[mkgmap-dev] Polyline base optimisation

Fri Apr 9 09:01:41 BST 2021

Hi Ticker,

OK, I've committed your patch as is. The (my) code in makeShortestBitStream() was already hard to read, maybe you find a better way to handle the backup/restore tricks of xSameSign and ySameSign?

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 18:03
An: Development list for mkgmap
Betreff: Re: [mkgmap-dev] Polyline base optimisation

Hi Gerd

It needs to know if there are deltas < 0, deltas > 0, and number of
bits to represent the largest delta, regardless of sign. If all the
deltas were, say between 10..20, max=20 is significant, but min=10 is
of no relevance; only if it was < 0 would it have an effect.

Ticker

On Thu, 2021-04-08 at 15:45 +0000, Gerd Petermann wrote:
> 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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