) For resolution 23 and 22 a lower level makes sense - it is not needed as much here to speed up the gps device map drawing speed, while for lower resolutions like 18-21 a high level is really needed else map panning becomes painfully slow (usually basemap kicks in at 17).

If I'm correct right now 2.6 at resolution 23 would be 5.4 at resolution 22, 10.8 at resolution 21 and so on. It's okay to keep that behaviour - but the value doubled just needs to be possible to be set for each resolution. It doesn't matter to me if we keep it like that or if we change the numbers than ourselves by resolution. Important thing is that it's possible to be set for each resolution

c) maybe also introduce the possibility to set DP for resolution 24. I think a value of 0.3-0.6 could be worth it. I'm not sure if there were some problems preventing DP to be used at 24. However as by now I think mkgmap is able to produce routable maps starting at 23 instead of 24, those problems should be fixed...
In general I think a map made for automotive use (car, motorcycle) would be fine using resolution 23 as a start instead of 24. Like Garmin City Navigator maps do. But then yes it's really important to not have a high DP value on resolution 23. Same as contourlines only maps which make sense at resolution 23 instead of 24.

--
Felix Hartman - Openmtbmap.org & VeloMap.org

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--
Felix Hartman - Openmtbmap.org & VeloMap.org

--
Felix Hartman - Openmtbmap.org & VeloMap.org