[OSM-talk] Color Problems with new Mapnik Style

[Andy Allan]

On Sat, Oct 25, 2008 at 10:29 AM, Michael Roth <mroth at nessie.de> wrote:
> Hello list,
>
> look at this:
>
> http://www.openstreetmap.org/?lat=47.73083&lon=9.02887&zoom=16&layers=B000FTF
>
> What is the cause of this problem?

I see that it's on a tile boundary, therefore I reckon [1] it's due to
colour compression. One of the utilities in the rendering toolchain
tries to minimize the number of colours used, in order to make the
tiles smaller. Usually it's only anti-aliasing that gets affected, so
it's hardly noticeable. But when there's two "main" colours that are
quite similar, occasionally it thinks that they can both be changed to
the same colour.

> How can it be fixed?

If colour compression is turned off, then tilesizes grow greatly. It's
turned off for the cycle map, and that's one of the reasons the tiles
are so much bigger. What would be neat is if the colour compressor
could be passed a list of "main" colours that it won't optimise away,
but that's development work for someone :-)

Cheers,
Andy

[1] probably - I could be completely wrong though :-)