[OSM-dev] Improvement of tile rendering

[Udo Giacomozzi]

I just set up my own Mapnik server that renders map tiles on demand
and saves them on disk so that they are efficiently cached. Currently
this is done using a PHP script (FastCGI handler with 5 instances)
that handles 404 errors and invokes a python script to render the
tile which then gets stored at the location that caused the 404 error.
Invoking python from PHP is suboptimal, I know, but I'm no Python
programmer, PHP lacks Mapnik support and so far this was only a proof
of concept. 

Anyway..

I noticed that to generate a tile, a 512x512 pixel map is generated,
which gets then cropped to 256x256 pixels and reduced to 255 colors by
using  'convert'.

I don't know if the mod_tile daemon does it the same way, but I guess
it does.

I guess the cropping is done to fix some text labelling issues (to
keep text readable across tiles), but in practice this does not work
well since I see lots of text cropped in Slippy Map.

Also, invoking the "convert" tool without doubt takes massive
resources compared to in-place color reduction.

Have any efforts been taken to use libmagick directly or avoid
external tools at least?

And, is it really impossible to avoid cropping at all? Can't Mapnik be
instructed (or extended) to render labels always at the same place, no
matter the viewport?

I'm sure this would improve both rendering performance /and/ quality.

Udo


PS: I'll be out of town until Friday, so can't reply any sooner...