Inspired by Watercolor map created by Stamen Design I'm trying to create custom OSM based map tiles. In the article the Stamen Design guys describe the way of post-processing their tiles (adding Gaussian blur, perlin noise, etc.). I have my own tile server with custom CartoCSS style. I'm pretty happy with it, but I'd also like to add some custom stuff (Gaussian blur etc.) just to a single layer of my CartoCSS style.
Is it possible to post-process layer of CartoCSS (in Tilemill editor) by some external application (e.g. Gimp command line)? What if I want to post-process the layer by some algorithm written in Java/Python/C++? How would I invoke such post-processing for a CartoCSS layer?
I searched through docs of Mapnik and TileMill but so far I didn't find any info regarding to my question:
Mapnik is based on AGG graphics library. From the mapnik sources one can find out how e.g. the lines (ways/paths/...) are rendered. One solution might be to write a custom "symbolizer" (as they call "renderer" in mapnik project). This "symbolizer" could be probably used to render a layer. I'm not sure if this is a correct assumption, but so far this seems to be the right way. I need to do some tests first.