[mkgmap-dev] Optimizing MapSplitter

Sat Nov 15 10:26:55 GMT 2014

Hi Steve,

I don't think that the current algo is completely wrong.
I think it is better to start with the complete data and divide it into 
smaller parts as that algo keeps the information of 
the location of elements, while your approach is likely
to compare many bboxes which are (too) far away from each other.

I think my last patch (mapsplit-mini-v2.patch) does not work because
of probems with high precision coordinates, but it also is too simple.
I am now working on a more complex one.

Gerd

> Date: Fri, 14 Nov 2014 12:05:08 +0000
> From: steve at parabola.me.uk
> To: mkgmap-dev at lists.mkgmap.org.uk
> Subject: Re: [mkgmap-dev] Optimizing MapSplitter
> 
> 
> Hi
> 
> > I think I'll try to change this:
> > 1) Put large objects (large bbox, maybe only few points as with 0x4b
> > background polygon)
> > into there own sub divs and don't add small objects to those sub divs
> > 2) Create smaller sub divisions in dense areas
> 
> I've been thinking of this way
> 1. Start with everything in its own subdivision.
> 2. For each subdivision:
>      Find other subdivisions that completely contain it.
> 	...but ignore subdivisions that are "too big".
> 	Combine the subdivision into the smallest of those found
>          that still have room.
> 3. Repeat "enough" times.
> 
> - "too big": a desktop zoomed to just the level covers less than
>    1000000 square units. My etrex 30 covers about 40000 square units
>    if I measured it correctly. So I would guess that divs can be
>    freely combined up to those kinds of sizes.
> 
>    But for larger containing divs, you would probably want to make
>    sure that all elements are at least 50% of the size of the division.
> 
> - "enough": I've no idea what that would mean or even if this would
>    work at all!
> 
> > I don't know yet how to do that, so I'll sleep about about again ;-)
> 
> Yes that is the problem!
> 
> ..Steve
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