[Tilesathome] Best fix for missing captionless tiles?

[Sebastian Spaeth]

Alan Millar wrote:
> I'd like to fill in many of the missing captionless layer tiles.  I'm
> mainly thinking of the ocean and coastal areas at the moment.
> 
> It seems to me more efficient to have a single blank area at the widest
> scope/lowest zoom possible.  So the middle of the ocean could have a
> single z4 blank tile instead of 65536 z12 tiles.  But that means a little
> recursion when reading the tiles.  Which is costlier: more blank tile
> records or more recursion?

Depends. It's a tradeoff. If you want to browse an empty area at high 
zoom, lot's of recursion will make that slow. However, storing each z17 
blankness is not going to be fun too. Most would argue, that we don't 
want to optimize for pannning at z17 over the Pacific ocean.

In case of doubt, I'd opt for the recursion, but I cannot back that up 
with performance numbers. There are some maintenance scripts for the 
blankness database at 
http://svn.openstreetmap.org/sites/other/tilesAtHome/Tools/CleanBlankDB/
which try to bubble up blankness information or remove redundant 
entries. You are welcome to have a go at improving their smartness which 
would be more effective as it would improve the storing of the 
information globally.


> Also, what happens when a blank tile has something placed over it?  For
> example, if a z8 blank sea tile has an island placed in one corner, will
> the blank tile stay in place "behind" the filled tiles, or will it be
> deleted and the rest of the area filled with a bunch of "unknown type"
> tiles?  I'm guessing the latter, but I don't want to assume.

Blankness stay's "behind" the tile and is not removed (at least that's 
how I think it should be, haven't checked the t at h code recently). 
blankness info is only looked up in case no physical tile file exists, 
so it doesn't hurt to have a blankness entry on an existing tile.

> I'm guessing that larger blank tiles are better, and partially blank areas
> need to be filled with higher-zoom blank tiles.  Based on that I'm
> considering writing a script to walk through the oceantiles.dat file
> recursively and generate captionless-layer blank sea tiles to upload.


Sounds good.

spaetz