[OSM-dev] osm2pgsql, zoom range for -e option?

[marqqs at gmx.eu]

Hi Stephan,

meanwhile I've asked Steve Hill (author which is mentioned in the source file header) for help. His explanation in brief:

The expired-tiles list which is written by osm2pgsql does not contain any redundant information. This is intended that way because he wanted to save memory: the quad-tree in the algorithm should not grow too much.

For this reason there are two groups of tiles excluded from the list:

1. Next lower zoom levels
For example, if a tile at zoom level 18 is considered as dirty, you will need to render the related tile at level 17 too. The list will only contain the level-18 tile.

2. Next higher zoom levels
If four neighboring tiles at zoom level 18 are dirty and they fit into a single level-17 tile, only the level-17 tile is marked as dirty, the four level-18 tiles are not.

Of course I can understand the need to save memory while the expired-tiles list is created but in my opinion this list should be written as a complete list of expired tiles...

Markus

-------- Original-Nachricht --------
> Datum: Wed, 10 Aug 2011 08:19:34 +0200
> Von: Stephan Knauss <osm at stephans-server.de>
> An: dev at openstreetmap.org
> Betreff: Re: [OSM-dev] osm2pgsql, zoom range for -e option?

> Hi Markus,
> 
> On 09.08.2011 19:53, marqqs at gmx.eu wrote:
> > OK, I fixed the bug. - If it really was a bug and not intended
> behaviour.
> > I'm still not sure...
> >
> > My patch (I added just 2 lines):
> > diff -C 5 -p expire-tiles_old.c expire-tiles.c
> > --- 139,151 ----
> > + 	if(this_zoom>=min_zoom)
> > + 		fprintf(outfile, "%i/%i/%i\n", this_zoom, x, y);
> > +
> 
> I would also expect the lowzoom tiles being expired. Having a quick look 
> at the source I don'T understand both. What do you try to fix with this 
> patch?
> 
> If the tile is dirty, I guess it would match one of the tree->complete 
> conditions.
> 
> What is the flow? It looks like it is recursing by increasing the zoom 
> level. So it would have started at the low-zooms already. Why did it not 
> output anything in this situation?
> 
> Is the _mark_tile() working as expected? Do you have data to reproduce 
> the problem?
> 
> Stephan
> 
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