[mkgmap-dev] Sensible resolutions - (or patch 5)

Sat Mar 26 11:50:43 GMT 2011

Actually, we do have a mean: if there are multiple parallel tracks (each
> drawn as a separate way with railway=*), it is a major railway. It
> should be doable to merge adjacent ways at lower resolutions and sum the
> "weights" of the ways, to decide what to draw. A style file extension
> could be useful, to specify e.g., the following:
>
> * draw individual railways at resolution 24
> * merge parallel tracks to one and draw them at resolution 22..23
> * merge parallel tracks to one and draw if count>1 at resolution 21
> * merge parallel tracks to one and draw if count>3 at resolution 20
>
> For polygons, it could be useful to specify the minimum size that
> qualifies for inclusion in a given resolution. Of course, small adjacent
> polygons would have to be merged together first.
>
> I am beginning to think that it could be useful to have low-resolution
> versions of the OpenStreetMap data, generated by an experimental
> algorithm that attempts to merge polygons and lines as suggested above.
> If it is not too CPU intensive to merge adjacent areas and lines,
> perhaps it could be implemented inside mkgmap after all.
>
I'm thinking since some time about an algorithm for merging small 
polygons. Up to now I have not found an suitable algorithm. In another 
case I have thought about merging parallel lines, especially the both 
tracks of highways.
While reading your mail I got the insight, that this are possibly two 
similar problems. In both cases there should be two objects with a small 
space between it be replaced by a merged single one.

The algorithm in general would be:
- Increase all polygons/lines with a outline.
- Check all increased polygons, if some of them overlap.
- If so, merge the original version of them.

But I expect this to be a really resource hungry task.

Regards,
Johann.