[mkgmap-dev] Polyline base optimisation

Thu Apr 8 16:45:36 BST 2021

Hi Ticker,

my understanding is that it is looking for the largest and the smallest delta value. It is possible to have a smallest value > 0.
It probably doesn't matter because of the way how xBtits and yBits is calculated but I still think that your version is even more confusing.

Gerd

________________________________________
Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Ticker Berkin <rwb-mkgmap at jagit.co.uk>
Gesendet: Donnerstag, 8. April 2021 17:34
An: Development list for mkgmap
Betreff: Re: [mkgmap-dev] Polyline base optimisation

Hi Gerd

I could have changed it to this, but it is looking for lowest val <= 0
and highest val >= 0 so 0 seemed the better choice.

The slight improvement is because it gives up after 2 tries that don't
show improvement, and before, for unsigned, one of these tries was
wasted.

Ticker

On Thu, 2021-04-08 at 15:11 +0000, Gerd Petermann wrote:
> Hi Ticker,
>
> Funny, I would have used Integer.MinValue for maxDx/maxDy to make it
> less confusing.
> Do you have an explanation for the change in size? My understanding
> is that your patch just skips a test which will never find a better
> encoding?
>
> Gerd
>
> ________________________________________
> Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag
> von Ticker Berkin <rwb-mkgmap at jagit.co.uk>
> Gesendet: Donnerstag, 8. April 2021 13:58
> An: mkgmap development
> Betreff: [mkgmap-dev] Polyline base optimisation
>
> Hi Gerd
>
> If starting with unsigned deltas in polyline encoding, and attempting
> to reduce the length by testing reduced x/yBase values, there isn't
> any
> point in testing Base-1, as the normal number of bits will be the
> same
> (added sign bit) and there will be some overflows.
>
> Patch attached to change this - it gives me a very slight reduction
> in
> RGN size and might give a performance improvement (I didn't detect
> any).
>
> I've also fixed a couple of log. logic errors and changed minDx/y
> initial to 0, which is much less confusing because they are symmetric
> with maxDx/y which do use 0.
>
> Ticker
> _______________________________________________
> mkgmap-dev mailing list
> mkgmap-dev at lists.mkgmap.org.uk
> http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
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