The On Screen Display (OSD) provides status information and controls blended directly onto the map.
Current (2008-06-21 svn) OSD types:
<osd type="button" x="0" y="0" command="zoom_in" src="osd_plus.png" /> <osd type="button" x="0" y="32" command="zoom_out" src="osd_minus.png" />
You will need to provide your own osd_plus.png and osd_minus.png to use this example (they are not in navit svn yet). The x and y coordinates depend on your screen resolution and the size of your bitmaps. The origin is at the upper lefthand corner of the screen.
You can determine the supported xml options for each OSD type by reading navit/osd/core/osd_core.c, searching for "attr_xxx" inside the osd_xxx_new() functions. Most types have at least an "x" and "y" option that determine the location of their OSD.
The OSD layer is still under development.