Api changes between v2.2 and v2.3 - plepe/mapnik GitHub Wiki
Removed
filter_factor removed from GDAL datasource options. This has always been available as a RasterSymbolizer property and can continue to be set there.
Changed
Default PNG output now is paletted using the high quality alpha-preserving hextree encoder (what used to be only possible with png8:m=h. This is now triggered when the png format string is used. Use png32 to maintain rendering of full color rgba images as previously. More details at this ticket and Image-IO
colorize-alpha filter now takes a single color argument (rather than requiring more than one) and will use this single color to paint in place of the existing alpha values.