Ok, here follows V3 of my patch. Needs some small code cleanup but works fine for now. I've now moved the code into the filter framework. First I've split up the SmoothingFilter into a SizeFilter and a SmothingFilter, as I consider the size filtering an task, which had nothing to do with the smoothing. Second I replaced the SmoothingFilter with the DouglasPeuckerFilter class. This does a much more convienient job at straightening out lines. For my test case it reduces the file size to 90% of the original one.
Well, I think you have to have a means of disabling it. Whether it is enabled by default or not doesn't worry me too much (however, I do think that by default, the map data should be frigged as little as possible).
The parameter is called --reduce-point-density=num. Without a given value it takes 2.6, which should lead only to invisible changes.
Would it make sense to do a simplification per level let's say 24 bits - 10m, 23 bits 20 meters, and so on ?
The allowed error distance gets shifted with the levels, so at lower zooms the filtering reduces more points. Problem at very low zoom levels is, that the MapLines becomes to short, soo a huge amount of short lines gets to be drawn. It would be very reasonable to merge the lines before straightening them out.