Have you got an example to show me?
I'm not quite sure I understand.
( magnum, Mon 11 Jul 2005, 14:30, archived)

*plays sound effect of trouser zipper*

( Enigmatic ™                 Voice No More                  ©, Mon 11 Jul 2005, 14:32, archived)

here
www.genarts.com/pics/HalfTone.jpg

the way it fades from black to white.
see what i mean?
imagine a strip from black -» white -» black again so it appears to fade in the middle.
(thisismyclone, Mon 11 Jul 2005, 14:45, archived)