Hi Jakob, the trick is used whenever it saves bytes. It is more likely that this happens when you have many nodes, it is not related to the type of line. Gerd Jakob Mühldorfer-2 wrote
Hi Gerd,
is this trick only good for polygons or does this work with normal streets/tracks as well?
Jakob
Am 16.07.2016 um 19:28 schrieb Gerd Petermann:
Hi all,
during the last days I tried to find more about the way how mkgmap stores
polygons, esp. why we have some limits. While doing that I stumbled over an
old comment in LinePreparer.java:
// I don't care about getting the smallest possible file size so // err on the side of caution.
which made me curious because I do care ;-)
I found out that Garmin offers a "trick" in the delta encoding which allows
to reduce the needed bytes in some cases, the imgformat-1.0.pdf
explains this in detail, search for " only the sign bit is set ".
This "trick" helps esp. well when coastline polygons or rivers are stored.
I can explain this more detailed if somebody likes to know more,
for now I'd like to hear if the trick causes any problems, I did not find any
so far.
A binary with this and a slightly improved overview_levels_v2.patch
can be found here:
http://files.mkgmap.org.uk/download/301/mkgmap.jar
Depending on the data a single tile with size ~5MB may be reduced by 50 - 500+kB.
Gerd
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