gdk-window-set-opacity (window opacity)

window -- a toplevel gdk-window object
opacity -- a :double with the opacity

Request the windowing system to make window partially transparent, with opacity 0 being fully transparent and 1 fully opaque. Values of the opacity parameter are clamped to the [0,1] range.

On X11, this works only on X screens with a compositing manager running.

For setting up per-pixel alpha, see the function gdk-screen-rgba-visual. For making non-toplevel windows translucent, see the function gdk-window-composited.