Okay I've done some comparisons between the two approximations.
For polygons the old version is much better because the patched version creates bad artifacts. For streets at resolution 23 (if used) to 21 I prefer it too. At resolution 20 I am indifferent and cannot really decide what is better. However from 19 to x the approach as given by the patch is far superior (drawing speed on GPS much higher, actually using the normal DP filter maps become more or less unusable for resolution 16,18 or 19 on etrex).
Can someone see how this behaviour could be implemented for lines only from resolution 20 or 19 downwards, while keeping the current approach of the DP filter for the rest?
Thanks for intensively testing my old patches and the propose for them being checked in. Glad to see them work fine and beeing useful. :-) I have looked at this patch again. I think your proposal of switching between them at a given resolution is a good one. This should preserve the accuracy at higher zooms and compress the data at lower zooms. Should be worth a try. I'm sorry, but at the moment I have no toolchain ready for compiling mkgmap and preparing patches. If I find the time, I will look into it the next days. Regards, Johann