[mkgmap-dev] [mkgmap-svn] Commit r572: MDR16 is some kind of codebook.

Tue Dec 21 19:10:07 GMT 2021

Hi Gerd

That's great. 

I don't have any maps with compression/Mdr16 or Mdr30-3.
http://gis.19327.n8.nabble.com no longer seems to work for me.
I found possible links to maps at:
http://www.garniak.pl/viewtopic.php?f=9&t=398
but interesting links were dead or went to generic page.

I suspect the #bytes after the delta for each level might be related to
the size of Mdr15 or some bits in byte 0/4/5/6 and related to letter
frequency count.

I didn't notice a different length sequence (6 or 5) starting at byte 4
in the mdr sections you sent (benelix, topo_d_v3 and the original)

I puzzled over the values before the level 5 data and couldn't see a
way of working out how many there were and what they might mean. The
benelux map is confusing because it is 0b 16 0b 16 0b. 

It is going to be interesting when re-generating this to find out how
much of all the unknown or redundant information is required by the
Garmin software!

Ticker

On Tue, 2021-12-21 at 16:49 +0000, Gerd Petermann wrote:
> Hi Ticker,
> 
> looks good.
> With your hints I was able to write a patch for MdrDisplay
> that seems able to decode compressed mdr15 for my
> maps which have mdr 16 section.
> Maybe you have another map...
> 
> It also changes some details for mdr 30/31 and 32/33.
> Open problem is the length of the gap between the end of data for the
> 6th level and the first bytes for level 5 (prefix 0b).
> I see either 7 or 12 or 5 bytes. Check the code that calculdates value
> for variable skip.
> The values might not at all depend on the depth of the tree.
> 
> Code is not at all optimized, it just started to work for me.
> Lots of bytes in MDR 16 are ignored.
> 
> Gerd