mapnik/Ruby-Mapnik — pull request #18 by alno (2012-03-14 17:45:07+00:00)
Title: Allow to specify image format when rendering to file

According to native save_to_file function - second argument is optional format name.

For example, with this patch 256-color png may be forced by passing 'png256' (by default .png rendered as 24-bit)