How can I "catch" these changes in a GtkWidget?

[Daniel Haude <dhaude physnet uni-hamburg de>]

Hello,

I'm writing an application which involves a video
"overlay" window (i.e., a screen region into which a vide grabber
card writes directly). My problem is that since this process
completely circumvents Gtk/Gdk, the window manager and even
(presumably) X itself, I need to be able to tell the grabber card
when the screen region (a GtkDrawingArea, incidentally) into
which it is dumping its data becomes invisible in whole or
partially. Essentially what I need to look for is the opposite of
the "expose" event.

I'm sure somthing like this exists somewhere within the X system,
otherwise XawTV couldn't work. But I haven't really figured out
how XawTV does this, and I wonder if this functionality exists
within gtk.

For clarification: My problem is not how to tell the grabber card
"Do not write to this region of the screen". That's easy. I need
some whay to get the necessary information so I can tell the
video driver about it.

Thanks for any suggestions,
--Daniel