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

Sat Jul 16 18:43:23 BST 2016

Great discovery Gerd!

Enviado do meu iPhone

> Em 16 de jul de 2016, às 14:28, Gerd Petermann <GPetermann_muenchen at hotmail.com> escreveu:
> 
> 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
> <reduceImgSize_v1.patch>
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