Find attached an patch (usable on R915), which reduces the point density of ways and roads to reduce the filesize of the resulting img file. It does it by using the douglas peucker algorithm. With this algorithm it is possible to set a max error distance and the resulting line will have filtered out all points without any points outside this distance band. For an exact explanation consult wikipedia. I tried it with an error distance of 10m. Higher distances don't gain much more. The saving is in some areas with bad osm data (useless high point density) up to 25%. In general it saves only 5% in file size. Consider this patch as an alpha release, which should only show the possibilities. For the release it will need at least an option at the commandline for setting the error distance. Also I'm not sure, if the StyledConverter.java is the correct file for this patch. I've seen there is some filter infrastructure in the mkgmap project, but I've no idea how to use it.