This pattern is generated by jsdotpattern script through this link:

http://www.imagico.de/map/jsdotpattern.php#x,128,jdp32360;gv,6,32,32;rd,1,0,0,pixel,0.125,4,4,0,jdp23814,749d6c,aedea3;

Since exported SVG file is not properly handled by Mapnik, it was converted to PNG file with alpha channel.
