[Tilesathome] color-reducing

[Frederik Ramm]

Hi,

> ImageMagick's convert knows a -map option with which one can specify a
> sample image from which it will take the colour palette and apply it
> to the image to be converted.  This should ensure uniform colors
> across the map.  The tradeoff is a potentially less optimal colour
> conversion for each individual tile.  But I think uniform colours are
> more important.

I'm currently doing some work on OSM animations (with Mapnik rendered 
images). I have had good success by combining a number of wildly 
different tiles into one image and then running

convert sample.png -depth 8 -colors 256 -unique-colors colormap.png

on it. The resulting colormap.png has one pixel per color. It could 
still be tweaked manually if need be, and then you run

convert tile.png -colors 256 -depth 8 -map colormap.png output.png

on the payload images. The resulting file size is, in my case, slightly 
smaller than a corresponding GIF image, and running an extra pngcrush on 
the resultant file gives only one or two percent additional saving.

Bye
Frederik

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