[OSM-dev] Tiles at home empty tile detection

Robert (Jamie) Munro rjmunro at arjam.net
Wed Mar 21 00:24:34 GMT 2007

Hash: SHA1

Sorry to reply to this thread so late...

Dirk-Lüder Kreie wrote:
> Hi
> There seems a lot of rumors and misconceptions around how the empty tile
> detection in tiles at home works.
> Some clarification about the why and how of ETD:
> - The server will be running out of harddrive space soon if there's
>   nothing done to prevent empty tiles cluttering the space.
> - The previous (crude) approach was to simply delete tiles smaller than
>   a certain size, based on the assumption that the less complex the
>   tile the smaller the png size, which is quite accurate, but...
> The Problem arose when comparing the outputs of different png
> implementations, some manage to squeeze 2 roads and a bridge in 654
> Bytes, others render a completely empty tile at 759 Bytes. So it became
> obvious that filesize is not accurate enough to see wether a tile is
> empty or not. This becomes even more clear when you consider large
> areas (forest), that produce uni-colored tiles, that will (by file size)
> look exactly like an empty tile.

In that case, we must make sure everyone is using the best PNG
implementation. Even if it doesn't help solve the above problem, it will
save disc space and bandwidth in on the server and make the site faster
for end users. One possible option is to run all the images through
optipng or similar (http://optipng.sourceforge.net/), preferably on the
tiles at home clients before they upload.

If the client knows an image is one colour, could it not upload a
message to the server telling it to redirect to a location that keeps
lots of one-colour tiles? Possibly the message could take the form of a
.htaccess file for the folder in question, or something that can be
picked up by an apache mod_rewrite rule.

Robert (Jamie) Munro
Version: GnuPG v1.4.6 (Darwin)
Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org


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