[OSM-dev] [PATCH] Delete empty t at h tiles on the server

Dirk-Lüder Kreie osm-list at deelkar.net
Wed May 2 09:45:08 BST 2007


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Jeroen Dekkers schrieb:
> At Tue, 01 May 2007 20:56:27 +0200,
> Frederik Ramm wrote:
>> Hi,
>>
>>  > With the
>>> attached patch those empty tiles get deleted, including all higher
>>> zoom level tiles.
>> Such a "cascading" deletion is probably incorrect in corner cases where 
>> there is something on a tile that only gets rendered at a very high zoom 
>> level (e.g. a lone pub node). This will lead to an emtpy Z-13 tile, but 
>> doesn't mean that corresponsing Z-17 tiles may be deleted.
> 
> Can't we assume that if there are things on Z-17 tiles, there will at
> least be something on the higher level zoom tiles?
> 
> If that assumption is wrong, then we should also change the t at h
> clients, because at the moment they don't generate subtiles of empty
> tiles.

I suggest handling the exact empty tile, marked with a 67 byte png, and
not make assumptions for other zoom levels.

So if you get a zoom 14 tile with 67 bytes delete exactly the
corresponding tile from the server (if existent) and not any tile from
other zoom levels.
The Client can be configured so it looks all the way to z17 even if the
tiles with lower zooms are all empty, which in many cases it already
does, because it renders the area in stripes, so any feature that runs
the entire tile area from north to south forces a rendering to z17 for
all tiles, and thus every tile is properly marked as empty. If so
desired the config variable RenderFullTileset[1] can be set to 1 so
every client will create empty tiles always, even if the complete stripe
is empty in lower zooms.


[1]http://trac.openstreetmap.org/changeset/2239
- --

Dirk-Lüder "Deelkar" Kreie
Bremen - 53.0952°N 8.8652°E

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