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

Tue Sep 6 21:37:19 BST 2016

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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> mkgmap-dev at lists.mkgmap.org.uk
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