Look at the following screenshot:
OSD from gnome-settings-daemon (http://git.gnome.org/browse/gnome-se...d-osd-window.c
), coded in C with Cairo and GTK
I coded this one in Python with PyCairo and PyGtk
The transparent black rounded rectangles have the same color. They appear to be the same when looked at on white background. But they differ significantly on the black background.
How did the gnome-programmers achieve, that their "leve of transparency" adapts to the background?