[OSM-dev] mod_tile - meta tile computation

Frederik Ramm frederik at remote.org
Tue Dec 28 22:04:19 GMT 2010


Hi,

Philipp Borgers wrote:
> What if we disable directory hashing but use meta tiling? Is this use
> case intended? 

Everybody uses meta tiles nowadays. This means that the last item of the 
hash quintuplet is somewhat degraded in that it only has one of the four 
values 0x00, 0x08, 0x80, 0x88 - whereas without metatiles you would see 
all 256 possible combinations there.

> In theory the computation should map METATILE * METATILE tiles to one
> meta file? Which should result for x in [0,7] and y in [0,7] in the new
> values z/0/0.meta and for x in[8,15] and y in [8,15] in the values
> z/1/1.meta and so on. I should try to divide the input by METATILE if
> I'm right.

Now you've lost me. What this computation basically does is, it takes a 
20-bit x value and a 20-bit y value (the coordinates of the tile):

xxxxxxxxxxxxxxxxxxxx
yyyyyyyyyyyyyyyyyyyy

(for 40 bits altogehter) and morphs that into a series of five 8-bit 
values (again 40 bits), intermingling components of x and y:

xxxxyyyy xxxxyyyy xxxxyyyy xxxxyyyy xxxxyyyy

This computation, and the resulting file names, are the same for 
metatile and non-metatile mode, except that in single tile mode, the 
file name of the tile is

xxxxyyyy/xxxxyyyy/xxxxyyyy/xxxxyyyy/xxxxyyyy.png

(of course with the xxxxyyyy replaced by matching integer numbers 0-255) 
whereas in metatile mode, the file name of the metatile is

xxxxyyyy/xxxxyyyy/xxxxyyyy/xxxxyyyy/x000y000.meta

and the three least significant bits of x and y make up the offset 
inside the metatile.

Bye
Frederik

-- 
Frederik Ramm  ##  eMail frederik at remote.org  ##  N49°00'09" E008°23'33"



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