[Tilesathome] Level 8 captions

[80n]

On Fri, Oct 3, 2008 at 1:59 PM, Sebastian Spaeth <Sebastian at sspaeth.de>wrote:

> 80n wrote:
> >     lowzoom captions are suppressed by the horizontalProximity="0.008"
> >     verticalProximity="0.00625" rules in the style sheets that suppress
> >     labels that are close to each other. which ones will be suppressed
> >     depends unfortunately on the requested area, which means that
> >     neighboring tiles can suppress different labels.
> >
> >     No, there is nobody who is about to fix this as far as I know.
> >
> >
> > Unless I'm missing something, this should be fixable quite easily by
> > just increasing the size of the bbox so that adjacent tiles are using
> > roughly the same data.  For cities at low zoom levels the bbox should be
> > several times larger than the size of the tile to be rendered.
>
> That's what I thought too. Unfortunately, it doesn't seem to work. The
> current lowzoom captions (at least those that I created) were done with
> a margin of 2 on each side, so I downloaded the area of 5x5 tiles just
> to create one caption tile. There seems to be an "avalanche" effect,
> once it starts suppressing different cities at the borders it will start
> suppressing different cities in the middle too...
>

Oh, ok, that makes sense.  If I remember correctly, the algorithm (for
Osmarender) starts by removing places that are extremely close together
first, then removing those that are still too close and iterating like
that.  This reduced the bias caused by working from left to right but
doesn't totally eliminate it.  Increasing the number of iterations might
help and/or reducing the size of each iteration.

If the sort order of the starting set of elements is different for each tile
then the effect might be exaggerated.  Perhaps sorting places by name or id
before processing might make it a little more consistent.

At z8 we are just rendering city names right?

It may be a bit extreme, but if *all* the cities on the planet were
downloaded for each z8 tileset then we would be guaranteed to get the same
result for each tile.

There are currently just over 5,000 cities in the database, resulting in a
1.5Mb download.  I don't know how well the renderers would cope with that,
but it might be worth a try.

If not practical then maybe using a bbox that is much much larger than the
tile (say half the size of Europe) so that there are enough sparse areas
(like seas and deserts) to dampen the avalanche.

80n




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