Hi Pilou,
I've made two illustrations which might help you to understand what extraodinary faces is.

A vertex surrounded by three (1) or more than four (3) faces is called extraordinary vertex. And a vertex surrounded by four faces (2) is called ordinary.

If one or more vertex of a face is extraordinary, this face is called extraordinary too.


My script can load meshes with following types of polygons:

A - Ordinary quads ( 4 ordinary vertices ) - full support, best quality
B - Extraordinary quads ( 1 or more extraordinary vertex ) full support, best quality
C - Ordinary triangles ( 3 ordinary vertices ) looks good, but sometimes an artifacts can be noticable,
D - Extraordinary triangles ( 1 or more extraordinary vertex ) doesn't support at all. produces noticable artifacts on all neighbour faces.
E - Polar triangles ( group of triangles with a common vertex, two other vertices of each triangle must be ordinary ) full support, best quality.

So, you should not use type C and type D polygons.