For my application, I need to produce tiles for only 3 or possibly 4 levels of zoom. Going by the OSM standard map with levels 1-18 (or is it 19 now?) I only need, e.g., 10, 16, and 17. If I’ve counted correctly, I can get by with 4 bits/pixel, which if I can use indexed color means I can cut the bandwidth consumption in half.
Are number of zoom levels and bits per pixel switch selectable, or selectable in the “css” (xml) descriptor file? I haven’t been able to find any mention of it anywhere, but surely I can’t be the first one ever to need to do such subsetting.
And if you don’t manage to find a solution, try http://mapnik.org/contact/ – yours is a very specialized question and probably beyond what you will get an answer for on this forum, unfortunately.
Someone who understands it all could do everyone a big favor by documenting the process to a professional standard. My perception is that it’s definitely not at that standard now. Everyone is so eager to write new code that there are skews all over the place, and obsolete, fragmentary, and idiosyncratic descriptions of how to do things. I note that tilemill expects mapnik 2.3, but 2.2 is current and 2.3 isn’t even mentioned at mapnik.org under Releases. Thumbing through the mapnik code to discover how things work, which is what I’m doing, is not a very productive way to gain information, particularly if the code changes out from under me while I’m still searching.
I can’t imagine how it will ever get better unless someone with professional qualifications takes it in hand.