Johann Gail wrote:
Hi,
I'm the coder of the original dp code. Some comments to it:
BTW - Is it possible that there is a difference with the douglas peucker
filter whether a road is routable or not?
Yes, looking at the code, it seems that it doesn't change points that
are routing nodes so routable ways will be "less smooth" than
non-routable ways.
This is true.
The reason for it laid in some possible inaccuracies at nodes.
Take for example a T-crossing. This means two lines.A straigth (long)
one and one ending at the node. In my first attempt the DP algorithm
handles all nodes indifferent. But this means, that the long line could
be straighened out at this node, whereas the ending line will never move
its end point. So the end node laid not on the line. This error could
become visible at some resolutions.
The only possibility to change this behaviour was to never move a node
connecting lines. This would affect all routing lines.
I think it is possible that non routable ways are smoothed heftier than
routable ways. This has nothing to do with this patch though, but the
difference only seems to become really visible on resolution <20
(maybe resolution 24 is filtered if not routable, but not filtered if
routable. Then the more filtering is applied on each successive level,
the bigger the difference).
It think the DP filter does nothing at res 24 whether the way is
routable or not.
Yes, as far as I can remember, there was some errors with the routing
tables, if I enable it at res 24. Also all other filters was disabled at
res 24. So I disabled it too.
Also I noticed that cgpsmapper maps, seem to have much higher smoothing
at lower resolutions compared to mkgmap (using 2.6). At resolution 24
and 22 it seem to be mostly identical though. Maybe we could also
increase the douglas peucker severity on resolution <20 compared to
right now. Even with very few lines mkgmap maps seem slow on GPS on
resolution 16 or 18.
That's worth experimenting with.
Perhaps Johann can comment on this, he's the original author of the DP
stuff.
Yes, at resolution 16 most of the nodes should be allowed to be
removable, as the connecting streets disappear at this level. But at the
time of writing I had no reasonable idea of how to get this information
inside the dp filter.
I don't think that this really matters. One will only use level 18 or 16
for orienation/map panning. Garmin City Navigator only goes down to 20,
then it's already basemap - of course in Mapsource they have a nice
filled overview map, which we are really missing. If we had a decent
overview map, I would have 18 as last level, as on my GPS I never use
level 17 or 16 cause it's too slow to be usable (even though it only has
motorways and trunk roads, plus cities more than 1Mio inhabitants or
Capitals.). Level 18 already could be much much more smoothed.
Have a look at the MalsingFreeMaps - I think their low resolutions are
really well done (smoothing as well as level of detail). (though level
22 and 20 is a bit too much information for my taste).
Maybe in the meanwhile this information is available?
Regards,
Johann
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