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[mkgmap-dev] [Patch v1] improve LinePreparer to reduce img size

From Gerd Petermann GPetermann_muenchen at hotmail.com on Sat Jul 16 18:28:24 BST 2016

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:


Depending on the data a single tile with size ~5MB may be reduced by 50 - 500+kB.

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