inverting pixels

[Rok Roskar <rroskar artsci wustl edu>]

I use a drawing_area to display an image... I read in a data array of
pixel values and the user has an option of seeing it as it is or
inverted. To invert the image, I use all the same data, but I call
gtk_gc_set_function(gc, GDK_OR_INVERT). This does not produce the
expected result... often the image becomes completely garbled, but I've
noticed that if I reload the image several times it eventually displays
correctly. All of this is very puzzling... obviously I am
misunderstanding how a gc function is applied... can someone please
provide some insight? Thanks,

Rok