parameter mask for pixmaps

[Ignacio Nodal <inodal teleline es>]

Hi, I'm adding a xpm into a button but i don't really understand how to
use the "mask" parameter.

The Reference Manual says:
"GdkPixmap*  gdk_pixmap_create_from_xpm      (GdkWindow *window,
                                             GdkBitmap **mask,
                                             GdkColor
*transparent_color,
                                             const gchar *filename);


Create a pixmap from a XPM file:

mask : a pointer to a place to store a bitmap representing
the transparency mask of the XPM file. Can be NULL, in which case
transparency will be ignored."

But I haven't found any example where the parameter "mask" is really
used. I mean, in the examples i've found it will be the same to put a
NULL instead.

Can anyone show me an example using another value for "mask"??

Thanks in advance

Ignacio Nodal