[OSM-dev] mod_tile/renderd serving metatiles

Lynn W. Deffenbaugh (Mr) ldeffenb at homeside.to
Sat Aug 25 02:27:31 UTC 2018

Greetings OSM developers,

Has anyone considered, designed, and/or implemented an extension to 
mod_tile to support serving raw metatile files to clients?

I've got my own tile server running using mod_tile and renderd and my 
own application that consumes OSM-compatible tiles.  I really don't like 
seeing the individual tiles arrive on my Android devices and would like 
to extend the knowledge of the metatile format into my client 
application.  That way, a single round-trip to the server brings back 
multiple tiles rather than one per round-trip.  The client caches tiles 
in an MBtiles database, so it can easily extract the individual tiles 
from the metatile and locally access them as needed.

I've looked at mod_tile and it appears to be an easy thing to add.  I'm 
considering extending the /status and /dirty approach to include a /meta 
option.  I'm not completely sold on this because I'm actually planning 
to return the entire meta tile for the specified /z/x/y.png that was 
requested along with http response headers describing the metatile 
parameters.  The latter part allows the client to not have to assume 
that the server is doing 8x8 metatiles but will be told on each response 
how many are in the file.

By returning a full metatile when a single tile is requested (with the 
/meta trailer), the client doesn't need to be aware of the metatile path 
naming, but only the format of extracting specific tiles from the 
metatile file.

I'm open to prior art in this area and/or comments on the thinking 
described above.  If I do this, is it something that anyone would be 
interested in having commited back to the repository?  In any case, it 
will be 100% backward compatible and possibly even a compile or runtime 
option for mod_tile to include this feature.

Lynn (D) - KJ4ERJ - Author of APRSISMO, an Amateur Radio APRS Client for 
Android under MOAI

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