[mkgmap-dev] Suggestion - Make Douglas Peucker Algorithm more Configurable

Thu Jan 7 15:19:16 GMT 2010

> Essentially the best option for drawing Polygons would be to determine 
> their "resolution" based on size. So make large forests appearing at 
> lower resolutions than small forests (well I think we all know that best 
> would be if resolution of any element were adapted to map density, but I 
> think that is even more complicated). I don't think this would be an 
> easy task. Depending on the area or country (e.g. France with the Corine 
> land cover import) putting forests at low resolutions really slows down 
> map panning.
>   
Yes, I know, DP works not perfect at the moment for poygons. I've spent 
some thoughts on it, how to improve the thing. Maybe the algorithm 
should not look at the distance of the points, but at the difference in 
area. So some small areas with sharp angles should be candidates for 
deleting. (The current algorithm will preserve them)
Another idea comes together with the rounding to resolution:
Maybe we should not round to the nearest point, but to the point which 
least changes the outline, i.e which do the smallest changes to the 
angle of the polygon.
> An "easy" compromise could be to put "stronger" douglas peucker filter 
> on polygons than on roads (I currently use 5.4, because 10 seems to be 
> too strong on roads). However for Polygons a value of 10 works pretty 
> well. It would be great if there would be support for that 
At the moment there is a commandline switch (-simplify-lines=xxx) which 
allows you to set the dp error distance for each call. It should be 
doable with nearly no effort to introduce a second option for polygon 
settings.
> (I don't know 
> if the douglas peucker filter is applied after or before the style-file, 
> if before of course the above will not be "easy"). 
For your information:
DP filter is applied after the style file. Its applied while building 
the img. It must be this late, because it has to filter each resolution 
with different settings. The lower resolutions are filtered stronger.
> The better mapped 
> countries are getting, the more importance choosing the proper 
> resolutions will have. The changes to the default style over the last 
> weeks were already a good step forward, but outside of cities OSM rarely 
> has more than 20-30% of ways and landuse covered (I dare people in 
> future will not draw few big forests, but split up more and more 
> detailed) so the solely based on tags approach will become more and more 
> difficult... (well at least I hope the difference between countryside 
> and cities will flatten out a bit).
>
>   
I see one big problem:
DP is or will in near future be disabled for polygons because of 
multipolygon problem. (Suggested by Mark, if I recall correctly). The 
reason for it is, that the multipolygons get split up into smaller 
polygons. This seems neccessary because of limitations of the img file 
format.
But if such splitted polygons gets simplified, the parallel edges may 
not be parallel any more, which leads to drawing errors.
So what to do here?

Regards,
Johann