*.xbg = geometry, *.xbm = material
They share the same format ('Geometry Resource File'), with different block types.
I've written code that can successfully read in the entire format, though much of it is unknown.
http://svn.gib.me/public/dunia/trunk/Gi ... rceFile.cs
http://svn.gib.me/public/dunia/trunk/Gi ... /Geometry/
Here's what I know about the vertex buffer formats so far: http://svn.gib.me/public/dunia/trunk/doc/vertex buffer formats.txt
Understanding the entirety of the format would be nice.
