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

Fri Apr 30 15:44:25 BST 2021

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> 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> im Auftrag von
> Felix Hartmann <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
>
> _______________________________________________
> mkgmap-dev mailing list
> mkgmap-dev at lists.mkgmap.org.uk
> https://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
>

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