gdk_gc_set_function() mystery

[mikecorn <mikecorn t-online de>]

I am having a problem with gdk_gc_set_function()

The following code draws black lines:

   gdk_draw_line(dwin1->window,gdkgc,x2,y2,x2+pw,y2);
   gdk_draw_line(dwin1->window,gdkgc,x2,y2,x2,y2+ph);
   gdk_draw_line(dwin1->window,gdkgc,x2,y2+ph,x2+pw,y2+ph);

My wish is to replace the constant black lines with lines that invert
whatever color they are being drawn over. 

I added gdk_gc_set_function() calls around the above code:

   gdk_gc_set_function(gdkgc,GDK_INVERT);
   gdk_draw_line(dwin1->window,gdkgc,x2,y2,x2+pw,y2);
   gdk_draw_line(dwin1->window,gdkgc,x2,y2,x2,y2+ph);
   gdk_draw_line(dwin1->window,gdkgc,x2,y2+ph,x2+pw,y2+ph);
   gdk_gc_set_function(gdkgc,GDK_COPY);

The drawn lines are the inverse of the foreground color, not the
inverse of the replaced RGB pixels. I tried the other functions
and I never got anything but the foreground color (black) or its
inverse (white).

I must not be understanding something in the documentation.
Can someone please help?

thanks

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