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

Tue Sep 6 21:47:38 BST 2016

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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>> 

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